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MACRO-8
PROGRAMMER’S REFERENCE MANUAL
FOR THE
PDP-8 FAMILY COMPUTERS

For additionaI copies order No. DEC—O8—CMAB-D from Program

Equipment Corporation, Maynard, Mass.
DIGITAL

EQUIPMENT

CORPORATION

Library, Digital
Price

$3.00

MAYNARD, MASSACHUSETTS

Ist Printing October 1965
2nd Printing November 1966
3rd Printing October 1967

4th Printing January 1969
5th Printing (Rev) November 1969

Your attention is invited to the last two pages of this

The Reader's Comments page, when filled in
manual
and returned, is beneficial to both you and DEC. All
.

comments received are considered when documenting

subsequent manuals, and when assistance is required,
knowledgeable DEC representative will contact you.

The Software Information page offers you a means of

keeping up-to—date with DEC's software

The following are trademarks of Digital
ment

Equip—
Corporation, Maynard, Massachusetts:

DEC

PDP

FLIP CHIP

FOCAL
COMPUTER LAB

DIGITAL

CONTENTS
PART I

BASIC INFORMATION

Page
CHAPTER I

INTRODUCTION

l—l

CHAPTER 2

THE BINARY SYSTEM

2-l

CHAPTER 3

THE PDP—8 INSTRUCTION SET

3—l

3.I

Instructions

3-l

3. I .l

Memory Reference Instructions

3-I

3. I .2

Augmented Instructions

3-3

3. I .3

Input-Output Transfer Instruction

3-3

3.I 4

Operate Instruction

3—4

3.2

The Organization of Memory

3-6

3.3

Complement Arithmetic

3-8

3 .3 .1

Addition

3—9

3 .3 .2

Subtraction

3-9

ORGANIZATION

4-I

4. I

Programming Steps

4-l

4.2

Coding

4—2

4 3

Comments

4—4

4.4

Address Tags

4-5

CHAPTER 4

4.4.l

4—6

Symbolic Addressing

4.5

Storage Techniques

4-6

4.6

Optimum Use

4—7

4.7

Subroutines

4—9

PART 2
THE MACRO LANGUAGE

CHAPTER 5

MACRO-8 PROGRAMMING LANGUAGE

5-I

5.l

Function

5-l

5.2

Compatibility

5-l

CO NTE NTS (Cont)

Page
Characters
l

mmmmmmmmmmm-h-Ah-pnwww

minow
I

#QN
.1

Links

CHAPTER 6
—l

Letters

5—2

Digits

5-2

Punctuation Characters

5—2

Ignored Characters

5—3

Coding Practices

5-4

Coding Structure

5—5

Elements

5-5

Integers

5-5

Symbols

5—5

Expressions

5-7

Special Symbols

5—10

Current Address Indicator

5—10

Origin Setting

5—10

Literals

5-H

Single Character Text Facility

5-12

PSEUDO-INSTRUCTIO NS
Functions

Current Location Counter

0~0~0~O~0\O~0~0~O~0~0~~00MO~U101014>0N

Extended Memory

Radix Control
Numbers
—I

Double Precision Integers

Floating Point Constants

CHAPTER 7

Text Facility
End of Program
End of Tape

Alterations to the Symbol Table

MACROS

7. I

Function

7.2

Defining

CONTENTS (Cont)

Page
Restrictions

7.3

CHAPTER 8
8

ERROR DIAGNOSTICS

Format

8 2

Error Messages

CHAPTER 9

OPERATING INSTRUCTIONS

9. I

Assembler Output

9.2

Versions

9 3

Procedures

9.4

Symbol Table ModifiCation

APPENDIX I

MACRO-8 SYMBOL TABLE

APPENDIX 2

ASCII CHARACTER SET

ILLUSTRATIONS
I

3—]

Memory Reference Instruction Format

3-2

IOT Instruction Format

3-3

Group I Operate Microinstruction Format

3—4

Group 2 Operate Microinstruction Format

4—]

Flow Chart of Program to Calculate Sum of Integers

4—2

Program Example

TABLES
2—I

First Sixteen Integers in Three Number Systems

3-]

Memory Reference Instructions

3—2

Group I Operate Microinstructions

3—3

Group 2 Operate Microinstructions

3—4

Effective Address Calculation

3-5

One's and Two's Complement Representations

9—]

Switch Options

3—2

PREFACE

This manual is directed both to the new user of programming systems and the expe—
rienced programmer.

Part I presents basic information on programming techniques

and an introduction to assembler concepts.

language.

Part II details the MACRO—8 assembler

The experienced programmer will find in Part II all the information

necessary for the use of MACRO-8.

DEC offers four symbolic assemblers for use on PDP-8 Family computers,

PAL III Symbolic Assembler, the basic 4K assembly system.

follows:

It is a

two-pass assembler with an'optional third pass which produces an octal/

symbolic assembly program listing, and is highly recommended for the
computer with 4K words of core memory. It is an excellent assembly
language for the less experienced programmer yet powerful enough to
satisfy the needs of the advanced programmer.
b.

MACRO—8 Symbolic Assembler, essentially PAL III with the following

additional features:

user-defined macros, double precision integers,

floating—point constants, arithmetic and Boolean operators, literals, text
facilities, and automatic off-page linkage generation. It is recommended
for the computer with 4K words of core memory when any of the additional
features listed above are desired.
c.

PAL—D Symbolic Assembler,

essentially MACRO—8 excluding macros.

It is used only in the PDP-8/I Disk Monitor System and requires 4K words

of core memory.
d.

8K SABR Symbolic Assembler,

to use with a

primarily for experienced programmers

computer that has 8K to 32K words of core memory.

It differs

from the preceding assemblers in its operating procedures, character set,
pseudo—ops, execution of the assembled program, and especially in its

assembled output (relocatable binary code).
It is assumed that the reader is familiar with assembly language programming.

For an elementary ap-

proach to this type of programming, we recommend DEC's publication, Introduction To Programming

(specify publication C-l8), available from the Program Library (address on Title page).

PART

BASIC

INFORMATION

CHAPTER

INTRODUCHON

In describing the solution oF the Following equation,

y=mx+b
one can

say,

"First, multiply the quantity x by m.

To this product, add the quantity b.

The result is the

value oF y.”
The same problem can be solved on an adding machine in the Following steps:

Clear the keyboard and registers.

Enter the value oF m and press the ADD key.

Enter the value oF x and press the keys which initiate the multiplication.

Enter the value oF b and press the ADD key.

The result, appearing in the totalizing register,

is the value oF y.

This sequence oF steps can be thought oF as a program For the solution ot the problem on an adding machine.
In similar Fashion, the steps can be written out For a solution to be perFormed by a digital computer.

In-

stead oF pressing buttons and keys, the programmer writes a sequence oF instructions to perForm operations
on

data stored in the computer.

Such a sequence (For a hypothetical computer) might appear as Follows:

clear
add m

multiply x
add b

deposit y
The variables m, x, and b represent quantities stored in the computer; the variable y represents a storage

The operations are carried out in a register called the accumulator, abbreviated AC.

instruction clears the AC.

The next adds the quantity m into the AC.

The third instruction multiplies

the contents 0F the AC by the quantity x, and the Fourth adds the quantity b to the result.
contents oF

The First

Finally, the

the AC are deposited into a storage register designated by the symbol y.

To be useFul to a computer, the instructions 0F 0 written program must be translated into a sequence oF
numeric codes, each oF which

represents a specific computer operation.

1—]

To do such

translating by hand

trom instructions to binary numbers would be tedious and
to perform

lengthy. Computer programs have been written

the translating task, interpreting the written program and producing from it a second program

which can be executed directly by the computer.

These translators are called assemblers because they

assemble from a source program written by the user, a working program instruction by instruction

This

output From the assembler is called an obiect, or binary program.
MACRO-8 is an assembler designed to accept input in the Form of a sequence of symbolic instructions

representing the operations capable of being executed on the PDP—8.
which may then be placed in the computer and executed.

It produces a binary program tape

The next chapter explains a Few basics of the

binary numbering system; succeeding chapters deal in more detail with the MACRO-8 assembly program.

PART

TH E

MACRO

LANG UAG E

CHAPTER

THE

BINARY

SYSTEM

Every item of information stored in or processed by a digital computer is encoded as a binary number.

Consequently, the user should become familiar with the binary system and be able to convert numbers
from binary to decimal representation and back, using the octal system as an intermediate.
as

the programmer gains more experience, he will find himself using the decimal system less and "thinking

in octal”

more.

This is a useful habit to cultivate.

Table 2-I gives the first I6 integers (and 0) as they are represented in the decimal,
number systems.

Note that in the decimal system there are ten different symbols,

used to represent any number.

binary, and octal
digits, 0-9 which are

In the binary system there are two, 0 and l; in the octal system,

The number of digits required in

0—7.
is

Eventually,

given system is called the radix.

eight,

Therefore, the decimal radix

I0; the octal radix is 8, and the binary radix is 2. A subscript is used to identify the radix of the sys-

tem in

and

which the number is represented.

IIOIOI2

Thus, 4196 I0 indicates a decimal number, 25478 an octal number,

binary number.

TABLE 2—I

FIRST SIXTEEN INTEGERS IN THREE NUMBER SYSTEMS

Decimal

Octal

Binary

000

IOOI

IOIO

00]
OIO

IOII

Decimal

Octal

Binary

Oil

IIOO

I00

HOT

IOI

IIIO

IIO

IIII

III

IOOOO

1000

All numbering systems using radices involve positional notation; that is, each successive digit position to
the left represents the next higher power of the radix.

For example, the decimal number

expressed algebraically as

4xI03+IxI02+9xIOI +6xl00

2—I

4I9610 may be

which when calculated becomes

4x1000+lxlOO+9xlO+6xl=4l96

Positionally, this appears as

102

TO]

100

1000

l00

(Radix to its respective powers)

(Units required of each value)

103

(Positioned radix)

Likewise, the octal number 25478 can be expressed as 2 x 83 + 5 x 82 + 4 x 81 + 7 x 8

0
.

In all systems the integral and Fractional parts of a number are separated by a radix point.

the system in use, this may be a decimal point, octal point,

Depending on

binary point.

As can be seen from the table, four binary digit positions are required to represent the decimal
up to 9.
are

The octal integers up to 7 require only three binary positions; furthermore, exactly three positions

needed.

octal digits.

In other words, three binary digit positions are necessary and sufficient to represent the eight

This fact makes binary—to-octal and octal-to—binary conversions quite simple.

Example: Binary—to-Octal Conversion
The binary integer

1011100110102
can

integers

be converted to its octal equivalent as follows:
1.

Divide the binary digits into groups of three,

starting from the right

l0l HO OH 010

Substitute for each group its octal equivalent
5

The result is the octal number

56328
To perform the reverse conversion, substitute the binary equivalent of each octal

4751

47518

=100Hi lOl 00]

=1001111010012

digit

To convert an octal fraction, group by threes in each direction away from the radix point

11101010100012 =1 HO iOl
2243.5578

010 0T0 TOO Oll

OTO 01(0)

=165.228

lOl l0l lll

OTOOTOTOOOT l

.lOllOllll2

Example: Decimal-to-Binary Conversion
The "remainder method” may be used to convert a decimal number to a binary number.

The decimal

number

may be converted as follows:

NN N00"
00 U1

\IOJ N—d
NN Q

N :l
CO 01

(binary radix)—>2

N \O \lO

‘— (remainders)

1011100110102

Notice that this method simply involves halving the number to be converted and noting the remainder after

each division.

Example:

Decimal—to-Octal Conversion

The remainder method may also be used for octal conversions.
2970

The decimal number

——I 0

would be converted as follows:

(octal radix)—98 T2970

2-3

prewar.»

4—— (remainders)

56328

In one sense, the conversion of a number From one representation to another is a way of encoding the

number; the octal integer 778 can be encoded as the binary integer llllll
a

binary code to any symbol, such as a letter of the alphabet.

Similarly, one can assign

The table in Appendix B shows the binary

codes assigned to all the characters of the Model ASR-33 Teletype.

A programmer may invent a symbolic name to refer to the location of a given word in the computer mem-

These symbolic names, or tags, are assembled together with the instruction mnemonic into a binary

ory.

number which indicates the memory location of the word, the instruction code, and the address of the
data

The association of binary code and symbol is the basis of a programming language

A programmer learns

the symbols for the computer's repertoire of operations and the rules for arranging a sequence of symbolic
instructions in a useful Format.
paper tape.

An assembly program accepts this source program

sequence of binary numbers,
on an

He prepares a symbolic program for input on a medium such as punched

input and translates it into an equivalent

producing a program which is usable directly by the computer by placing it

output medium, which again may be punched tape.

be read into the computer and executed.

This binary,

object program tape may then

CHAPTER

THE

3.]

PDP-8

INSTRUCTION

SET

INSTRUCTIONS

Every PDP-8 operation is specified by a unique combination of l's and 0's stored in the twelve bits of one
memory register.

Such an instruction word can be one of two types:

memory reference instructions per-

form operations which require access to the information stored in a memory register; augmented instructions
do not refer to memory cells.

The operation code of an instruction is contained in bits 0, l, and 2 of the word.

Since three binary digits

correspond to one octal digit, it is apparent that there can be no more than eight operation codes, corre—
sponding to the octal digits 0—7. Codes 0-5 are reserved for memory reference instructions. Operation
codes 6 and 7 are for augmented instructions.

These two types of instructions are defined, and the instruc-

tions described, in the following sections.

The following special symbols are used in the instruction lists below.
Definition

Mel
C(A)

The contents of register A

C(A) 3» C(B)

The contents of register A replace the contents of

The address or location of any memory register

The

ith bit of register Y

Y]_4

Bits

l-4, inclusive, of register Y

C(AO__5) => C(Y6_] 1)

The contents of bits 0-5 of register A replace the contents of bits 6-] l of register Y.
not affected

The contents of A are

lnclusive OR

O><
3.1 .1

AND
>

(Boolean)

The l's complement of the contents of register A

Memory Reference Instructions

Word format of memory reference instructions is shown in Figure 3-l , and the instructions are explained
in Table 3-l

3-1

MEMORY
PAGE

OPERATION
CODES 0-5

LW_J

TABLE 3-I

Mnemonic

Octal

Time

Symbol

Code

(psec)

AND Y

3.0

‘II

ADDRESS

INDIRECT
ADDRESSING

Figure 3—l

Memory Reference Instruction Format
MEMORY REFERENCE INSTRUCTIONS

Operation

Logical AND. The AND operation is performed between
the C(Y) and the C(AC). The result is left in the AC,
C(AC) are lost. The C(Y) are unchanged.
Corresponding bits are compared independently. This in—
struction, often called extract or mask, can be considered
=>
/\
as a bit-by—bit multiplication.

and the original

C(Yl) C(ACi)

C(ACi)

Example

TAD Y

3.0

C(ACi) original

C(Yl)

Two's complement add.

—'OO

The C(Y) are added to the C(AC)

in 2's complement arithmetic

and the original

final

C(ACi)

The result is left in the AC

C(AC) are lost.

The C(Y) are unchanged.

If there is a carry from AC0, the link is complemented.

This feature is useful in multiple precision arithmetic

C(Y) + C(AC) => C(AC)
ISZ Y

3.0

Index and skip if zero.

The C(Y) are incremented by one

in 2's complement arithmetic.

If the resultant C(Y)

the next instruction is skipped.

If the resultant C(Y) 75 0,

the program proceeds to the next instruction.
are

The C(AC)

unaffected.

C(Y) + 1 => C(Y)
lf result
0, C(PC) + l => C(PC)
=

DCA Y

3.0

Deposit and clear AC. The C(AC) are deposited in core
memory location Y and the AC is then cleared.

The previous C(Y) are lost.

C(AC) => C(Y), then 0 => C(AC)

TABLE 3-]

MEMORY REFERENCE INSTRUCTIONS

Mnemonic

Octal

Time

Symbol

Code

(psec)

JMS Y

3.0

(continued)

0 pera t'Ion

Jump to subroutine. The C(PC) are deposited in core mem—
ory location Y.
Y + I

The next instruction is taken from location

C(PC) + I => C(Y)
Y + I => C(PC)

JMP Y

I .5

Jump to Y.

The next instruction is taken from core mem—

ory location Y.

Y => C(PC)

3. I .2

Augmented Instructions

There are two classes of augmented instructions, or instructions which do not reference core memory.

They are the input-output transfer (IOT) which has an operation code of 6, and the operate (OPR), which
Bits 3 through II within these instructions function as an extension of the

has an operation code of 7.

operation code and can be microprogrammed to perform several operations with one instruction. Augmented
instructions are

3.1 .3

I-cycle instructions requiring I .5 psec for execution

Input-Output Transfer Instruction

Microinstructions of the input-output transfer (IOT) instruction effect information transfers between the

arithmetic and control element and aninput—output device via the input—output control element.
format of the IOT instruction is shown in Figure 3-2.

Bits 3 through 8 are used to select the

and bits 9 through II enable generation of I/O pulses during event times 4,

The

I/O device;

2, and I, respectively.

Operations performed by IOT microinstructions are explained in Chapter 4 of the PDP-8 Users Handbook.

GENERATES
AN IOP 4
PULSE AT
EVENT TIME 5
IF A I

OPERATION
C

GENERATES
AN IOP I
PULSE AT
EVENT TIMEI
IF A 1

W4
DEVICE
SELECTION

Figure 3-2

IOT Instruction Format

3-3

GENERATES
AN IOP 2
PULSE AT
EVENT TIME 2
A 1

3.1.4

Operate Instruction

The operate instruction consists of two groups of microinstructions.

Group I is principally for clear,

complement, rotate, and increment operations and is designated by the presence of a 0 in bit 3. Group 2
is used principally for checking the contents of the accumulator and link and continuing to or skipping the
next instruction

A l in bit 3 and a 0 in bit ll

based on the check.

designate a Group 2 microinstruction.

Group I operate microinstruction Format is shown in Figure 3-3, and the microinstructions are listed in the
table below.

Any logical combination of bits within this group can be combined into one microinstruction.

For example, it is possible to assign l's to bits 5,

6, and II; but it is not logical to assign l's to bit 8 and

9 simultaneously since they specify conflicting operations.

If RAL, RAR,

RTL, or RTR is specified, lAC

may not be specified, and conversely.

OPERATION
CODE 7

CLA

ROTATE 1
POSITION IF A O.
2 POSITIONS
IF A I

ROTATE
AC AND L
RIGHT

CMA

WF—LflF—gfif‘k‘fi
O

CONTAINS
A 0 TO
SPECIFY
GROUPI

Figure 3-3

TABLE 3—2

CLL

CML

ROTATE

IAC

LEFT

Group I Operate Microinstruction Format

GROUP I OPERATE MICROINSTRUCTIONS

Mnemonic

Octal

Event

Symbol

Code

Time

CLA

7200

O pera f.Ion

Clear AC

0 => C(AC)

CLL

7l00

Clear L.
0 => C(L)

CMA

7040

Complement AC. The C(AC) are set to the T's complement of C(AC).
C(AC) => C(AC)

CML

7020

Complement L.
C(L) => C(L)

RAR

70l0

Rotate AC and L right.

rotated right one place

C(AC.) => C(AC. + 1)
):> C(L)l
C(L) =l>I C(ACO)

C(Ac'

The C(AC) and the C(L) are
.

TABLE 3-2

GROUP I OPERATE MICROINSTRUCTIONS

Mnemonic

Octal

Event

Symbol

Code

Time

RAL

7004

(continued)

0 per at'Ion

Rotate AC and L left.

The C(AC) and the C(L) are

rotated left one place.

C(ACi) =>
C(Aci
=>

])

C(L)

C(ACO)

C(L) => C(ACH)
70I2

RTR

Rotate two places to the right.

Equivalent to two

successive RAR operations.

7006

RTL

Rotate

fwo places to the left. Equivalent to two

successive RAL operations.

700i

IAC

Index AC.

The C(AC) are incremented by one in

2's complement arithmetic.

C(AC) +1 $ C(AC)
7000

NOP

Causes a I .5 psec program delay.

format is shown in Figure 3-4, and the microinstructions are listed in

Group 2 operate microinstruction
the table below.

No operation

—

Any logical combination of bits within this group can be composed in one microinstruction.

REVERSE

OPERATION
CODE 7

CLA

CONTAINS A 1
TO SPECIFY
GROUP 2

Figure 3-4

SKIP
SENSING OF
BITS 5,6,7

SZA

SNL

SMA

HLT

OSR

CONTAINS A 0
TO SPECIFY
GROUP

Group 2 Operate Microinstruction Format

If skips are combined in a single instruction, the inclusive OR of the conditions determines the skip.

For

example, if I's are designated in bits 6 and 7 (SZA and SNL), the next instruction is skipped if either
C(AC)

0, or C(L)

I, or both

If two reverse sense skip instructions are combined (bit 8 is set), the

logical AND of the conditions determines the skip.

For example, if I's are designated in bits 6, 7, and

8 (SNA and SZL), the next instruction is skipped if C(AC) 7’é 0 and C(L)
from
to

The CLA microinstruction

Group I can be combined with Group 2 commands. This command occurs at event time 2 with respect

the event times listed in the following table.

3—5

GROUP 2 OPERATE MICROINSTRUCTIONS

TABLE 3—3

Mnemonic

Octal

Event

Symbol

Code

Time

CLA

7600

O peratlon

Clear AC.
0 => C(AC)

SPA

7510

Skip on positive AC. If the C(AC) is a positive
number, the next instruction is skipped.
:r
C(ACO) 0, then C(PC) + 1 => C(PC)
=

SMA

7500

Skip on minus AC.

If the C(AC) is a negative num—

ber, the next instruction is skipped.
'I , then C(PC) + 1 => C(PC)
If C(ACO)
=

SNA

7450

Skip on non-zero AC.
If C(AC) 75 0, then C(PC) +1 => C(PC)

SZA

7440

Skip on zero AC.
If C(AC)
0, then C(PC) +1 => C(PC)
=

SZL

7430

Skip on zero L. If C(L) O, the next instruction is
skipped.
If C(L)
0, the C(PC) + 1 => C(PC)
=

SNL

7420

Skip on non-zero L.
If C(L) =1, then C(PC) +i => C(PC)

SKP

74l0

Skip, unconditional. The next instruction is skipped.
C(PC) + 1 => C(PC)

OSR

7404

OR with switch register.

(May be combined with

CLA.)
C(SR) V C(AC) => C(AC)
HLT

7402

Halt.

Stops the program.

If this instruction is com—

bined with others in the CPR 2 group, the computer
stops immediately after completion of the cycle in
process.

3.2

THE ORGANIZATION OF MEMORY

A PDP—8 memory field can be likened to a book.

The 4,096 words of the memory field correspond to the

lines of text, and if we divide the memory into segments of l28 words each, we have an analogy to a

32-page book in which each page contains 128 lines.
The memory field is in fact segmented in this fashion, and the analogy to a book is affirmed by the fact
that each of the l28—word blocks is called a page.
to allow direct reference to all

be provided.

Because the PDP—8 instruction word is not long enough

of the registers in a memory field,

special type of address reference must

This can be illustrated by pursuing our book analogy a little bit farther.

3-6

Suppose that one is reading a text, and taking notes in the margins of each page.
some

of the notes will refer to other parts of the page,

to information

One can expect that

other pages.

if one of the notes refers to a line of text on the same page, write only the line number.

For convenience,

If the note re—

fers to a line on some other page, write the page number followed by the number of the line on that page.

Alternatively, of course, it is possible to begin on the first page and number all the lines of the book con—
secutively, and refer to them by these numbers alone.
In similar fashion,

given word of memory may be referred to by its page address; that is, its address

within a page, or by its absolute address, which designates its position in the whole of memory.

analogy ends

Here the

A PDP-8 memory field is organized as follows:

the 4,096 words are arranged sequentially, with absolute

77778. The field is divided into 32 pages numbered from 0-378. Each page
tains I28 registers, with page addresses of
O-I778. As Figure 3-I shows, the address field of memory
field of
to
memory reference instruction contains 7 bits, which is iust enough to allow
2008

addresses of 0 through

con-

locations.

access

If bit 4 of the instruction contains a l, the address field of the instruction refers to one of the

addresses on the current page, that is, the page in which the instruction is stored.

If bit 4 contains a O,

the reference is to an address on page 0.

The state of bit 3 of the instruction determines what is done with the contents of the memory register specified by bits 4—I I

If this bit is 0, the contents of the cell addressed by the instruction are taken as the

operand, and the operation is completed.
effective address of the instruction.
are

In this case, the address specified by the instruction is the

If, however, bit 3 contains a I, the contents of the cell addressed

treated, not as the operand, but as the I2—bit absolute address of the register containing the operand.

In other words, the cell addressed contains the effective address of the instruction.
ory reference instruction can indirectly address any register in the memory field,

In this way,

mem-

regardless of which page

it is on.

Thus, there are four ways, depending on the states of bits 3 and 4 of a memory reference instruction, in
which the effective address may be obtained.

TABLE 3-4
Bit 3

Bit 4

EFFECTIVE ADDRESS CALCULATION
Effective Address

The operand is in page 0 at the address specified by bits 5

through I I
O

The operand is in the current page at the address specified by

bits 5 through I I
3—7

TABLE 3—4

EFFECTIVE ADDRESS CALCULATION

Effective Address

Bit 4

Bit 3

(continued)

The absolute address of the operand is taken from the contents
of the location in page 0 designated by bits 5 through II

The absolute address of the operand is taken from the contents
of the location in the current page designated by bits 5 through II.

3.3

COMPLEMENT ARITHMETIC

In the PDP-8, as in other machines utilizing complementation techniques,

negative numbers are repre-

sented as the complement of positive numbers, and subtraction is achieved by complement addition.

Representation of negative values in 2's complement arithmetic is slightly different from that in I's com—

plement arithmetic

The I's complement of a number is the complement of the absolute positive value; that is, all I's are re—

placed by 0's and all 0's are replaced by I's.
plement of the positive value plus one

The 2's complement of a number is equal to the I's com-

In I's complement arithmetic a carry from the sign bit (most significant bit) is added to the least significant bit in an end-around carry.

In 2's complement arithmetic a carry from the sign bit complements the

link (a carry would set the link to I if it were properly cleared before the operation), and there is no'
end—around carry

A I's complement representation of a negative number is always one less than the 2's complement representation of the same number.

Differences between I's and 2's complement representations are indicated

in the following table.

TABLE 3-5

ONE'S AND TWO'S COMPLEMENT REPRESENTATIONS

Number

One's Complement

Two's Complement

OOOOOOOOOI OI

000000000I OI

000000000 I 00

+ 3

00000000001 I

0000000000I I

0000000000 I 0

+ I

00000000000I

+ 0

000000000000

—0

IIIIIIIIIIII

Nonexistent

—I

IIIIIIIIIIIO

IIIIIIIIIIII

3-8

TABLE 3—5

ONE'S AND TWO'S COMPLEMENT REPRESENTATIONS (continued)

Number

One's Complement

Two's Complement

-2

HHHHHOT

HHHTHHO

—3

HHHHHOO

THHHTHOT

—4

THHHHOH

111111111100

-5

HHHTHOTO

THHHHOH

Note that in 2's complement there is only one representation for the number which has the value zero,

while in T's complement there are two representations.

Note also that complementation does not interfere

with sign notation in either T's or 2's complement arithmetic; bit 0 remains a O for positive numbers and a
i for negative numbers.

PDP—8 arithmetic operations (as exemplified by the TAD

instruction) are carried out in 2's complement.

This means that the operands involved in the arithmetic must be in correct 2's complement representation.
The 2‘s complement of any number is formed by taking the T's complement, and adding T

to it.

The two

operate class instructions, CMA and IAC, may be combined into a single instruction to perform the opera—
tion of taking a 2'5 complement.

Because the operation is often required, the mnemonic CIA is given to

the combined instruction.

3 3. T
.

Addition

The addition of a number contained in a core memory location and the number contained in the accumu-

lator is performed directly by using the TAD Y instruction, assuming that the binary point is in the same

position and that both numbers are properly represented in 2's complement arithmetic. Addition can be

performed without regard for the sign of either the augend or the addend. Overflow is possible, in which
case

the result will have an incorrect sign,

carry from bit 0 will

3.3.2

although the eleven least significant bits will be correct.

complement the link bit.

Subtraction

Subtraction is performed by complementing the subtrahend and adding the minuend.

As in addition, if

both numbers are represented by their 2's complement, subtraction can be performed without regard for
the signs of either number.

3—9

CHAPTER

ORGANIZATION
4.1

PROGRAMMING STEPS

The rules for writing a program can best be introduced by tracing the steps from the statement of a problem
to the
n

completed routine. Consider, for-example, this problem definition: "compute the sum of the first

integers.

The problem may also be stated as:

s=n+

After defining the problem,

the problem, should be made.

(n—l)+ (n-2)+ (n-3)+ ...+3+2+l

flow chart, that is,

block diagram of the general steps to the solution of

It might look something like Figure 4-1.

moment, consider the second, third, and fourth boxes.

perform the main computation of the sum.
sum.

Ignoring the first box for the

The operations specified in these three boxes

Box 2 specifies the actual arithmetic of computing the partial

Box 3 is a counter, which counts the number of partial additions that have been made.

Box 4 is a

decision point; if the count indicates that the sum is complete, the program goes on to box 5; if it is not

complete, the program returns to the beginning of the main computation.

This sort of continuous recycling

through a section of program is called a loop.
1

CLEAR TEMPORARY
REGISTERS
AND INITIALIZE

CALCULATE
PARHAL sum

DECREMENT
INDEX

N0
4

YES

Figure 4-]

HALT

Flow Chart of Program to Calculate Sum of Integers

The first box in this chart specifies the operations which prepare the program for operation
different data each
every program written will be used more than once, with

4—1

Since nearly

time, it is necessary at the

start of the routine to clear out old results from

previous use of the program so that one can start fresh

with new data, in the same way that one clears the keyboard and registers of an adding machine before

starting a new calculation.

ln programming terms,

this preparation is called initializing.

Besides indicating the general sequence of operations, the flow chart also gives an idea of storage require-

ments.

Space will be needed not only for the instructions which perform the computation, but also for the
One register will be needed to hold the partial sum accumulated through re-

data used by the routine.

peated additions; this register will naturally hold the correct total when the calculation is finished.
Another cell is needed for the index, which is the counter specified in box 3 of the flow chart.
one

The program,

then, must supply two types of information. First, it must include the executable instructions,

data, and temporary registers which will occupy memory cells when the program is executed.
must

Finally,

include a certain amount of information for the MACRO Assembler itself,

and extent of the program in memory.

to establish

Second, it

the locations

These assembler directives are called pseudo-instructions; they are

included in the program along with the executable instructions.

A pseudo-instruction is rather like a

proofreader's mark on a manuscript; the mark provides information to the editor, but does not itself cause
additional text to appear in the printed book.

Similarly, a pseudo-instruction provides information to the

Assembler, but does not itself cause any coding to be inserted into the obiect program.

4.2

CODING
First one must decide where the program is to be stored.

Now to write the program.

This word is called the m,

necessary only to establish the location of the first register of the program.

and can be designated in two ways.
to

To do this, it is

The most common way is to designate the page in which the program is

be stored; the origin is automatically set at the first register (page address 0) in that page.

Thus, to put

the routine in page 2, the pseudo-instruction, PAGE, would be used as follows:
PAGE 2

The program would then begin in location 0 of page 2.

To set the origin to any register other than the first one in a page, the absolute address of the starting

location must be specified.

This is done by using the special character

“9:"
.

To place the origin at loca-

tion 2T0, the coding would be:

*210

In establishing the origin,

index within the Assembler, called the current location counter (CLC), has

been set equal to the absolute address of the origin.

For example, the use of PAGE 2 would set the CLC

4—2

to a value of 400,

which is the absolute address of the first register in page 2.

the CLC to be set to a value of 2T0.

Likewise, *210 causes

ln programming terms the CLC points to the origin; such an index

is called a pointer.

The coding for the program may now be written.

First three registers for the three items of data must be

Usually, data storage is set aside by placing a O in each register, thus allocating a cell.

reserved.

The

coding looks like this:
PAGE 2
O
O
0

Since the origin has been set at location 400, the three data cells occupy locations 400, 40l , and 402

(page addresses 0, l, and 2, respectively).
Next, write the coding for the computation.
PAGE 2
O
O
0

CLA
DCA 400
TAD 40]

DCA 402
TAD 400
TAD 402
DCA 400
STA
TAD 402
SZA

JMP 406
HLT

$
The program now occupies locations 400-4l6 (O-lé on page 2).

The special character $ indicates to the Assembler that it is a complete program and that nothing else is
to follow.

This program is now complete, and can be assembled into a working routine.

The coding,

however, is

rather stark, and not very useful to another programmer, should he wish to discover what the program does.
Of course, he could Figure out the function of the routine by going through it step by step with pencil

4-3

and paper, but if this program were much
a

longer, this task would be tedious and impractical. Comments,

method of explaining the Functions of the program, make the work immediately useful to someone else.

4.3

COMMENTS

Comments are included in a PDP—8 program in a simple way.
our

program.

Consider the three data storage locations of

A comment would identify each register's function immediately; they might appear as:

/PART|AL SUM AND FINAL TOTAL
/INTEGER N
/INDEX

0 0'

The slash preceding each comment is the character which identifies a comment field.

All information on

that line following the slash is considered to be a comment, and is ignored by the Assembler.
and neatness,

For clarity

tab has been inserted between the instruction and the comment.

Since a slash identifies all the subsequent information on a line as a comment,

for a title by placing the slash in the first space after the left margin.

one

full line can be used

With a title, the program might

look like this:

/|NTE GER SUMMATION ROUTINE
PAGE 2

/PARTIAL SUM AND FINAL TOTAL

Now, an immediate identification of what the program is to be used for is included in the listing.
More comments explain the workings of the program:

/|NTEGER SUMMATION ROUTlNE
PAGE 2
o
0
0

/PART|AL SUM AND FINAL TOTAL
/|NTEGER N
/INDEX

CLA

DCA 400

/ZERO TO SUM

TAD 401

DCA 402
TAD 400

/SET INDEX
/MAIN LOOP

TAD 402
DCA 400
STA

/DECREMENT INDEX

4—4

TAD 402

/IS IT 0?
/NO: KEEP GOING
/YES: HALT

SZA
JMP 406
HLT

$
Now it is a little easier to understand the program, but some limitations remain.
lar place in the computer's memory; to put it in some other
addresses would be necessary.

It is tied to one particu-

location, recoding the entire routine with new

Nor are these octal page addresses much help in keeping track of program

flow; even in a short routine such as this, it is necessary to do some counting to find out what location the
instruction STA is in, for example.

-4.4

A simple,

meaningful way of identifying storage addresses is needed.

ADDRESS TAGS

As described above, a symbol is assigned to each of the PDP-8 instruction codes.
mer can

Similarly, the program-

assign a'symbol to any one of the storage addresses in the computer memory.

routine under discussion contains three important registers:

For instance, the

the beginning of data storage, the beginning

of the program sequence, and the beginning of the main loop.

Each one of these locations can be labelled with a symbolic tag, in the following way:

/INTE GER SUMMATION ROUTINE
PAGE 2

/PART|AL SUM AND FINAL TOTAL
/INTEGER N
/INDEX

DATA, 0
0
0

BEGIN, CLA
DCA 400

/ZERO TO SUM

TAD 40I

Go,

DCA 402
TAD 400

/SET INDEX
/MAIN LOOP

TAD 402

DCA 400
STA

/DECREMENT INDEX

TAD 402

SZA
JMP 406
HLT

/IS IT 0?
/NO: KEEP GOING
/YES; HALT

$
Three reference points have been established.

Note that the tags are roughly mnemonic, thus offering an

additional measure of program identification and clarification.
been defined.

In programming terms, three symbols have

In this case, each symbol has a value that is equal to the absolute address of the register in

which the associated item (data word or instruction) is stored.
of BEGIN, 403; of GO, 406.
4-5

Thus, the value of the symbol DATA is 400;

'4.4.l

Symbolic Addressing

Locating sections of a program has been simplified, but full advantage of address tags has not yet been
taken.

The instruction addresses themselves are still tied to the absolute addresses of the program.

ever, since

How-

each tag has a numeric value corresponding to an address, a tag may also be used as a symbol

in the address part of

instruction.

For example, the absolute address 402 is clearly equivalent to the

expression, BEGIN—l (BEGIN=403; 403-l=402). Similarly, the address 4ll is equivalent to GO+3.

Replacing absolute addresses with symbolic expressions, the program will look like this:
/INTE GER SUMMATION ROUTINE
PAGE 2

/PARTIAL SUM AND FINAL TOTAL
/lNTEGER N
/INDEX

DATA, 0
0

BEGIN, CLA
DCA DATA

/ZERO TO SUM

TAD DATA+I

GO,

DCA DATA+2
TAD DATA

/SET INDEX
/MA|N LOOP

TAD DATA+2

DCA DATA
STA

/DECREMENT INDEX

TAD DATA+2
SZA
JMP GO
HLT

/IS IT 0?
/NO: KEEP GOING
/YES: HALT

This program that is easily readable, includes a running commentary for explaining its functions, and
carries its own symbolic references that further assist in understanding and keeping track of the routine.

Moreover, the routine has been freed from a specific place in memory by the use of symbolic addresses.
To change the location of the program, it is necessary only to change the origin setting by the PAGE

pseudo-instructions

4.5

STORAGE TECHNIQUES

The program above will function but it may be improved.

One of its drawbacks is the fact that wherever

the program is stored, the data must be stored right along with it.

When space is at a premium, as it

often is, it would be desirable to put all of the data in a fixed place, and let the working parts of the
program be arranged as necessary.

In the

PDP-8, data that is used by several parts of a program is often stored in page 0.

If each of the

three data words in the program is given a name, page 0 addresses can be assigned to them.

4-6

Call the first

word TOTAL, the second INDEX, and the third N

In effect, the data words have been labeled with

address tags, but since the names can be used in symbolic

eXpressions (for instance, in

address), programmers usually refer to them as variables.

If the variables are assigned to page 0 and the

instruction

instruction address in the program changed to match, the coding will look like this:

/| NTE GER SUMMATION ROUTINE
*20

TOTAL, 0
INDEX, 0
0
N,

/THESE REGISTERS ARE
/NOW ON PAGE 0

PAGE 2

BEGIN, CLA
DCA TOTAL

/ZERO TO SUM

TAD N

GO,

DCA INDEX
TAD TOTAL

/SET INDEX
/MAIN LOOP

TAD INDEX

DCA TOTAL
STA

/DECREMENT INDEX

TAD INDEX
SZA

JMP GO

HLT

/IS IT 0?
/NO: KEEP GOING
/YES: HALT

When the program is loaded, the three items of data (having been assigned addresses on page 0) will be
stored in page 0.

A large step toward our goal of a refined, useful routine has been taken; i.e.,

program which may be

placed anywhere in memory and is easily understood and interpreted by someone other than the author,
because of liberal comments and symbolic addressing.

4.6

OPTIMUM USE

It remains only to relate this sequence of coding to the context in which it is likely to operate.

Obviously

the summation program is useless in core by itself, waiting for an integer summation to be carried out.
Since the example has served its purpose in illustrating how a complete program can be written, all the

coding except the minimum necessary for the actual computation will be removed, leaving this (the

ellipsis always indicates the presence of unspecified coding):

4—7

BEGIN, CLA
DCA TOTAL

/ZERO TO SUM

TAD N

GO,

DCA INDEX
TAD TOTAL

/SET INDEX
/MAIN LOOP

TAD INDEX

DCA TOTAL

/DECREMENT INDEX

STA
TAD INDEX

/IS IT 0?
/NO: KEEP GOING
/YES: HALT

SZA
JMP GO
HLT

This sequence can now be placed within a larger program wherever it may be needed.

example of a long program which uses this routine two times.

Figure 4-2 is an

Whenever the summation routine occurs,

it is preceded by an instruction which takes the value of N from the AC and places it on the proper loca-

tion in data storage

One other change is also necessary.

The address tag GO has been eliminated from

all but the first occurrence in the coding sequence; it is obvious that one symbol cannot be defined with
more

than one value, or there would be confusion as to which of the values was meant at any one time.

As a result, it is necessary to eliminate the reference to GO in the JMP instruction at the end of the
sequence

Notice that the instruction, DCA INDEX always remains exactly seven locations ahead of the JMP in-

struction, regardless of where the routine is stored.
to the
a

Evidently, an address expression which could refer

location of DCA INDEX relative to the location of the JMP instruction is needed.

In other words,

construction is desired that will perform the same function as the following but without having to define

the symbol HERE:

DCA INDEX

TAD INDEX

HERE,

JMP HERE-2

Remembering that the current location counter always "points" to the location of the instruction currently
being assembled, it becomes apparent that what is desired is a symbol which, whenever it is used, M
takes the value of the CLC

In MACRO-8, this symbol is the period,

Whenever this character is

encountered in an address expression, the value of the CLC is substituted for it.

In the example in

Figure 4-2 it is used in the address of the JMP instruction wherever it appears in the routine.
the necessity of having to define a new set of symbols each time the routine is used is avoided.

In this way,

PAGE 10

BEGIN,

DCA

TOTAL

TAD

/IN|T|AL|ZAT|ON
'

GO,

DCA

INDEX

TAD

TOTAL

TAD

INDEX

DCA

TOTAL

/SET INDEX
/MA|N LOOP
-

/INCREMENT INDEX

STA
TAD

INDEX
_

/Is IT 0?
/NO: KEEP GOING
/YES: ALL DONE

SZA

JMP

DCA

TOTAL

TAD

DCA

INDEX

TAD

TOTAL
INDEX
TOTAL

TAD

DCA

/SET INDEX
/MAIN LOOP

/DECREMENT INDEX

STA
TAD

INDEX

/IS IT 0?
/NO: KEEP GOING
/YES: ALL DONE

SZA
JMP

.-7

Figure 4-2

4.7

/INITIALIZATION

Program Example

SUBROUTINES

Now the program example is useful, but one more difficulty must be overcome.

An examination of

Figure 2-4 will make this problem obvious: If the routine is used very many times, an extremely large
amount of storage space will be used

simply in repetitions of the same coding sequence

the general aim of writing a concise, efficient program.

This counteracts

If the routine could be written only once,

placed

in a separate section of memory and called into operation each time it was needed, the whole procedure

would be more efficient.

4—9

A program that may be used in this manner is called a subroutine.

It is a sequence of coding with the

fol lowing properties:
I.

It is self-contained, that is, it can be assembled by itself without having to be part

of a larger program.

logically separate from other coding sequences.

It occupies its own section of memory,

It performs only one task.

It is called into operation only by another program, and when it has finished its

Each subroutine has a definite purpose.

task, it returns to that program.

It can be called any number of times, and upon com-

pletion always returns control to the point from which it was last called.
5
a

When necessary, it uses data supplied by the calling program, and returns results to

place accessible by that program

When a subroutine is written, four requirements which are implied by the last two properties listed above
must be

fullfilled:

The data must be accessible by the subroutine

0—

The results must be accessible by the calling program.

There must be a way of calling the subroutine into operation

There must be a way of returning control to the calling program

The problem of data transmission has been solved by placing the three data storage registers in page 0,

thereby making them accessible to any program or subroutine in memory.
return is not quite

obvious.

The transfer and control of

First, restore the symbolic tags to the subroutine as well as the title and

other pseudo-instructions to make it a self-contained program giving:

/|NTE GER SUMMATION SUBROUTINE
PAGE 2

INTSUM, DCA N
DCA TOTAL

/SAVE INPUT IN C(AC)
/ZERO TO SUM

TAD N

GO,

DCA INDEX
TAD TOTAL

/SET INDEX
/MA|N LOOP

TAD INDEX

DCA TOTAL
STA

/DECREMENT INDEX

TAD INDEX

/IS IT 0?
/NO: CONTINUE
/YES

SZA
JMP GO

At a first glance, it could seem easy to eliminate the problem of transferring control to the subroutine.
A JMP INTSUM in the calling program every time the subroutine was called would provide the path

what happens at the end of the calculation?

The subroutine has no way of knowing where to return.

But

The

subroutine cannot merely halt as this violates requirement d above.
The solution lies in the use of the JMS instruction.

Remembering from Chapter 2 when the JMS instruction

is executed, the contents of the program counter are saved in the

location addressed by the instruction,

and control is transferred to the register immediately following the one addressed.
contains the address of the next instruction following the JMS is available

Thus, a register which

This can be used to return to

the proper point in the calling program when the subroutine has finished its computation.

Rearrange the

first part of our subroutine as follows:

/lNTE GER SUMMATION ROUTINE
PAGE 2

INTSUM,

DCA N
DCA TOTAL

/SAVE PC HERE
/SAVE INPUT NUMBER
/ZERO TO SUM

TAD N

GO,

DCA INDEX

/SET INDEX

Now, whenever the instruction JMS INTSUM is used, the contents of the program counter are stored in the
location tagged with the name INTSUM and the computer continues operation with the instruction imme-

diately following which is the first instruction of the subroutine.
Now that the subroutine has been called, a means to exit from it must be provided.
stores the program counter which

JMS

"points" to the instruction following the JMS

The JMS instruction

After the execution of a

INTSUM, the register INTSUM contains the contents of the program counter at the time the JMS was

executed.
return to

This can be used as an effective address (See Chapter 3

the point of call.

the ”Organization of Memory") to

To do this, the subroutine may be terminated with a JMP instruction which

references the register INTSUM as an indirect address as follows:

4—H

/INTEGER SUMMATION SUBROUTINE
PAGE 2

/SAVE PC HERE
/SAVE INPUT NUMBER

INTSUM, o
DCA N

JMP I

The character

INTSUM

mas;

is used to signify indirect addressing.

EXIT SUBROUTINE

In use here it means,

”Jump to the register whose

address is contained in the register tagged INTSUM."

The complete subroutine is shown below:

(not including the page 0 definition of the variables).

/INTE GER SUMMATION SUBROUTINE
PAGE 2

INTSUM,

DCA N
DCA TOTAL

/SAVE PC HERE
/SAVE INPUT NUMBER
/ZERO TO SUM

TAD N

GO,

DCA INDEX
TAD TOTAL

/SET INDEX
/MAIN LOOP

TAD INDEX

DCA TOTAL
STA

/DECREMENT INDEX

TAD INDEX

SZA
JMP GO
JMP I INTSUM

/IS IT 0?
/NO: CONTINUE
/YES: ALL DONE

The reader is referred to the PDP-8 Library for other examples of subroutine calling techniques.
The MACRO—8 Programming Language may now be discussed in detail.

CHAPTER
MACRO-8

5.I

PROGRAMMING

LANGUAGE

FUNCTION

The MACRO-8 Symbolic Assembler accepts source programs written in symbolic language and translates
them into binary form.

MACRO-8 produces an ‘obiect program tape (binary), a symbol table defining

memory locations (for use with

5. 2

DDT), an octal/symbolic assembly listing, and useful diagnostic messages.

COMPATIBILITY

MACRO—8 is compatible with PAL III, but has the Following additional features:

User-Defined Macro's

Groups of computer instructions required for the
the solution of a specific problem can be defined
instruction by the user.

as a macro

Double Precision Integers

Positive or negative double precision integers are

allotted two consecutive core locations.

Floating Point Constants

The format and rules for defining these constants
are

compatible with the format used by the PDP—8

Floating Point Package (See Digital-8—5—S).
Operators

Symbols and integers may be combined with a
number of operators.

Literals

Addition

Boolean AND

Subtraction

Boolean Inclusive OR

Symbolic or integer literals (constants) are auto—
matical ly assigned

Text Facility

There are text facilities for single characters and

blocks of text.

Link Generation

Links are automatically generated for out of page
references

5—l

To incorporate these new features, it was necessary to decrease the size of the symbol table and because

of this, programs that were originally coded to be assembled by PAL III might have too many symbols to
be assembled by MACRO-8.

If Switch Register switches 10 and H

are

set to "l

during assembly (See

Table 9—1) MACRO—8's external(user) symbol table will be extended.

5.3

CHARACTERS

Programs in the MACRO-8 language are written using characters from the ASCII character set. The

following characters are used:

5.3.l

Letters

ABCD...XYZ

5.3.2

Digits

1234567890

5.3.3

Punctuation Characters

Since a number of characters are invisible (i .e

nonprinting), the Following notation is used to represent

them in the examples:

space

)

tab

carriage return

The Following characters are used to specify operations to be performed upon symbols or numbers:

Character

Hie

1—!

space

Combine symbols or numbers-

plus

Combine symbols or numbers (add)

minus

Combine symbols or numbers (subtract)

exclamation point

Combine symbols or numbers (OR)

)

carriage return

Terminate line

._.>l

tab

Combine symbols or numbers or format source

tape

5-2

comma

Assign symbolic address

equals

Define parameters

;

semicolon

Terminate coding line

dollar sign

End of pass

asterisk

Set location counter

point

Has value equal to current location counter

slash

Indicates start of a comment

ampersand

Combine symbols or numbers (AND)

quote

Generate ASCll constant

( )

parentheses

Define literal on current page

[ ]

brackets

Define page 0 literal

< >

angle brackets

Define a macro

5 3 4
.

Ignored Characters
Form-feed

End of a

logical page of a source program

(see Symbolic Editor)
Blank tape

Used for leader/trailer

Rubouts

Used for deleting characters

Code 200

Used for leader/trailer

Line-feed

Follows carriage-return

All other characters are illegal when not in a comment or a TEXT field, and cause the error message lC
to be

printed. The form-feed is used at the end of a page of program for editing purposes. The functions

of leader and trailer are self-explanatory.

This may be either blank tape or code 200.

Tabulations are usually used in the body of a program to provide a neat page for printing; they can separate
fields from one another, as between an instruction and an associated comment.

For example,

line could

be written as:

GO,

TAD TOTAL/MAIN

LOOP)

but it is far easier to read if tabs are inserted:

GO, —->i TAD TOTAL —>| /MA|N

(the characters —§l and
Either

;

are

LOOP)

nonprinting)

(semicolon) or the combination carriage-return/line-feed may be used as line terminators.

The

semicolon is considered identical to carriage-return/line-feed except that it will not terminate a comment.

5-3

Example:

/THIS lS A COMMENT

TAD A

TAD

;

The entire expression up to the carriage return is considered a comment.
neat appearance for
As was noted previously, the tabulation is used as a formatting device to provide a

Use of the semicolon allows the programmer to place several lines of coding

the printed program listing.
on a

to rotate
single line. If, for example, he wishes to write a sequence of instructions

the C(AC) and

C(L) six places to the right, it might look like:

RTR)
RTR)
RTR)

He may place all three instructions as a single line by substituting the control character ";"

for the line terminator

RTR;

”)

RTR;

(carriage return).

(semicolon)

The above sequence of instructions may be rewritten as:

RTR‘)

This type of format is particularly useful when setting aside a section of data storage for a list.

For ex-

ample, a l2-word list could be reserved by:
LIST,

5.3.5

0‘)

Coding Practices

A neat printout

(or program listing, as it is usually called) makes subsequent editing, debugging, and

interpretation much easier than if the coding were laid out in a haphazard fashion.
listed below are in general use, and will result in a readable, orderly listing.
a

program to produce

A title comment begins at the left hand margin.

Pseudo-instructions may begin at the left margin; often,

one

tab stop to line up with the executable instructions.

Address tags begin at the left margin

tabulation

(See Digital-8—21—U for

listings of this form.)

The coding practices

however, they are indented

They are separated from succeeding fields by

Instruction fields, whether or not they are preceded by a tag field,

one

tab stop.

5—4

are

indented

A comment is separated from the preceding field by a tabulation, unless it occupies

the whole

line, in which case it usually begins at the left margin.

CODING STRUCTURE

5.4

and restrictions applicable

Following is the hierarchy of assembler language components with definitions
to each

level.

5.4.l

Elements

Any group of letters, digits, and punctuation characters which represents binary values less than 2

IS an

element.

5.4.2

Integers

Any sequence of numbers delimited by punctuation characters forms a number. Example:
1
l2

4372

The radix control pseudo-instructions indicate to the Assembler the radix to be used in number
tation.

The pseudo-instruction DEClMAL indicates that all numbers are to be

interpre-

interpreted as decimal until

the next occurrence of the pseudo-instruction OCTAL.
The pseudo-instruction OCTAL indicates that all numbers are to be
occurrence

of the pseudo-instruction DEClMAL.

interpreted as octal until the next

The radix is initially set to octal and remains octal un—

less otherwise specified.
5.4.3

Symbols

A symbol is a string of one or more alphanumeric characters delimited by a punctuation character.
are

Symbols

composed according to the following rules:
l

The characters must be either alphabetic (A-Z) or numeric (0—9)

The first character must be alphabetic.

Only the first six characters of any symbol are meaningful; the remainder, if any,

are

ignored.

A symbol is terminated by any nonalphanumeric character.

The MACRO Assembler has, in its permanent symbol table, definitions of the symbols for all PDP-8 operation

codes, operate commands, and many lOT commands (see Appendix A for a complete list).

5—5

These may be

used without prior definition by the user.

Example:

is a symbol whose value of 4000 is taken from the operation code

JMS

definitions.
is a user—created symbol.

When used as a symbolic address tag, its value

is the address of the instruction it tags.

Assembler

This value is assigned by the

Note that because of rule 3, a symbol such as INTEGER, for instance, would be interpreted as lNTEGE
since the seventh letter is ignored.

If symbols of more than six characters are used, the programmer should

be careful to avoid the error of defining two apparently different symbols whose first six characters are,
in fact, identical.

For example, the two symbols, GEORGE] and GEORGEZ, differ only in the seventh

character, so that the Assembler would treat them as being the same symbol, GEORGE
It is not necessary to define a symbol before it is used in an expression.
end of PASS l, however.

They must be defined before the

Thus, one may refer to a number of registers by their address tags, and then

define the symbols later.

Parameter Assignments

5.4.3.1

A symbol may be assigned a value by means of a parameter assignment statement which looks like an alge-

braic statement.

The single symbol to the left of the equal sign is assigned the value of the expression on

the right. No space(s) or tab(s) may appear between the single symbol to the left of the equal sign and the

equal sign

Examples:

EXIT
C

JMP I 0

A + B

All symbols to the right of an ”=" sign must be already defined.

The symbol to the left of the "=" sign

and its associated value is stored in the Assembler's symbol table.

The use of the "=" does not increment the current location counter.

Assembler itself rather than a part of the output binary.
5.4.3.2

A symbol may be defined by the user in one of three ways:

By use of a parameter assignment. Example:
DISMIS

JMP RESTOR

5—6

rather, an instruction to the

The equal sign may be used to redefine a symbol.

Symbol Definition

It is,

As a macro name.

Example:

DEFINE LOAD A
< CLA

TAD A >

3.
a

By use of the comma. When a symbol is terminated by a comma, it is assigned

value equal to the current location counter.

Example:

/SET CLC TO 100

*100

TAG,

CLA

B,
A,

JMPA
DCAB

The symbol I'TAG" is assigned a value of OlOO, the symbol "B" a value of 0102, and the symbol ”A" a

value of 0103.
5.4.4

Expressions

All elements, i.e., symbols and numbers (exclusive of pseudo-instruction symbols, macro names, and
double precision or Floating point constants) may be combined with certain operators to form expressions.
These opera tors are:

This signifies 2's complement addition (modulo

plus

409610).
—

This signifies 2's complement subtraction (modulo

minus

409610).
1

exclamation point

This signifies Boolean inclusive OR (union).

ampersand

This signifies Boolean AND (intersection).

~—-I

space

Space is interpreted in context.

It may signify

inclusive OR or act as a field delimiter.

Symbols and integers may be combined with any of the above operators.
ated from left to right; no grouping of terms is permitted.

Value
Value
Value

A symbolic expression is evalu—

Example:

_B_

A+ B

A-B

A 1 B

0002

0003

0005

7777

0003

0002

0007

0005

0014

0002

0007

0005

0700

0007

0707

067]

0707

0000

5-7

The MACRO-8 Assembler makes a distinction between the types of symbols it is processing.
are

These types

l) permanent symbols, 2) user defined symbols, and 3) macro names. The character "space" is inter—

preted written in the context of the expression.

If a space is used to delimit two or more permanent sym-

bols, space signifies inclusive OR. Example:
CLA

is a permanent symbol whose value is 7200.

CMA

is a permanent symbol whose value is 7040.

The expression:
CLA

has a value of 7240.

CMA

.__.

If the symbol following the space is a user defined symbol, space acts as an address field delimiter.

Example:
*2117

CLA

JMP

.._J

"A" is a user defined symbol whose value is 2] T7.

The expression JMP

L_.

A is evaluated as follows:

The seven address bits of A are taken, i.e.:
A

010

00100]

1001

iii

The remaining five bits of A are tested to see if they are 0‘s (page 0 reference); if they are not, the
current page bit is set.

000

Oil

001

ill

The operation code is ORed into the expression:
10]

000

Address A 000

Oil

00]

ill

Oil

001

ill

JMP

or, written

5317

101

concisely:

000

In addition to the above outlined tests, the page bits of the address field are compared with the page bits

of the current location counter.

If the page bits of the address field are nonzero and do not equal the

page bits of the current location counter,

If the reference

out-of—page reference is being attempted.

is to an address not on the page where the instruction will be

located, the Assembler will set the indirect

bit (bit 3) and an indirect address linkage will be generated on the current memory page.
page reference is already an indirect one, the error diagnostic II

PASS 2.
case

If the out—of-

(Illegal Indirect) will be typed on

When the link is generated, the LG (Link Generated) message will be typed on PASS 2.

In the

of several out—of—page references to the same address, the link will be generated only once, but the

LG message will be printed each time.

Example:

*2117

CLA

*2600
JMP

._.

The space preceding the user defined symbol "A" acts as an address field delimiter.

The Assembler will

recognize that the register tagged "A" is not on the current page (in this case 2600-2777) and will
generate a link to it as follows:
in location 2600 the Assembler will

place the word

5777 which is JMP I 2777
in address 2777 (the last location on the current page), the word 2I I7 (the actual

address of ”A”) will be placed.

The address field of a memory reference instruction may be any valid expression.

Example:

A=270
*
200
TAD A—20

would

produce, in location 200, the word
OOI OIO I01 000 or 1250 (TAD 250)

Although the Assembler will recognize and generate an indirect address linkage when necessary, the
programmer may indicate an explicit indirect address by using the special

between the operation code and the address field.

This must be

The Assembler cannot generate a link for an instruction

that is already specified as being an indirect reference.
II

symbol I'I'l

(Illegal Indirect).
5-9

In this case, the Assembler will type the message

5 5
.

SPECIAL SYM BO LS

The period, asterisk, left parenthesis and quote have special usage which is adhered to universally in
PDP—8 assembler programs.

5.5. 1

Current Address Indicator

The single character period (.) has, at all times, a value equal to the value of the current location
It may be used as any integer or symbol

counter.

(except to the left of an equal sign). Example:

*200

JMP .+2

ls equivalent to JMP 202.
*

300

+2400

would produce, in register 0300, the quantity 2700.
*2200

CALL=J MS I

0027
Since the second line, CALL=JMP |

does not increment the current location counter, 0027 would be

placed in register 2200 and CALL would be placed in the symbol table with an associated value of
100 110 000 000 or 4600.
5.5.2

Origin Setting

The origin (current location counter) is reset by use of the special character asterisk (*).

location counter is set to the value of the expression following the
0200.

All symbols to the right of

"*"

must

"*”
.

The current

The origin is initially set to

already have been defined. Example:

If D has the value 250
then
*D+lO will set the location counter to 0260.

To simplify page handling, the pseudo—instruction PAGE may be used.

the origin is reset to the first location of the next page.

When ”PAGE" is encountered,

A page number may be specified by a legal

expression Following the page pseudo-instruction. Example:
*

at this

270

point, either

400

PA GE
or

PAGE 2

will reset the origin to 0400.

5.5.3

Literals

Since the symbolic expressions which appear in the address part of an instruction usually refer to the loca—
tions of registers containing the quantities being operated upon, the programmer must explicitly reserve

the registers holding his constants.

The MACRO-8 language provides a means for using a constant directly.

Suppose, for example, that the programmer has an index which is incremented by two.

One way of coding

this operation would be as follows:

cLA
TAD INDEX
TAD C2

DCA INDEX

C2, 2
Using a literal, this would become

CLA
TAD

INDEX

TAD

(2)

DCA

INDEX

The left parenthesis is a signal to the Assembler that the expression following is to be evaluated and as-

signed a register in the constants table of the current page. This is the same table in which the indirect
address linkages are stored.

In the above example, the quantity 2 is stored in a register in a list begin-

ning at the top of the memory page (page address 177), and the instruction in which it appears is encoded

5-H

with an address referring to that address.

A literal is assigned to storage the first time it is encountered;

subsequent references will be to the same register
If the programmer wishes to assign literals to

page 0 rather than the current page, he may use square

brackets, "E" and "J “, in place of the parentheses. However, in both cases, the right of closing member
may be omitted.

TAD

AND

The Following examples are acceptable:

(777)
UMP)

Note that in the second example, the instruction AND [JMP has the same effect as AND [5000.

Literals may be nested.

For example:

*200
TAD

(TAD

(30

will generate
0200

l 376

0376

l 377

0377

0030

This type of nesting may be carried to as many levels as desired.
at page

address W7 and extending toward page address 0.

page 0).

Literals are stored on each page starting

(Only 12710 or 1778 literals may be placed on

If a literal is generated for a nonzero page and then the origin is set to another page, the current

page literal buffer is punched out (during PASS 2).

If the origin is then reset to the previously used page,

the same literal will be generated if used again.

5.5.4

Single Character Text Facility

If a single character is preceded by a double quote, the 8—bit value of the ASCII code for the character
is inserted instead of taking the letter as a symbol.

CLA
TAD

("A

will place the constant 030] in the accumulator.

Example:

CHAPTER

PSEUDO-INSTRUCTIONS
6.1

FUNCTIONS

The pseudo-instructions are directions to the Assembler to perform certain tasks or to interpret subsequent

coding in a certain way. By themselves pseudo-instructions do not generate coding or (in general) effect
the current location counter.

The functions of each pseudo-instruction are described in this chapter.

CURRENT LOCATION COUNTER

6.2

PAGE

This pseudo-instruction is used to set the current location counter.

PAGE n

This will reset the current location counter to the first address of page n, where n is an

integer, a previously defined symbol, or a symbolic expression. Examples:
PAGE 2 will set the CLC to 0400

PAGE 6 will set the CLC to 1400

When used without an argument, PAGE will reset the CLC to the first location on the

PAGE

next succeeding page.

Thus, if a program is being assembled into page l and the pro-

grammer wishes to begin the next segment on page 2, he need only insert the

instruction PAGE,

pseudo-

follows:

JMP

—7

PAGE
CLA

The current location counter may be explicitly set by use of the asterisk.
6.3

EXTENDED MEMORY

On PDP—8's equipped with more than one

FIELD n

memory bank, the pseudo—instruction

may be used where n is an

integer, a previously defined symbol, or a symbolic expression

within the range 03n57.
This pseudo—instruction causes a word of the form

XXX

000

where

OOOSXXX 5 ill
6-1

to be punched on the binary tape

during PASS 2. This word is interpreted by the Extended Memory Binary

Loaders (see Digital‘8-2A-U; Digital-8—ZB—U).

6.4

RADIX CONTROL

Normally, all integers used in a program are taken as octal numbers. If, however, the programmer wishes
numbers treated as decimal, he may use the pseudo-instructions:

to have certain

When this pseudo-instruction occurs, all integers encountered in subsequent coding will

DECIMAL

be taken as decimal until the occurrence of the pseudo-instruction

which will reset the radix to its original

OCTAL

(octal) base.

NUMBERS

6.5

The types of numbers allowed are integers (See Chapter 5), double precision integers, and double pre—
cision floating point numbers.

6.5.]

Double Precision Integers

Double precision integers may be positive or negative (2's complement) according to their sign but may
not be

combined with operators.

not disturbed.

They are always taken as decimal radix although the current radixis

Each double precision integer is allotted two consecutive registers with the sign indicated

by bit 0 of the first word.
The double precision integer mode is entered through the use of pseudo-instruction DUBL and all numbers
encountered will be taken as double precision integers until an alphabetic character is encountered.
Each number is terminated by the carriage return

(d )

*400‘)
679467)
44)

DUBL

_3)

TAG,

CLA)

would produce
0400

0245

0401

705 3

0402

0000

0403

0054

6-2

the semicolon (;) or by a comment.

Example:

0404

7777

0405

7775

0406

7200

and the symbol ”TAG” would have a value equal to 0406.

6.5.2

Floating Point Constants

Double precision floating point constants may be positive or negative according to their sign but may not
Decimal radix is assumed but the current radix is not altered.

be combined with operators.

point constants are each assigned three registers and are stored in normalized form

Floating

(See Digital—8-5—S

for a description of floating point arithmetic.)

The double precision floating point mode is entered through use of the pseudo-instruction FLTG.

All

numbers encountered after the use of FLTG will be taken as double precision Floating point constants until
the occurrence of an alphabetic character

otherthan E

The general input format of a floating point

number is

iddd ddddEidd
-

where the d's are decimal digits.

Any character which is not legally part of the above format (except

rubouts) terminates input of the number. Example:
Produces
*400

FLTG

+509

32E-02)

-62 97E
.

TAG2,

00E

04)

-2)

CLA)

0400

0003

040]

2427

0402

6670

0403

0024

0404
0405

5462
0740

0406

7772

0407

2436

041 0

5574

041 i

7200

and the symbol ”TAG2" would be assigned a value of 0411

6.6

TEXT FACILITY

There is a text facility for single characters and text strings.
mode (double quote),

see

Chapter 5.

For a description of the single character

.A string of text may be entered by giving the pseudo-instruction TEXT followed by a space, a delimiting

character, a string of text, and repeating the same delimiting character.

Example:

ATEXTA

TEXT

The character codes are stored two to a register in ASCII code that has been trimmed to six bits.

lowing the last character, a 6—bit zero is inserted as a stop code.

Fol-

The above statement would produce:

2405

3024
0000

/B0 B/

TEXT L4

would produce
0217

0200

The TEXT pseudo—op could also be used'as part of a calling sequence to a subroutine:
JMS MESS

TEXT /

JMS MESS

NOWDS
ADDMESS

ADDMESS,

TEXT /

/NO WDS IN MESSAGE
/ADDRESS OF MESSAGE

Note that while the TEXT pseudo—instruction causes characters to be stored in a trimmed code, the use

of the single-character control code (") causes characters to be stored as a Full 8—bit ASCII code.

6.7

END OF PROGRAM

The special symbol ”35" indicates the end of a program.
nates the

PASS.

When the Assembler encounters the "$" it termi—

6.8

END OF TAPE

When several tapes are to be assembled together,

each, except the last (which ends in "$"), should have

its last symbol the pseudo—instruction PAUSE.

This causes the MACRO—8 Assembler to stop processing

and halt the computer.

After placing a new tape in the reader, assembly can be continued by depressing

CONTINUE.

6.9

ALTERATIONS TO THE SYMBOL TABLE

There are two pseudo—instructions that may be used to alter the permanent symbol table (during PASS 1):
EXPUNGE

EX PUNGE the entire symbol

FIXTAB

FIX the symbol TABle.

table, except for the pseudo-instructions.

All symbols that are currently in the symbol table are fixed.

Example:
CLSF

6141

FIXTAB

would define CLSF as a permanent symbol.
EX PUNGE
TAD=lOOO
FIXTAB

would place the symbol TAD in the assembler's permanent symbol table.
been expunged

All other symbols would have

To append the following card reader IOT's to the symbol

table, the programmer generates an ASCII tape

of:

RCSF=663I
RCSP=667I

RCRD=6674
FIXTAB
PAUSE

The ASCII tape is then read into core ahead of the symbolic program tape during pass I.

The PAUSE

pseudo-op stops assembly, and the Loader waits for the programmer to put the symbolic program tape
into the tape reader and press CONTinue.

CHAPTER

MACROS

7.1

FUNCTION

When writing a program, it often happens that certain coding sequences are used several times with iust
If so, it is convenient if the entire sequence can be generated by a single state-

the arguments changed.
ment.

To do this, the coding sequence is defined with dummy arguments as a macro.

A single statement

referringto the macro by name, along with a list of real arguments, will generate the correct sequence
in line with the rest of the coding.

7.2

DEFINING

The macro name must be defined before it is used.

The macro is defined by means of the pseudo-instruction

DEFINE followed by the macro's name and a list of dummy arguments.

For example:

A macro to move the contents of register A to register B and also leave the result in the

accumulator, would be coded as follows:
DEFINE

MOVE

DUMMYI'

DUMMY2

<CLA
TAD

DUMMYI

DCA

DUMMY2

TAD

DUMMY2>

The actual choice of symbols used as dummy arguments is arbitrary; however,
not be defined

they may

referenced prior to the macro definition.

The above definition of the macro MOVE is identical to the following:
DEFINE

E_.

<CLA;TAD

MOVE

ARGI;

ARGIEE

ARGZ

DCA ARGZ;

TAD ARG2>

The actual definition of the macro is enclosed in angle brackets.
When a macro name is processed by the assembler, the real arguments will replace the dummy arguments.
For example:

Assuming that the macro MOVE has been defined as above,
*400

A,0
B, -6

0400

0000

0401

7772

MOVE

A, B

0402

7200

0403
0404
0405

3201
l 201

l 200

A macro need not have any arguments:

NOTE:

sequence of
coding to rotate the C(AC) and C(L) six places to the left might be encoded
as a macro

For example,

by means of

DEFINE.“ ROTL 6

<RTL; RTL; RTL>
The entire macro definition is placed in the Macro Table, two characters per word, with a dummy argument value

replacing the symbolic name. Example:

DEFINE

LOAD.“ A

<CLA
TAD A>
is stored, in the Macro Table,

|CL|A)| TA|

D...

roughly as follows:

l77oo|>ool

where the vertical lines indicate successive l2—bit words.
are

not stored

Comments and line-feeds

The macro definition can consist of any valid coding except for TEXT or

7.3

type statements.

RESTRICTIONS
l

Macros cannot be nested; i.e. , another macro name or definition cannot appear in

a macro

definition and cannot be brought in as an argument at reference time.
”

type statements cannot appear in a macro definition.

TEXT or

Arguments cannot be:

Macro name

TEXT pseudo—instruction or

‘'

special character

The symbols used as dummy arguments must not have been previously defined or

referenced

A macro may not be redefined.

DEFINE

LOOP“ A

<TAD

DCA

TAD

COUNT

ISZ

JMP

.-2>

Example:

The symbol "COUNT" is not a dummy argument but an actual symbol.
A macro is referenced by giving the macro name,

by commas.
macro

space, and then the list of real arguments,

separated

There must be at least as many arguments in the macro reference as in the corresponding

definition.

When a macro is referenced, its definition is found, expanded, and the real arguments

replace the dummy arguments.

LOOP

The expanded macro is then processed in the normal fashion.

X, Y2

is equivalent to:

TAD

DCA

TAD

COUNT

ISZ
JMP

Y2
.

NOTE:

—2

The macro table shares the available space (60410 registers) with the

symbol table. Thus the programmer must be aware of the amount of room required by his macros and the fact that each symbol occupies four words of memory. Also, the arguments of a macro call are temporarily stored in this buffer
space while the macro is being expanded.

CHAPTER
ERROR

8.]

DIAGNOSTICS

FO RMAT

The format of the error messages is:
ADDRESS

ERROR CODE

Where ERROR CODE is a 2-character code which specifies the type of error, and ADDRESS is either the

absolute octal address where the error occurred or the address of the error relative to the last symbolic
tag (if there was one) on this page

Assembly will continue or may be continued after all errors except SE (Symbol Table Exceeded).
the Assembler will halt and may not be restarted.

error

occurs,

8. 2

ERROR MESSAGES

If an SE

Current, Non-Zero Page Exceeded
An attempt was made to

override a literal with an instruction or

override an instruction with a literal.

This can be corrected by

decreasing the number of literals on the page

decreasing the number of instructions on the page

Zero Page Exceeded

Same as PE only with reference to page 0

Illegal Redefinition of a Symbol
An attempt was made to give a previously defined symbol a new value not via the "="

The

symbol was not redefined. (This is similar to the Duplicate Tag diagnostic of PAL Ill).

illegal Character
1

# % '

? @\

were

processed neither in a comment nor a TEXT field. The character

is ignored and the assembly continued.

A non-valid character was processed

The computer will halt with the illegal character

displayed in the accumulator. Assembly may be continued by putting the desired character
in the SWITCH REGISTER and depressing CONTINUE

Illegal Equals
An equal

A +

TAD
A + B

sign was used in the wrong context. Examples:

(The expression to the left of the equal sign is not a single symbol)

Illegal Indirect
An out of page reference was made, and a link could not be generated because the indirect

bit was already set.

Example:

*200
TAD

PAGE

CLL

CMA

Link Generated
Not an error

——

link was generated for an out of page reference at this address.

*200

Generated Binary
A

TAD

Example:

0200

I 777

0377

0400

7I 40

PA GE

A,
SE

CMA,

CLL

Symbol Table Exceeded
The Symbol Table overlaps the Macro Table or vice versa.
be continued

Assembly is halted and cannot

Illegal Format in a Macro Definition
The expression after the DEFINE pseudo-instruction does not comply with the macro definition,

position, or structural rules. Example:
A macro name is referenced before the macro definition.

8—2

Undefined Symbol
A symbol has been processed during PASS 2 that was not defined by the end of PASS 1

Missing Parameter in a Macro Call
An argument, called For by the macro definition, is missing.

Example:
MAC

DEFINE
< TAD

CIA
DCA
MAC

A
B >

SUM

Two MACRO-8 internal tables have overlapped

This situation can usually be corrected by de—

creasing the number of current page literals used prior to this point on the page

If the error

persists, please contact the Small Computer Systems Programming Group at Digital Equipment

Corporation for assistance

8-3

CHAPTER

OPERATING

9.1

INSTRUCTIONS

ASSEMBLER OUTPUT

MACRO—8 is a 2—pass assembler with an optional third pass which produces an octal/symbolic assembly

listing. During the first pass, MACRO-8 processes the source tape and places all symbol definitions and
macro

definitions in its symbol table and macro table,

respectively. During the second pass, MACRO-8

processes the source tape and punches the Binary Format Tape.

At the end of PASS 2, MACRO-8 prints

the Symbol Table (it is also punched if the 33-ASR PUNCH is turned on).
read by DDT (See Digital-8—4—S)

This punched table may be

The third pass provides a listing of the generated octal code and the

original source language.
9.2

VERSIONS

There are two versions of MACRO-8 which differ with respect to their use of input/output equipment:
the low speed version uses the 33-ASR Reader for all input and the 33—ASR Punch for all output; the high

speed version uses the Type 750 Photoelectric Reader for all input, the Type 75 High Speed Punch for

binary output, and the 33—ASR Punch for printable output such as error printouts, symbol table punching
and listing, and third pass assembly listing.

NOTE: In the high speed version of MACRO—8, the Type 75 Punch may be
used as the printable output device by changing the contents of location 0004
from 2600 to 0600.

This is useful for long third pass listings, since the punched
It is advised that

output from the 75 Punch can be subsequently listed off-line.

this change not be made until pass 3, so that pass l and pass 2 error messages
will come out on the 33-ASR.
9.3

PROCEDURES
1.

Load MACRO—8 with the Binary Loader (See Digital—8—2-U).

Put the source tape in the reader.

Set the SWITCH REGISTER to 0200.

Depress LOAD ADDRESS

Set switch options (See Table 9—l)

Depress START

Turn on the 33-ASR reader (if using the low speed version).

9—]

When MACRO-8 stops reading (after processing a PAUSE statement),

next tape in

the reader and depress CONTINUE

been processed

place the

Repeat this step until all tapes have

When MACRO-8 encounters the terminating character, dollar sign (35), it performs

one

of the following sets of events depending upon what pass has iust been completed.

Proper operator intervention is then required.

Pass Just Completed
l

Events

Operator Intervention
Turn on 33—ASR punch (in high speed

Set up for PASS 2

MACRO—8, symbol table is output via
33—ASR). Put source tape in reader,
and proceed from Step 2 of the oper—
ating procedures, above.
2

(a) If PASS 3 is desired:
(In the high speed version of MACRO-8,

Terminate current assembly;

punch out page 0 constants,

the contents of register 0004 could be
altered at this point to change output
devices). Go to step 2 of the operating

checksum and trailer code on

binary tape; print and punch
rubout, the alphanumerically
ordered symbol table, an EOT

procedures, above.

code, a rubout, and trailer
code; Set up for PASS l
.

(b)

If PASS 3 not desired:

Turn off 33-ASR punch, put next program
Go to
to be assembled in the reader.

step 2 of the operating procedures, above.
3

Terminate assembly listing:
Set up for PASS l

Turn off 33-ASR punch; put next
program to be assembled in the reader;

hit CONTINUE to enter PASS 1.

TABLE 9-l

SWITCH OPTIONS

Switch Up

None

Result

MACRO-8 will enter the next pass as defined in the preceding table.

example:

For

if the previous assembly was terminated during or at the end of PASS

restarting MACRO-8 with no switches up would cause PASS 2 to be entered.
MACRO-8 initially starts at PASS 1

Restore symbol table to only the permanent symbols and enter PASS 1
Enter PASS 2

TABLE 9—]

SWITCH OPTIONS (continued)

Switch Up

Result

Enter PASS l without erasing any previously defined symbols.
Enter PASS 3.

During PASS 3, MACRO-8 outputs an octal/symbolic listing

of the assembled program.

If this pass is terminated before completion, either

switch options 0 or 2 may be used to return to PASS l for subsequent assemblies.
MACRO-8 will output as much of the source statement (symbolic) as its internal

storage capacity will allow.

Because of the internal operations during the

processing of macro statements, the symbolic output may be meaningless.
The double precision integer and double precision Floating point processors

are

deleted and may be used for storage of user defined symbols.

creases

the size of the symbol table by

6410 symbols.

The macro processor and the number processors (above) are deleted and may be

used for storage of user defined symbols.

table by 125
NOTE:
to

This in-

Switches l0 and ll

are

This increases the size of the symbol

l0 symbols.

sensed whenever PASS l is entered.

MACRO-8 would have to be reloaded

handle subsequent programs that use macros, double precision integers,

floating point numbers.

The Binary Format Tape produced during PASS 2 may be loaded by the Binary Loader.

When the

is completed, the accumulator should contain zero which indicates that it has loaded correctly.

The PASS 3 output is of the following format:
AAAA

NNNN

(Symbolic)

CR/LF

Where AAAA is the absolute octal address and NNNN is the generated code.
of phase with the octal

Example:
*200
TAD (l

0200

T377

020]

3776

DCA A

0376

4000

*4000

0377

0001

4000

0000

A, 0

Literals are somewhat out

SYMBOL TABLE MODIFICATION

9.4

Because of the small amount of core
macro

(60410 registers) remaining to be used for programmer symbols and the

table, the following suggestions are offered which may allow a particular installation or individual

to conserve on

table space

By use of the pseudo-ops EXPUNGE and FIXTAB, unnecessary instruction mnemonics can be removed from
the symbol table thus making more space available for programmer defined symbols and macros.

This also

decreases assembly time as the never used instruction symbols are not involved in the symbol table searches.

The most often used instruction mnemonics should be assembled first,

the special characters and pseudo-instructions.

that they will be in core next to

This is desirable because the symbol search routine starts

searching at the top of the table and works down.
At an installation that does not have a piece of optional equipment available, the corresponding instruction
set

can

be removed.

A symbolic tape beginning with EXPUNGE,

containing all necessary instruction mne-

monics, and ending with FIXTAB and the 35 sign could be assembled (only PASS l

prior to any other assemblies.

is needed) by MACRO-8

Example:

EXPUNGE

AND=OOOO
TAD=I 000
C LA=7200

FIXTAB

(The pseudo-op PAUSE could also be used with this tape, the
first of a multiple tape assembly.)

9—4

APPENDIX I

MACRO—8 SYMBOL TABLE

/MEMORY

REFERENCE

INSTRUCTIONS

KCC=6D32

AND=DOOO

KRS=6O34

TAD=IOOD

KRB=6O36

Isz=2DOO

ITELETYPE

DCA=30OD

TSF=6®41

JM5=ADOD

TCF=6D42

JMP=SDDD

TPC=6O44

IOT=OOOD

TLS=6®46

0PR=TDOO

/FLOATING

/MICROINSTRUCTIONS

EEXT=ODOO

POINT

CLL=71DO

EMPY=OOOO

CMA=7O4D

EDIV=4OOO

CML=TD2O

EOET=5OOD

RAR=7DID

PPUT=OOOD

RTR=7D12

PNOR=7DOD

RAL=7Z®4

IOSCILLOSCOPE

RTL=7DO6

/DISPLAY

IAC=TOOI

DCX=6$51

5MA=75OO

DXL=6O53

SZA=744O

DCY=6O61

SPA=751O

DYL=6®63

SNA=7450

DIx=6DSA

SNL=742®

D1Y=6D64

52L=743D

DXS=6057

SKP=TAIO

DY5=6D67

05R=74O4

DSE=6O71

HLT=7402

DCF=6072

/COMBINED

MICROINSTRUCTIONS

/INCREMENTAL

LAS=76$4

PLSE=65D1

STA=724D

PLCE=65O2

STL=7120

PLPU=6SOA

GLK=72D4

PLPR=6511

/PROGRAM

I NTERRUPT

PLOTTER

PLDD=6514

IOF=6002

PLUD=6522

DIGITAL

PLPL=6521

CONVERTER

PLPD=6524

ADC=6@@4
SPEED

PEREORATED

TAPE

READER

/LINE

PRINTER

RSE=6O11

LCF=6652

RRB=6O12

LPR=6655

LSF=6661

REC=6O14
SPEED

PEREORATED

TAPE

PUNCH

LCB=6662

PSE=6D21

LLD=6664

PCE=6O22

/CARD

PPC=6D24

CRSF=6632

PLS=6O26
ITELETYPE

KSF=6®31

PRECISION

PLDU=6512

ION=6®®1

/HIGH

AND

DLB=6O74

CIA=7041

/HIGH

COMMANDS

PSUB=2ODO

CLA=72OO

/ANALOG

INTERPRETIVE

EADD=1OOO

NOP=7DOD

TELEPRINTER/PUNCH

READER

CERS=6634

KEYBOARD/READER

CRRB=6671
CRSA=6672

AI-l

AND

CONTROL

CRT

CR§E=6674
/CARD

MMLC=6766

PUNCH

MMML=6766

CONTROL

CPSF=6631

IMEMORY

CPCF=6641

SMP=61®1

CPSE=6642

CMP=61®2
/TYPE

CPLB=6644
/AUTOMATIC

MAGNETIC

TAPE

CONTROL

PA RITY

138/139

MSCR=67®1

ADSF=6531

MCD=67®2

ADCV=6532

MTS=67®6

ADRB=6534

MSUR=6711

ADCC=6S41

MNC=6712

ADSC=6542

MTC=6716

ADIC=6S44
MAGNETIC

MSWF=6721

ISERIAL

MDWF=6722

DRCR;6603

MCWF=6722

DRCW=6605

MEWF=6722

DRCF=6611

MIWF=6722

DREF=6612

MSEF=6731

DRTS=6615

MDEF=6732

DRSE=6621

MCED=6732

DRSC=6622

MEEF=6732

DRCN=6624

MIEF=6732

IEXTENDED

MTRS=6734

MUY=7405

MCC=6741

DVI=7407

MRWC=6742

NMI=7411

MRCA=6744

SHL=7413

MCA=6745

ASR=7415

/COMBINED

INSTRUCTIONS

MQL=7421

MMMF=67S7

SCA=7441

MULTIPLY-DIVIDE

ARITHMETIC

MQA=7501

CAM=6101

/MAGNETIC

LAR=6104

TIFM=67Q7

LMO=6102

TSRD=6716

RDA=6112

TSWR=6716

MUL=6111

TSDF=6721

DIV=6121

TSSR=6722

SZO=6114
SAF=6124
RDM=6122

FSST=6724

/MICROTAPE

TAPE

SYSTEM

TSRS=6734
TWRT=6731

INSTRUCTIONS

DRUM

LSR=7417

MMMM=67S7

/AUTOMATIC

ANALOG

DIGITAL

/CONVERTER

TCTI=6732

MMLS=67SI
MMLM=6752

RDF=6214

MMLF=6754

RIF=6224

MMSF=6761

RMF=6244

/MEMORY

MMCF=6772

RIB=6234

MMSC=6771

CDF=6201

MMRS=6774

CIF=62Q2

MMCC=6762

A1-2

EXTENSION

SYSTEM

ELEMENT

APPE NDIX 2

ASCII CHARACTER SET

These characters may be used in symbols.
Character

8-Bit From

6-Bit From

323

324

303

325

304

326

305

327

306

330

307

331

310

332

311

260

312

261

313

262

263

264

316

265

317

266

320

14
15
16
17
20

314
315

267

321

270

322

271

Character

8—Bit From

6-Bit From

301

302

These characters are special.
Character

8-Bit From

6—Bit From

41
42

241
242
243
244

245

246

247

(
)

250

252

45
46
47
50
51
52
53

+
,

251
253

Meaning
Inclusive OR
Character pseudo-instruction
Illegal outside of (TEXT) or (") or comment

43
44

End of PASS

Illegal outside of (TEXT) or (") or comment
Logical AND
Illegal outside of (TEXT) or (") or comment
Define literal
Teminate literal
Set origin

2'5 complement addition

254

Define symbol

255

256

2'5 complement subtraction
Has value of CLC

257

Start of comment

272

Illegal outside of (TEXT) or (") or comment

A2—1

Character

8-Bit From

6-Bit From

273
274

Start macro definition

275
276

Define Parameter

End macro definition

277

300

Illegal outside of (TEXT) or (") or comment
Illegal outside of (TEXT) or (") or comment

333

Define page 0 literal

334

Illegal outside of (TEXT) or (") or comment

335

End page 0 literal

336

337

Illegal
Illegal

~>‘—‘/I-1@.°V
1*

Meaning
Terminate expression

Li ne -feed

212

Used For Formatting (ignored)

Return

2 I5

Terminate line

Space

240
377

Address delimiter or inclusive OR

Ru bout

Form—feed

2 I4

Blank

000

Code 200

200

Ignored
Ignored
Ignored
Ignored

A2—2

INDEX

Binary—to—octal conversions, 2—2
Bits, 3-3, 3—7

Absolute address, 3—7, 3—8, 4-2, 4-3, 4-5,

4-6, 9—3
Absolute positive value, 3-8

Accumulator, 3-4, 9—3
Addend, 3-9
Addition, 3-9

Bit zero, 3—9

Block diagram, 4—]
Boolean AND, 5—7

Boolean inclusive OR, 5-7

Brackets, 5—I I

5—12

Address field delimiter, 5—9

Address part of instruction, 4—6
Address tags, 4-5, 4-6, 5-4

Carriage return, 5-2, 5-3, 6—2

AND, 3-2, 3—5, 5-7, 5—12

Carry, 3-8, 3—9

Angle brackets, 7—1

Cell, 4—3

Argument, missing, 8-3

Character code storage, 6—4

Arithmetic operations, 3—9

Character string, 5—5

ASCII character set, 5—2, A2—I

Characters, 5—2

ASCII code, 5—I2

Characters, special, A2-I

ASR—33 Teletype binary codes, 2-4

CIA, 3—9

Assembler pseudo—instructions, 4-2

CLA (clear AC), 3-4, 3-5, 3—6

Assembly, 4-3

CLC (current location counter), 4-2

Assembly listing, 5—I, 9-1

CLL (clear L), 3—4

Asterisk, 5—IO, 6—I

CMA (complement AC), 3—4, 3-9

Augend, 3—9

CML (complement L), 3—4

Augmented instructions, 3-l, 3-3

Coding practices, 5-4
Comma, 5—7
Comments, 4—4, 5—3, 5-4, 6—2, 7-2

BE (overlapped MACRO—8 internal tables), 8-3

Binary codes, ASR—33 Teletype, 2—4

Binary Format tape, 9—]
Binary integers, 2'-I
Binary Loader, 9-1, 9-3
Binary point, 3—9
Binary program tape, I-2

Comment field, 4—4

Complement, 3-9
Complement arithmetic, 3-8
Constants table, 5-II

CONTINUE, 6—5, 9—2
Count, 4-]

Counter, 4—2

INDEX

(Cont)

Current address indicator, 5-IO

Encoding, 2—4

Current location counter,

End of tape, 6-5

4—2, 5—9, 6-1

Current Non—Zero Page Exceeded, 8—1

Equal sign, 5—6

Current page, 3—7, 5—11

Error code, 8—]

Current page bit, 5-8

Error diagnostic II, 5-9

Current radix, 6—2, 6-3

Error diagnostics, 8—1
Error message format, 8-]

Error message IC, 5—3
Error messages, 8—]

Data cells, 4—3

Data storage registers, 4—10

Event times, 3—3, 3—5

Expression, 4—6, 5—6, 5—7

DCA, 3-2

EXPUNGE, 6—5, 9—4

DECIMAL, 6—2

Extended memory, 6-]

Decimal integers, 2-1
DECIMAL pseudo—instruction, 5-5

Extended memory binary loaders, 6—2

Decimal—to-binary conversion, 2—3
Decimal-to-octal conversion, 2—3

DEFINE, 7-]

FIELD n, 6-l

Definition of macro, 7—l

FIXTAB (fix symbol table), 6—5, 9—4

Definition of macro name, 7—]

Floating point constant registers, 6—3

Deposit and clear AC, 3—2

Floating point constants, 5—1, 6-3

Diagnostic messages, 5—]

Floating point constants normalized, 6-3

Dollar Sign ($), 4-3,

Floating point input format, 6-3

6-4, 9-2, 9—4

Double precision Floating point, 6—2

FLTG (floating point pseudo-instruction), 6-3

Double precision floating point constants, 6-3

Flow chart, 4—l

Double precision integers, 5-1, 6-2

Form—feed, 5-3

Dummy arguments, 7-l
G

Dummy argument symbol, 7-]

Group I operate microinstructions, 3—4
Group 2 operate microinstructions, 3-4, 3—5, 3—6
E, 6-3
H

Effective address, 3—7

Elements, 5—5, 5-7

Halt, 6-5

INDEX

HLT (halt), 3—5, 3-6

(Cont)

Intersection, 5—7

1/0 device, 3—3
1/0 pulses, 3-3
IOT instruction format, 3-3

I, 4—12
IAC (index AC), 3-4, 3—9
IC (illegal character), 8-I
ID

(illegal redefinition of symbol), 8-]

IOT instructions, 3—3
IOT microinstruction operations, 3—3
ISZ (index and skip if zero), 3—2

IE (illegal equals), 8-2

Ignored characters, 5—3
II

(illegal indirect), 5—9, 8-2

JMP, 3—3

Illegal characters, 5—3

JMP I, 5—9

Illegal equals, 8—2

JMP instruction, 4-H

Illegal format, 8-2

JMS (jump to subroutine), 3-3

Illegal indirect, 5—9, 8—2

JMS instruction, 4-H

IM (illegal format), 8—2

Inclusive OR, 3-5, 5—8
Inclusive OR (space), 5-8

Incrementing location counter, 5-6
Index, 4—2, 4—3

3—7, 4-1], 4—12

Indirect address linkage,

5-9, 5-”

Indirect bit, 5-9

Initializing,

Leader, 5—3
Least significant bits, 3-9

Index and skip if zero, 3-2

Indirect address,

Labelled locations, 4-5

(link generated) message, 5—9, 8—2

Line—feeds, 7-2
Line terminators, 5—3

4-2

Link, 3-4, 3-8, 5-9

Input, I—2

Link generation, 5-], 5—9, 8-2

Input—output transfer instructions, 3-3
Instruction address, 4—6
Instruction Fields, 5-4
Instruction operation code, 3—]

Instructions, 3-3

Listing, 5-4
List storage, 5—4

Literal buffer, 5—12

Literal regeneration, 5—l2
Literal storage, 5-H, 5-12

Integers, 5-5, 6-2
Integers, decimal,

Left parenthesis, 5-“

Literals, 5-I, 5-H, 9—3

binary, and octal, 2-]

Logical AND, 3-2, 3-5

INDEX

(Cont)

Location counter, 5—7

Module, 5-7

Locations labelled, 4—5

MP (missing parameter), 8—3

Loop, 4—]
N
M

Negative numbers, 3-8
MACRO Assembler, 4—2

Nested literals, 5-12

MACRO-8 Programming Language, 5-l

Nonprinting characters, 5-2

MACRO—8 symbol table, Al-l

Nonzero page literal, 5-12

MACRO-8 versions, 9—]

Numbers, 6-2

Macro definition, 7-]
Macro definition storage, 7—2

Macro expansion, 7-3
Macro instruction, 5-l

Macro name, 5-7, 5—8
Macro name definition, 7—l
Macro nesting, 7-2

Macro redefinition, 7-3
Macro referencing, 7—3

Macro restrictions, 7—2
Macro table, 7-2, 9—4
Macro table storage, 7-3

Macros, 7-]

Memory field, 3-6, 3—7
Memory field page, 3—6
Memory field Words, 3—6

Memory organization, 3—6
Memory reference instructions (MRI), 3-], 5-9
Memory register, 3-]
Microprogramming, 3—3
Minuend, 3—9

Missing parameter, 8-3
Mnemonic, 2-4

Obiect program, 2—4
Obiect program tape, 5-l
OCTAL, 6—2
Octal fraction conversion, 2—3
Octal integers, 2—l

Octal page addresses, 4-5
OCTAL pseudo—instruction, 5—5

Octal-to-binary conversions, 2—2
l—cycle instructions, 3—3
l's complement, 3—8, 3—9

Operand, 3—7

Operate instructions, 3—3, 3—4
Operating instructions, 9—]
Operation code, 3—3, 3-4
Operation code, instruction, 3-]
Operation characters, 5-2

Operator intervention, 9—2

Operators, 5-1, 5—7, 5-10, 6-2, 6—3
OPR, 3—3

Optimum use, 4-7
Organization of memory, 3-6

INDEX

(Cont)

Origin, 4—2, 4—3

Programmer symbols, 9—4

Origin reset, 5-l0

Programming language, 2—4

Origin setting, 5-10

Pseudo—instruction, 4-2, 4—10, 5—4, 6-'l

OSR, 3—5

Pseudo-instruction PAGE, 5—10

OST, 3-6

Punctuation characters, 5—2

Out-of—page reference, 5—9
Q

Overflow, 3—9

Quote, 5-12, 6—3, 6—4, 7—2

Page, 3—7
PAGE, 4-2, 6—]

Radix, 2-1

Page address, 3—7

Radix control, 6-2

Page bits, 5—9

Radix point,

PAGE n, 6—]

Radix pseudo—instructions, 5-5

PAGE pseudo-instruction, 5—l0

Page zero, 3—7, 4—6, 4-10, 4—12,

2-2, 2-3

RAL (rotate AC and L left), 3—4

5-H, 5-12

Page zero addresses, 4—6
Parameter assignment statement, 5—6

PASS 1, 5—6, 6—5, 9—], 9-2, 9—4
PASS 2, 5—9, 5—12, 6-2, 9—l, 9—2
PASS 3, 9—1, 9—3
PASS 3 format, 9-3

PAUSE pseudo—instruction, 6-5, 9—2, 9-4
PE (Current, Non—Zero page Exceeded), 8-]

RAR (rotate AC and L right), 3—4

Recycling, 4—]
Redefinition of symbol, 5-6

Registers, 3—6, 3—7, 4-2, 4-3, 4-5
Relative location, 4—8
Reverse skip sensing, 3-5

ROTATE AC and L, 3-4
RTL (rotate left), 3—4
RTR (rotate right), 3-4

Period, 4—8, 5-10
Permanent symbols, 5-8
Permanent symbol table, 5-5, 6—5

Pointer, 4—3, 4—11
Points, 4-“
Positional notation, 2—]
Positive numbers, 3—8

Program (Location) Counter, 4-2, 4—11

SE (symbol table exceeded), 8—2

Semicolon, 5-3, 5—4, 6-2

Sign, 6-2
Sign bit, 3—8
Sign notation, 3—9

Single character mode, 6-3

(Cont)

INDEX

Single character text facility, 5-12

Symbol rules, 5-5

Skipping, 3—4

Symbol table, 5-1, 5-6, 9—]

Skips, 3-5

Symbol table alterations, 6—5

SKP (skip unconditional), 3-6

Symbol table exceeded, 8—2

Slash, 4-4

Symbol table modification, 9—4

SMA (skip on minus AC), 3-5, 3—6

Symbol table punching, 9—]

SNA (skip on non—zero AC), 3-6

Symbol value, 4—5

SNL (skip on non—zero L), 3-6

Symbolic addressing, 4-6

Source programs,

2-4, 5-]

Symbolic expression, 5-H

Source tape, 9-]

Symbolic expression evaluation, 5-7
'

SPA (skip on positive AC), 3—6

Symbolic name, 2-4

Space address field delimiter, 5—8

Symbolic program, 2-4

Space interpretation, 5-7, 5-8

Symbolic tag, 4—5, 4-10

Space, 5—6

SZA (skip on zero AC), 3-5, 3—6

Special characters, A2-l

SZL (skip on zero L), 3—6

Special symbol (dollar sign), 4-3, 6—4, 9—2, 9—4
Special symbols (text), 3—]
Storage for list, 5-4

Storage requirements, 4—2
Storage techniques, 4—6
Stored literals, 5-12

Subroutine properties, 4-10
Subroutine requirements, 4—10

Subroutines,

4-9, 4-10

Subscript, 2-]

Tabulations, 5-3, 5—4
TAD instruction, 3-2, 3-8, 3—9

Tags, 2-4
TEXT pseudo-instruction, 6-4, 7—2
Text facility,

3-9

Subtrahend, 3-9
Switch options, 9-2
Switch register, 9—]

5—1, 6—3

TEXT field, 5-3
Text strings,

Subtraction, 3-8,

Symbol, 4—5,

Tab, 4-4, 5—6

6—3, 6—4

Title, 4-4, 5—4

Trailer, 5—3

Transferring control, 4-H
Two's complement add,

3-2, 3—8, 3—9, 6-2

5—5

Symbol and integer combination, 5-7

Symbol definition, 5-5, 5-6
Symbol redefinition, 5—6

Undefined symbol, 8—3

INDEX

Union, 5-7
US

(undefined symbol), 8-3

User defined symbols, 5—8

Value zero, 3—9

Variables, 4—7, 4-12
W

Words, 3-7
Z

(zero page exceeded), 8—]

(Con’r)

PDP-8/I

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