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Document:	PDP-7 AsmMan
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DIGITAL -7-3-S

PDP-7
SYMBOLIC ASSEMBLER
PROGRAMMING MANUAL

DIGITAL EQUIPMENT CORPORATION • MAYNARD. MASSACHUSETTS

CONTENTS
Chapter
INTRODUCTION ••..•.....•......•.............•••......•...•

I LLUSTRATI ON OF PDP-7 ASSEMBLER FEATURES ........•........
The Location Cou nter •••.•••.•••••...•.•••••
Coding Illustrations •.•.•..

The Character Set .......•.•.•............................

Elements •••.•......•.•.•.•...•..........

Comments

•

••

••••

0......

•

...........•........•.•.....•..................

12
13

Instructions

Statements

PROGRAM PREPARATION ..•.•.......•.•.•.•.•.....•....••....

Program Tape. . . . . . . . . . . . . . . . . . . . . . . . . . . . • . . . • . . . . . . . . . . . .

Location Assignment .•.•.•.•.....•...•.••••..•••..••••••••

Symbolic Address Tag •...........•.•.••..•••••.•..••..••..

Program Example. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

ASSEMBLE R OUTPUT

•...•...•.............................•..

Object Tape ...••...•••••..•....•.....••.................

Symbol Punch ......•..........•...........•..............

Pu nc h ...•...•.•.•...•.•.•.•.....•.......•.............•

Symbol Printouts •....

Error Messages ..

•

••••••••••••••••••••••••••

•

•••

•

Expressions .•.•....

•

.....

•

•••••••••••

•

THE SOURCE LAN GUAGE

•••

••

•

••••

••••••••••••••••••

••

•

•••••••••

•

•••

•

••

•

OPERATING THE ASSEMBLER ................................. .
Operating Instructions.

iii

0.0.0

••••

0.....................

31
31

CONTENTS (continued)
Chapter

Page
Loading a Symbol Punch............................ . . . . . . •

Ha Its Duri ng Assemb Iy .•..................................

The FF Loader. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Loading the Object Program ...............................

PSE UDO-I N STRUC TION ••..•.•..•......•......••.•.•.•.•..••..

PERMANENT SYMBOLS •••••••.•••.•............•.............

THE FORTRAN ASSEMBLY SYSTEM ..•.•...••...................

CHARACTER SETS ....•.......•................••..............

Appendix

CHAPTER 1
INTRODUCTION

The use of an assembly program has become a standard practice in the programming of digital
computers.

Th is type of processor perm its a programmer to code in a more convenient language

than the 18-bit binary numbers which are util ized by a PDP-7.
are widely recognized:

The advantages of this practice

Easily recognized mnemonic codes are used instead of numeric codes;

instructions or data may be referred to by a symbol ic name; decimal or alphabetic data may be
expressed in a more convenient form than in a binary number; programs may be altered without extensive changes in the source language; and debugging is simpl ified.
The basic process performed by the Assembler is the substitution of numeric values for symbols,
according to associations defined in the symbol table.

In addition, the user may request that

the Assembler itself assign values to the user's own symbols at assembly time.

These symbols

are normally used to name memory locations, which may then be referenced by name.
The value of the ability to use mnemonic names to represent machine instructions cannot be
overestimated.

For example, the name ADD reminds the user of the addition function, while

the number 300000 does not. Consequently, the instructions are easier to remember when
mnemon ics are used.

The same is true of location names.

It is much easier to associate the

name TOTAL with the location containing the accumulated total, than it is to remember that
location 13764 contains the total.
Another advantage is that, since the assignment of absolute numbers to symbol ic locations is
done by the Assembler, the updating of a program by adding or removing instructions is easy.
In addition to translating statements directly into their binary equivalents, the Assembler will
accept instructions for performing translations.

These instructions may not look different from

other instructions, but they do not generate binary codes.
to as pseudo-instructions.

For this reason, they are referred

For example the pseudo-instruction DECIMAL tells the Assembler

that all numbers following in the program are to be taken as decimal rather than octal.

This

instruction is important to the assembly process but has no binary equivalent in the object program.

Certain other features of assembly can be directed at assembly time by setting the console
ACCUMULATOR switches, abbreviated ACS.
The PDP-7 Assembly system consists of the Assembler and the FF Loader {Digital 7-1-1}. A
source program tape prepared in the source language using ASCII ,or FIODEC code, produces
an object tape punched in FF Binary code in one pass through the Assembler.

The object tape

is loaded by the FF Loader (included on the object tape), which completes the assembly functions that could not be performed in one pass and loads the resulting binary program representation into the computer ready for execution.
The Assembler requires a basic 4K memory configuration with a teleprinter, perforated tape
reader and perforated tape punch. Use of the basic Assembler allows 2202 locations for additional symbol storage during assembly in a 4K machine or 12202 locations in an 8K machine.
The extended version of the Assembler has the abi! ity to process the I ist of extended symbols
in Appendix 2, in addition to the basic symbols handled by the basic version. The extended
Assembler allows 505 locations and 10,505 locations when used with 4K and 8K machines,
respectively.

If this storage capacity is exceeded, the user must segment his program and as-

semble it in sections. When used with extended memory (more than 8K), the Assembler occupies
the same area of memory as in an 8K machine.
The source program may be prepared in either ASCII or FIODEC code. Although ASCII has
been used throughout this manual in programming examples, equivalent FIODEC characters
as shown in Appendix 4 are equally val id.

CHAPTER
ILLUSTRATION

OF PDP-7

ASSEMBLER

FEATURES

THE LOCATION COUNTER
In general, statements generate 18-bit binary words which are placed into consecutive memory
locations. The location counter is a register used by the PDP-7 Assembler to keep track of the
next memory location available.

It is updated after processing each statement. A statement

which is assembled into a single machine instruction would update the location counter by one;
a statement which is assembled into six binary words would update it by six. The location
counter may be expl icitly set by an element or expression followed by a slash. The element or
expression preceding a slash sets the location counter to the value of that element or expression.
Subsequent instructions are assembled into subsequent locations.
Example:
100/

The next instruction is placed in location 100.

CODING ILLUSTRATIONS
To ill ustrate some of the features of the PD P-7 Assembler, a small routine has been chosen and
coded in a number of different ways. The routine continually adds one to the contents of a
location unti I the resu It is pos itive I then ha I ts. The instructions used are represented as the ir
octal codes (more compact than the binary actually used).

The code is the sum of the operation

code (200000 for the first instruction) and the address in memory of the quantity to be operated
on. The number be i ng incremented is in location 200. The notation C (A) means contents of A.
Example 1:
100/
101/
102/
103/
104/
105/
200/
201/

200200
300201
040200
741100
600100
740040

o
1

/C(200) INTO AC
/ADD C(201) TO AC
/STORE AC IN 200
/SKIP ON PLUS AC
/JUMP TO LOCATION 100
/HALT
/WILL CONTAIN NUMBER TO BE INCREMENTED
/CONSTANT 1

Since the location counter is automatically incremented, specifying sequential addresses could
have been avoided after the first address in the progression.

In addition, the names of the PDP-7

instructions could be used in place of the octal codes. The octal representation of these Instructions is substituted by the Assembler whenever symbols appear in the program.
Example 2:
100/

200/

LAC 200
ADD 201
DAC 200
SPA
JMP 100
HLT
0

1
The same program could have been written using symbolic address tags.

The comma after the

symbol A indicates to the Assembler that the location in which it places the instruction LAC B
is to be named A.

Information associating the symbol A with the number of the actual location

is placed in the symbol table . Consequently, when processing the instruction JMP A, the
Assembler finds the values of the symbols JMP and A in the symbol table and uses these values
to form the binary equivalent of the instruction JMP A.
Example 3:
100/
A,

LAC B
ADD ONE
DAC B
SPA
JMPA
HLT

200/

B,
ONE,

0
1

Unless the user specifically wanted to use locations 200 and 201 for storage, he could let the
Assembler assign the locations.

Example 4:

100/

LACB
ADD ONE
DAC B
SPA
JMPA

HLT

ONE,

The Assembler also handles literals for the user.

The value of the expression contained in pa-

rentheses is called a constant and is automatically placed in an unused location by the Assembler.

The address of that location is inserted into the instruction.

In the example beiow,

the address of the register containing 1 is substituted in place of (1).
The user may also request the Assembler to assign variable storage for him by placing a # within
the first six characters of the variable. A symbol which includes this character is automatically
assigned a register at the end of the program, and a a is placed in that register.
Example 5:

100/
A,

LAC #B
ADD (l)
DAC B
SPA
JMPA

HLT
Even though the actual locations used may not be the same, the results of the program assembled
from the above examples will be the same in all cases.

CHAPTER
THE

SOURCE

LANGUAGE

This section explains the features of the ASCII source language available to the user of the
PDP-7 Assembler; for equivalent FIODEC characters, see Appendix 4.

THE CHARACTER SET

Letters
ABCDEFGHIJKLMNOPQRSTUVWXYZ.
Use of the lower case letters is not perm itted. The character period is treated as a letter, and
for purposes of discussion, is considered a letter.

Digits

1234567890
Punctuation Characters
NOTE: Since a number of characters are invisible, the following
notation is used to represent them in the examples.
space

-I

tab

carriage return, I ine feed

The following characters are used to specify operations to be performed upon symbols or numbers.
Character

Use

'--I

space

combine symbols or numbers

plus

combine symbols or numbers

minus

combine symbols or numbers

logical AND

combine symbols or numbers

Character

Use

inclusive OR

combine symbols or numbers

carriage return,
Iine feed

term inate word

-1

tab

term i na te word

comma

assign symbol ic address

equals

defi ne parameter

slash

comment, or assign location

(

left parenthesis

initiate constant

)

right parenthesis

term inate constant

Ignored Characters
form feed

end of a page of a source program

Special Characters
#

designates single register variable

designates multiregister variable
(normally three registers)

Illegal Characters
All other characters are illegal to the Assembler except in comments and cause the error print
ICH.

Illegal characters are ignored.

ELEMENTS
Any group of letters, digits, and parentheses which represent binary values less than 2
elements. Values are assigned to elements by the Assembler or the Loader.

Number
Any sequence of digits delimited by punctuation characters forms a number.

are

Examples:

1
12

4371
The radix control pseudo-instructions indicate to the Assembler the radix to be used in number
interpretation.

The initial radix is octal.

The pseudo-instruction DECIMAL indicates that all

numbers are to be interpreted as decimal until the next occurrence of the pseudo-instruction
OCTAL.
The pseudo-instruction OCTAL indicates that all numbers are to be interpreted as octal until
the next occurrence of the pseudo-instruction DEC IMAL.

Symbol
Any sequence of letters and digits beginning with a letter and delimited by punctuation characters is a symbol. Although a symbol may be any length, only the first six characters are
considered, and any additional characters are ignored; symbols which are identical in their
first six characters are considered identical.

Note that period (.) is treated like a letter.

However the user should neither begin nor end a symbol with the character period, since this
use has been reserved for the FORTRAN system.
The Assembler has, in its permanent symbol table, definitions of the symbols for all the PDP-7
operation codes, operate commands, and many lOT commands (see Appendix 2 for a complete
list). These may be used without prior definition by the user.
Examples:

LAC

is a symbol whose value of 2000008 is taken from the
operation code definitions.

SQRT

is a user-created symbol. When used as a symbol ic
address tag, its value is the address of the instruction
it tags. This value is assigned by the Assembler.

Parameter Assignments
A parameter may be defined by use of the equal sign. The symbol to the left of the equal sign
is assigned the value of the expression to the right.

Examples:

A =6
EXIT = JMP I 20
TST = 477
If the parameter is used in a program as an operand address, the contents of the corresponding
register are taken as the operand.
LAC TST

loads the AC with the contents of register 477

LAC (TST

loads the AC with the value 477

If the expression to the left of the equal sign does not consist of a single symbol or the expression to the right is not term inated by a tab or carriage return, the error message I FP is printed.
An undefined symbol appearing in the expression to the right of the equal sign causes the error
print UPA.

No error message is printed if a defined symbol is redefined in a parameter assignment

unless it is a permanent symbol.

In this case, if the old value is not equal to the new, the error

message RPS is printed and the symbol is redefined.

Variables
The user may request the Assembler to assign storage registers for him.

These registers, whose

value may be changed while the program is running, are designated as variables. A symbol
(permanent symbols and pseudo-instructions must not be used) containing # or $ as one of its
first six characters, which is not explicitly defined elsewhere by use of a comma or equal
sign, is a variable. A symbol may contain a # any number of times without becoming multidefined, but this character is required only once, not necessarily on the first occurrence of
the variable. Currently unassigned variables are defined as the addresses of successive registers at the occurrence of the pseudo-instruction VARIABLES.
VARIABLES may be used repeatedly in a program.
Examples of variables:
#WHAT
WAI#T2
LEVEL#

The pseudo-instruction

If the pseudo-instruction VARIABLES is not used, the table of variables is automatically assigned
a storage block at the end of the program.

Upon loading the program, the contents of all lo-

cations assigned by use of the variable facil ity are zeros.
A variable containing $ causes a multiple number of registers to be reserved.

The number of

registers to be reserved may be controlled by the pseudo-instruction BAR n.

The element or

expression!2. specifies how many locations are to be allocated for $ variables. !2. is initially
set to three.
Example:
To reserve seven registers for WANT and three register for GET
at the occurrence of VARIABLES, use these instructions:
DAC $GET

VARIABLES

DAC $WANT

BAR 7
VARIABLES

Undefined Symbols
If any symbols remain undefined at the termination of assembly, they are automatically defined
as the addresses of successive registers following the variables storage block and their definitions printed.

That is, they are treated as variables containing #, but the user is informed of

the assignment.

Current Address Indicator
The single character period (.) is assigned the current value of the location counter (see page 3)
at assembly time.

(Note that if a letter or digit appears on either side of the period, a symbol

is formed, defeating the address indicating function of the period.)

Examples:
200/

JMP .+2

This instruction is identical to a jump
to location 202.

JMP .2

However, this instruction is a jump to
the address represented by the symbol .2.

Literal
A storage register whose contents remain the same throughout the running of a program is designated as a constant. A constant can be represented by using a literal: an element or expression contained in parentheses.

This type of element causes a register to be reserved in the

constants table by the Assembler.
Example:
ADD (1)
or
ONE=l

ADD (ONE)

is equivalent to
ADD ONE

ONE,
except that in the first cases, the number 1 is stored automatically and its address substituted
during loading.

Unique constants are stored only once in the table, so that many uses of the

same constant result in only one memory location being allocated for that constant.

For ex-

ample, the three statements,
ADD (1)
SAD (-1)

LAC (1)
result in two registers being allocated in the constants table, one for I and one for -1. The
contents of the parentheses may be any element or expression:

LAC (JMP .-4)

The closing parenthesis may be deleted for brevity:

LAC (JMP .-4
Only one level of parenthesis may be used. Constants are automatically stored beginninf1 in
the register indicated by the location counter when START or PAUSE is encountered.

Indirect Addressing

If bit 4 in the binary representation of a memory reference instruction is 1, instead of taking
the memory location referenced as the effective address of the operand, the contents of the
location referenced are taken as the effective address of the operand.

Indirect addressing is

represented in the source language by the character I following the operation symbol thus:

LAC 1500
500/ 407
407/ -40
This instruction does not place the contents of location 500 in the accumulator as it would
without the I. Rather the contents of location 500 are taken as the location of the quantity
to be placed in the accumulator. After the execution of this instruction, the accumulator
conta ins -40.
Since indirect addressing sets bit 4, the value of the element I may be represented as 020000.

EX PRESS IONS
Expressions are strings of elements separated by arithmetic or logical operators. Expressions
18
represent numeric values less than 2
in magnitude. The value of an expression is calculated
by first substituting the numeric values for each of the elements and then performing the operations.

The allowable operators are:
Combine by

Operators
L-J

space

addition (lis complement)

plus

addition (lis complement)

minus

subtraction (lis compl ement)

logical AND
inclusive OR

When combining elements, operations are performed as encountered from left to right.

In gen-

erol, expressions may be of two classes: those expressions (called storage words or instructions)
which occupy space in the binory version of the progrom; and those expressions wh ich are used
during the assembly process.

Examples of such expressions would be symbolic address tags,

locotion assignments, or parometer assignments.

COMMENTS

If the character slash V) occurs, not immediotely preceded by an element or expression, all
characters between the slosh and the next carriage return are ignored.
page 7) are permitted within comments.

Illegal characters (see

Parity errors are ignored within comments also.

Examples:
ITHIS IS A COMMENT
LAC A
4001 0

lAS IS THIS
lAND THIS ALSO~

INSTRUCTIONS
Instructions are elements or expressions wh ich make up the binary program (storage words).
Memory reference instructions always have an effective operand address.
operand address itself or the I address modifier and an address.

Th is may be the

In addition, if the address

portion of the memory reference instruction is 0 I iteral, the effective address is the location
in memory which contains that literal.
Examples:
SZA

A symbol ic instruction from the operate group. An
element.

LAC (407

An expression, consisting of the operation LAC and
the litera I (407.

DAC IXIT

An expression, consisting of the operation DAC, address modifier, and address. The effective address
is a combination of the last two elements.

060342

An element, numerical representation of the preceding instruction where XIT is the tag for location
342.

STATEMENTS
Statements are combinations of elements, expressions, and comments del imited by carriage return, line feed pairs

(1 ~). To achieve clarity, the components of a statement normally

appear in three areas or fields on a line delimited by tabs (-I) or the carriage return, line
feed pair

(1 ~). In the leftmost field are parameter assignments, location assignments, or

symbol ic address tags. In the middle field are instructions or constants. Though the rightmost
field is usually used for comments, they may appear in any of the fields.
Examples:
STORE = 30
TST = 777776
/PLACE -1 IN TEST WORD
/THIS IS A ROUTI NE TO SORT A TABLE

40/
BEGIN,

/START PROGRAM; CLEAR AC

REMOVE,

CLA
LAC XIT -4
AND (TST

400/

126

/LENGTH OF 1ST LIST

The Assembler interprets both carriage return, Iine feed

(..J ~) and tabs ( -I ) as field del imiters.

Lonsequently, in addition to the statement format suggested above, any format can be used
which separates statement components with a carriage return or tab, regardless of line length.
For example, when assembling tables or repetitive instructions, which would take large amounts
of space if listed on individual lines, the following format could be employed.
table,

-t 15~ ~
-f 35~ ~
-l 31~ ~

-1 20~
-J 12~
-1 01~
-I 25;
-1 34;

~
~

may be I isted thus:
table,

-1 15

-1 20
-1 25

-1 35
-1 12

-1 31) ~
-1 01; ~

-1 34)~

Similar treatment may be given instructions:

RR9,

RTR
RTR

RTR
RAR

RTR

CHAPTER 4
PROGRAM

PREPARATION

A program is prepared in ASCII or FIODEC code on 8-channel punched paper tape, usin
off-line typewriter or the on-line program EDITOR with the PDP-7.

_In

In general, a prograr r

should begin with about 2 feet of tape feed (only the feed hole punched) to allow easy placement in the reader.

Deleted characters (seventh hole punched) and tape feed may be used

freely throughout the tape and are always ignored.

PROGRAM TAPE
The program tape itself consists of three sections, described below.

Title
All text between the first character (other than initial carriage return, I ine feed pairs) and the
next carriage return, I ine feed is taken as the title of the tape. The first Iine of a II symbol ic
programs should be a title line. This title is printed on the teleprinter at assembly time, as
well as punched in readable format on the front of the binary tape. The title is not subject to
normal program conventions; it need not begin with a slash to indicate it is not a part of the
program.

Program Body
The text consisting of statements and pseudo-instructions, follows the title. Redundant carriage
return, I ine feeds and tabs are ignored and may be used for formatting. A suggested program
body format is described in the preceding section.
The character form feed should be used as a page separator (with both the tape ED ITOR and
Teletype Model 33KSR) although pages have no meaning to the Assembler.

New pages should

begin with a carriage return, line feed.
Deleted characters (rubout overpunch), tape feed, and form feeds are ignored by the Assembler
during processing.

NOTE: To avoid erroneous assembly, the first statement in a
program or the first statement after an absolute address assignment
must not contain more than one symbol which is undefined at
that time.

Terminating Pseudo-Instruction
The last line of a program consists of the pseudo-instruction START, or PAUSE, followed by
either the starting address of the program and a carriage return, I ine feed or by a carriage
return, line feed alone. Either pseudo-instruction indicates the end of the symbolic program.
If START is followed by an address, control is transferred to that address when loading is completed.

In this manner, the program is immediately executed.

dress, the computer halts.

If PAUSE is followed by an ad-

By depressing CONTINUE,control is transferred to the specified ad-

dress and the program executed.

If either START or PAUSE is used with no ensuing address, the

computer halts after loading the program.

To execute the program, the user must place the

starting address in the ADDRESS switches and depress START.
Constants are stored starting at the address in the location counter when START or PAUSE is
encountered on the last source tape. This normally follows the program unless the current
location was reset (using the slash) immediately preceding the START.

LOCATION ASSIGNMENT
The use of a slash V), if immediately preceded by an element or expression, sets the location
counter equal to the value of that element or expression.
Examples:
300/

LAC (56

BEG-240+A/

LAC (56

The instruction is stored in location number
BEG-240+A.

SYMBOLIC ADDRESS TAG
An element or expression which represents a location in memory may be assigned a value in a
number of ways.

The user could util ize the parameter assignment feature thus:

A=.

ADD 100

The symbol A is assigned the value equal to the location in which the instruction ADD 100 is
placed by the Assembler.

If the symbol already has a definition, it would be redefined.

The

user can reference this location later by the symbol A:
JMPA
The sImplest way to assign a value to a tag is by use of the comma.

ADD 100

The value of A would be the same as in the first case; the only difference would occur if A
had previously been defined, which would result in the diagnostic MDT.
The Assembler, if possible, sets the element or expression to the left of a comma equal to the
current value of the location counter.

If the comma is not preceded by an expression, the di-

agnostic IFe occurs. A single undefined symbol or an expression containing only one undefined
symbol preceding the comma has its value set equal to the current location.
/\n expression preceding the comma which contains more than one undefined symbol causes the
error print TUA.

If the expression to the left of the comma contains no undefined symbols but

is equal in value to the current location, it is ignored; otherwise the error print MDT occurs.
This feature is useful for verifying table lengths.
Examples:

A,
B+ 1,
101,
GEORGE+HARRY-4,

Where A and B are previously undefined symbols.
.
Where the number is the same as the
current val ue of the location counter.
Where either GEORGE or HARRY are
previously undefined symbols.

PROGRAM EXAMPLE
THIS IS A SAMPLE PROGRAM.
/IT ROTATES A BIT THROUGH THE AC AT A RATE
/DETERMINED BY THE AC SWITCHES
GO,

LAS
SPA:CMA
JMP GO
DAC CNTSET
lAC ONE
DAC BIT
Cll

lOOP,

lAC CNTSET
DAC CNT
LAC BIT

lOOP1,

ISZ CNT
JMP lOOPl
RAl
DAC BIT
LAS
SMA
JMP LOOP
JMP GO

/EXAMINE AC SWITCHES
/WAIT UNTil ACSO=O

/1 IS A CONSTANT
/ClEAR THE LINK

/lOOP UNTIL CNT GOES TO ZERO
/JUMP TO PRECEDING lOCATION
/ROTATE BIT
/IF ACSO=l, RESET TIME CONSTANT

/STORAGE FOR PROGRAM DATA
CNT,
0
BIT,
0
CNTSET,
0
ONE,
1
START GO
The result of assembling this and requesting a symbol print would be:

THIS IS A SAMPLE PROGRAM
GO
lOOP
lOOPl
CNT
BIT
CNTSET
ONE

22
31
34
44

46
47

The same program could be written using the additional fdcilities for constants, variables, and
current location indicator.

THIS IS A SAMPLE PROGRAM
GO,

LAS
SPA:CMA
JMP GO
DAC #CNTSET
LAC (1
DAC #BIT
Cll

LOOP,

lAC CNTSET
DAC CNT
LAC BIT
ISZ #CNT
JMP .-1
RAL
DAC BIT
LAS
SMA
JMP LOOP
JMP GO

START GO
1n th is case I the constant 1 occupies location 47 in the constant area following the program.

CHAPTER 5
ASSEMBLER OUTPUT

The Assembler processes the symbol ic source tape, types the program title, and punches a binary
obiect tape.

Err~r m7ssages are typed out during assembly in the formats described on page 27.

A printout of user-defined symbol values may be req,uested at the completion of assembly .
The user's program is punched on .the obiect tape'in a code called FF Binary'~ While storage
.'

words on binary tapes may be read directly from bits 6-1, a FF Binary tape is p'~nched in a
compl icated form to be used by a sophistica~ed loader: to produce the storage:words of the user's
program.

For this-'~eason, no attempt is made to explain how to read or interpret the FF

representation o{ a program. See the FF'Loader (Digita I 7-19-1) for th is i nfor~ation.
In the ensuing sections, memory locations are given for the 8K memory configuration.

These

locations also work properly with 4K memories or extended memory configurations, 'though
the 4K locations are actually 10000 less.

OBJECT TAPE

The tape' is punched ,in the reverse direction from which it will be loaded {the termination block
.;

is punched first; the readable title, last}.

Thetape consists of five sections, explained below

in order of appeara~ce when loading. See Figure 1 for an example.

Title
The first data on the object tape is the title, punched in readable form.

Loader
Following the title is the FF Loader in binary, preceded by a 6-instruction loader-loader in
read-in-mode also punched in binary. See page 35 for descriptions of the role of each loader.

•
•
•
•
••
•
•

B. FF
Loader (in
binary)

•••••••
•• ••
••

••••• •
•

••••••••
• •

•• •• ••
•••••
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•
••
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•

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•• •• •
•• •• •••• ••
•

•••••••••
• ••
•
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•

A. Readable
Title

•

•
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•
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•
•
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•

•••••• •
••
••
••
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•
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• • D. Block
• •••

•• •••
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• • •
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•
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• •••
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• •
•

••

•
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E. Block Body

C. Start Block

•

••••••• ••

• •• • •
• ••
••• •••
•
•
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•• ••• •••••
•• ••• • ••
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•

•••••••

Heading

•• • ••
F. Termination
••
•
••
Block
•• ••••••••
•
(in binary)
•
•• •••••
•
•• •••••••••••
• • ••• •••
••
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• •

• • •• •
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E. Block Body
•••
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(in
Funny
•
•••••••
Format binary) •• • ••••
•••••••
•••
•• •••
•••
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• •
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••
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••
Figure 1

An Object Tape

•••
•
•

••

•
••
•

....
•
•

If the FF Loader is expected to be in memory when the object tape is loaded, the pseudoinstruction NOI NPUT can be used to instruct the Assembler to suppress punching of the loader
and punch instead a JMP 17600, the starting address of the FF Loader. A tape in this form may
be loaded by the loading commands in DDT-7 as well as through a FF Loader already in memory.

Starting Block
Three binary words are punched:
1. The instruction HLT or NOP depending upon whether PAUSE or START
was used.
2. A JMP to the starting address following the START or PAUSE.

If no

address followed I a HLT is punched.
3. The address of the register preceding the constants table (location counter
when START or PAUSE was encountered). Constants are stored beyond this
locationi the program is loaded backward from th is location.
The following table summarizes the first two words of the starting block produced by the possible pseudo-instruction forms.
Address
Not Specified

Address
Specified
START

NOP
JMP ADDRESS

NOP
HLT

PAUSE

HLT
JMP ADDRESS

HLT
HLT

Data Blocks
The program is punched in data blocks, each consisting of the following components.

Block Heading
Three binary words:

DAC LA where LA is the largest address in which an instruction from the

block will be loaded (the first instruction encountered by the loader).

The

block is loaded backward from this location.

2. - N where N is the number of words in the b lock on tape.
only N words in FF Binary, then looks for new commands.

The loader reads

Since every line is

read, N words are equiva lent to 3N I ines on tape.
3.

The checksum, the sum of all words in the data block excluding the check-

sum.

This is compared to a computed sum to insure correct loading.

Block Body
The words in the block body are punched in the alphanumeric mode, delivering 24 information
bits to each 3 lines of tape (FF Binary).

Eighteen of these form a data word; the remaining

six are a code word which indicates to the FF Loader the type (or use) of the data word.
words are punched as the symbol i c program is read.

The

Since th is tape is produced backward, the

first instruction in the block body when loading would correspond to the last instructior, in this
block of the symbol ic program.

Termination Block
Two binary words:
1. A SKP instruction to indicate the end of the tape to the loader.

This also

causes the loader to execute the instructions generated by the terminating
pseudo-instruction (see Starting Block).
2. A dummy word to stop the reader.

This second word is necessary since the

loader commands the reader to read the next word wh i Ie the previous word is
be i ng processed.

SYMBOL PUNCH
The ensuing sections are concerned with how to obtain and use a symbol punch.

No effort is

made to explain the internal procedures used by DDT or the Assembler to assimilate a symbol
punch or how the formats as punched in FF Binary differ.
A symbol punch is the definition of source program symbols, punched on the object tape in one
of two formats.

When encountered by the loader, the symbol definitions are ignored; the only

advantage of punching symbol definitions is for use in debugging work with DDT -7 (to enable the
user to refer to his own symbol ic tags) or for the Assembler.

The Assembler can load Assembler

format symbol definitions into its permanent table from a symbol punch.

When assembl ing pro-

grams which refer to an often-used subroutine, a printout routine for example, the subroutine
need not be assembled with each program to obtain definitions for the subroutine symbols.

In-

stead a symbol punch of the necessary symbols can be loaded into the Assembler and these symbols referred to without further definition by programs being assembled.
The symbol definition punching formats are different for DOT-7 and the Assembler; the user must
be careful to obtain the punching format required for his purpose.

Symbol Punch for DDT -7
The Assembler normally punches symbol definitions for DDT-7.

Each symbol defined during as-

sembly is punched on the object tape in ODT-7 format at time of definition unless suppressed
by the AC switch settings (see page 32).

Symbol Punch for the Assembler
The pseudo-instruction SYMBOLS causes the Assembler to 'suspend DDT symbol punching, as
controlled by the AC switches, until the occurrence of NOSYMBOLS.
are defined, they are punched in Assembler format.

In the interim as symbols

The pseudo-instruction NOSYMBOLS

terminates the punching of Assembler format symbol definitions begun by SYMBOLS and restores
the DDT format specified by the AC switches (see Appendix 2 for an example).

Because of internal operating procedures, for a symbol punch to be loaded into the Assembler,
the tape must not contain the FF Loader. Consequently, the pseudo-instruction NOINPUT

must be included on any tape which will be used to add symbol definitions to the Assembler's
table at a later time. The symbol punch may be added to the Assembler's symbol table by
following the directions for loading a symbol punch on page 32. Upon loading, the Assembler
reads the tape ignoring all data except symbol definitions which are added to the previous
symbol table. Symbols which appear both on tape and in the Assembler symbol table are
redefined to the tape definitions (see page 32).

Symbol Punch by PUNDEF
The definitions of symbols occurring in the list following the pseudo-instruction PUNDEF are
punched on the output tape in Assembler format. Such symbols must be defined prior to the
use of PUNDEF. This is equivalent to surrounding the specified symbols with the SYMBOLS NOSYMBOLS combination except that with PUNDEF, DDT symbols can also be punched when
the symbol is defined.

In addition, PUNDEF automatically invokes the pseudo-instruction

NOINPUT suppressing the punching of the FF Loader.
The PUNDEF list must consist of the symbols the user wishes punched, separated by commas,
and terminated by a carriage return. The error prints LIT, IFL, LNS may occur indicating,
respectively, illegal terminating punctuation and two types of illegal format in a list.
Note that the PUNDEF tape may be physically separate to avoid having to load an entire program tape to obtain a few definitions.

In this case, the proper format is:

TITLE
PUNDEF SYMB 1, SYMB2, SYMB3, etc.
START
This tape should be assembled before symbol definitions are erased from the Assembler symbol
table (that is, by depressing START to begin assembling the tape). ACS 15 must have been
left down during the assembly which produced the desired symbols (see page 32).

PUNCH
The pseudo-instruction PUNCH causes the expression following punch to be punched on the
tape in binary. With PUNCH, the user can obtain a tape which can be loaded by a binary

loader such as the RIM Loader (see PDP-7 manual) directly into memory.

(Binary, not FF Binary,

is read three lines at a time, six bits per line (holes 1-6) to form an 18-bit word.

Only lines

whose eighth hole is punched and whose seventh, delete, hole is unpunched are read.) The
FF Loader is not needed to load binary tapes. Consequently, after the iob of the FF Loader
is completed, other data could be loaded into locations 17600 to 17761 by using PUNCH with
RIM.

(See page 35 for a description of the role of the various loaders.)

SYMBOL PRINTOUTS
After the completion of assembly, symbol printouts can be requested in alphabetic or numeric
order {see operating instructions}.

These give all symbol definitions which were added to the

Assembler1s permanent symbols during the assembly.

Symbols can be printed only oncei after

printing, an internal indicator is set to suppress the printing of any symbol which was printed
previously.

Both printouts can be obtained in succession, however.

Undefined Symbol Assignments
At the end of assembly, before the loader is punched, any undefined symbols are automatically
defined.

Each undefined symbol which was used in a storage word is defined as the address of

a register at the end of the program and the definition printed.

If the symbol was not used in

a storage word, the symbol is printed but not defined.

ERROR MESSAGES
The error message appears in one of the following three formats.

With the exception of SCE

(storage capacity exceeded) and ILP (illegal parity), assembly continues automatically after
the error message has been printed.

Format A
The appearance of a diagnosti c printed in format A:
ERROR

PREVIOUS VALUE

SYMBOL

NEW VALUE

Whether the new value was actually incorporated into the symbol table depends upon the particular error.

Error

Meaning

MDT

A previously defined symbol was redefined
with a comma. (See page 17 .)

RDA

An attempt was made to redefine a permanent symbol with a comma. The symbol was not redefined.

RPS

A permanent symbol was redefined. (See
page 8.)

Format B
The appearance of a format B diagnostic is:
ERROR

OCTAL ADDRESS

SYMBOLIC ADDRESS

The general error message is printed in Format B.
Error

Meaning

IFC

Illegal format in symbolic address tag. The
tag is ignored. (See page 17.)

IFI

An expression using CHAR or FLEX was formed
improperly. (See page 38.)

IFL

Illegal format in a PUNDEF list. (See page 26.)

IFP

Illega I format in a parameter assignment.
The ass ignment is ignored. (See page 8.)

IFS

START or PAUSE used incorrectly. Assembly
continues as if START or PAUSE had been
used with no expression following. (See
page 17.)

ILF

Illegal format in a pseudo-instruction such
as BAR. The pseudo- instruction is ignored.

INS

An illegal format in a PUNDEF list--two commas
appeared ina row or a dig i t appeared.

LIT

An illegal character was found in a PUNDEF
list. The character is taken as a term i nator .

MDT

The value of the complex symbol ic address
assignment (tag) and the location counter
disagree. The symbol ic address tag is redefined
if possible. (See page 17.)

Error

Meaning

SCE

Storage capacity of the symbol table was
exceeded. No recovery is possible.

TUA

Too many undefined symbols appeared in
a symbol ic address assignment (tag). Location counter remains unchanged. (See
page 17.)

UBR

An undefined symbol appeared in a BAR
pseudo- instruction. The setting of BAR
remains unchanged.

Format C
The appearance of a format C diagnostic is:
ERROR

OCTAL ADDRESS

SYMBOLIC ADDRESS

CAUSE

Format C is an expanded version of Format B. CAUSE is additional information to help the
programmer ascertain the cause by an undefined symbol which will be printed. ASCII codes
are printed when the cause is a character.
Error

Cause

Meaning

ICH

character

A character not part of the Assembler's source language was
used. The character is ignored.
(See page 7.)

ILP

character

A character read from tape did
not have an odd number of holes
across the line. Place the correct
character (if possible) in bits 12-17
of the ACS and press CONTI NUE.

UAA

symbol

An undefined symbol appeared in an
absolute address assignment V). The
current address indicator remains unchanged.

UPA

symbol

An undefined symbol appeared in a
parameter ass ignment. The ass ignment is ignored. ~ee page 8.)

UPN

symbol

An undefined symbol appeared in a
PUNCH pseudo-instruction. The symbol is ignored.

Error
UST

Meaning

Cause
symbol

An undefined symbol appeared in a
START or PAUSE instruction. The symbol is ignored and the START or PAUSE
f>aken a lone. (See page 17.)

CHAPTER 6
OPERATING THE ASSEMBLER

OPERATING INSTRUCTIONS
1.

Load the Assembler by placing the binary tape of the Assembler in the

reader and starting the RIM (Readin Mode) Loader in location 17770.

It is

assumed that the RIM Loader will be prestored in core memory. Press START.
2.

Place the symbol ic source language tape in the reader, and set the ADDRESS

switches to 20. Set AC switch 10 up to indicate ASCII symbolic tape (down for
FIODEC).
3.

The operator may choose, at this point, to begin a normal assembly or

command the Assembler to execute special functions as indicated by the
AC switches.
a.

Normal assembly, restore symbol table to permanent symbols--

depress CONTI NUE.
b.

Special functions--set ACS (see page 32) and depress START.

When the pseudo-instruction START or PAUSE in the source tape
is encountered, the Assembler stops with all ones in the AC.
NOTE 1: To assemble more than one symbol ic tape into one binary
output tape (a main program and subroutines, for example), the
sequence of steps in assembly is altered. After Step 3, the next
symbol ic tape is put in the reader. With 20 in the ADDRESS switches,
depress the START key. Repeat these steps for remaining symbolic
tapes. The title of the first tape and the START from the last tape
are incorporated into the binary output tape unless otherwise specified
by ACS3. When all desired symbolic tapes have been assembled,
continue with Step 4.

4. To complete the normal assembly, depress CONTlNUEo The Assembler
punches the variables, the undefined symbols (I isting these on the on-I ine
Teletype), the starting block, and the loader and punches the title in
readable form.

Then the Assembler stops with all ones in the AC.

The

assembly of a loadable obiect tape is complete at this point.

NOTE 2: To restore the Assembler's symbol table to permanent symbols
before beginning another assembly, put up AC switch 15. Then after
completing Step 5, return to Step 2. If no symbol printouts are
desired, depress CONTI NUE and return to Step 2.

To print the symbol definitions, set the AC switches (see below) and

depress the CONTINUE key. When the printouts are completed, a halt
occurs with all zeros in the AC.

LOADING A SYMBOL PUNCH
A symbol punch in Assembler format (see page 25) can be loaded into the Assembler at any time,
but the suggested time is prior to assembl ing the first tape (before Step 2). To load a symbol
punch, place the tape in the tape reader, set ADDRESS switches to 4, and depress the START
key.

The symbol definitions are added to the Assembler's permanent symbol table; restoring

the Assembler's symbol table has no effect on them.

To start an assembly, return to Step 2

above.

AC Switch Control
Throughout the assembly of a program, ACS 10 indicates the symbol ic tape code: ASCII (up)
or FIODEC (down).

This may be reset if desired for each program or subprogram assembled.

In Step 3, the AC switches perform the following functions:
AC Switch Up

Meaning
Exam i ne AC sw itches 1-4 further.
Suppress punching.

Suppress punching of symbols for DDT-7. Save
space on tape unless needed for DDT work.

AC Switch Up

Meaning

Take the title on this tape. The title from
the current tape replaces the first tape's
title on a single binary output tape (see
Note 1).

Restore the Assembler when restarting an
unfinished assembly.

In Step 5, the switches have the following meaning:
AC Switch Up

Meaning

Restore symbol table to permanent symbols
(after symbol printouts if requested). Starting the next assembly with CONTINUE has
the same effect.

Symbol printout, numerical order.

Symbol printout, alphabetical order.

In addition, the following switches have meaning throughout an assembly.
Meaning

AC Switch Up

ASC II symbol ic tape

Causes all printing to be done on the high
speed line printer.
HALTS DURING ASSEMBLY

The following are all possible abnormal halts during assembly, the cause, and the action which
can be taken.
Cause
i II ega I parity

AC Contents

Action

character

storage capacity

Place correct character in ACS.

Depress CONTI NUE.

Segment program and reassemble.

exceeded, printout SCE
offensive interrupt

See below.

status register

When a device other than the reader, punch, or Teletype causes a program interrupt, the Assembler halts with the status register displayed in the AC. CONTINUE clears some standard
device flags, not including those of the devices used by the Assembler, and proceeds.

If this fails to clear the offending device1s flag, the instruction to clear that flag must be
loaded and executed.

To accomplish this, deposit the required lOT in location 6.

In location 7,

deposit a JMP to the register specified by the program counter when the halt occurred.

Then

set the ADDRESS switches to 6 and depress START.
Since the Assembler uses the program interrupt, users at installations which have special equipment connected to the program interrupt system must take special care to insure that the associated flags are cleared before assembly storts.

The devices which are cleared by the Assembler

are:
Perforoted Tope Reader
Perforated Tape Punch
Teleprinter
Clock
Type 30D Display
Light Pen
Character Generator
Type 57A Mag Tape Control
Cord Reader
Cord Punch
Line Printer

TH E FF LOAD ER
The Assembler performs its action in one pass; that is, the source language tape is processed
only once to produce the binary object tape including a 162 (octal) location FF Loader. Certain functions which cannot be handled at assembly time must be handled by this loader when
the program is loaded into memory.
The first of these is the insertion of symbol definitions for symbols wh ich were undefined during
assembly. When the Assembler first encounters an undefined symbol, the symbol is togged as

undefined and assigned a register.

Each time the symbol is used before it is defined, the ad-

dress of the assigned register along with an identifying code is punched on the binary object
tape. When the symbol is subsequently defined, both the defined value and the address of the
assigned register are punched on the binary tape.
defined value during loading.

The assigned register is used to contain the

When loading, since the end ot the binary tape which is punched

last is the end read first, the definition of a symbol is encountered before any use of the undefined symbol.

Thus, the loading process is accompl ished correctly.

The second problem handled by the loader is the setting up of constant tables. When constants
are encountered during assembly, the Assembler does not know where they are to be stored.
Thus, the constants are punched on the binary output tape with an identifying code. When a
constant is encountered by the loader, the constant table, which is built up by the loader at
the end of Assembler assigned storage, is searched for previous assignments.
is found, the new constant is added to the table.

If no assignment

The address of the constant is recorded, and

th is address replaces the constant.
The FF Loader uses registers 7 and 10 during the loading process.

Upon completion of loading,

reg ister 7 contains the address of the first location after the constant table, wh ich is normally
the first free location avai labl e following the program.

Th is number is a Iso in the AC at the

time the first instruction of the program is executed to allow the program to set up storage
areas after the program.

LOADING THE OBJECT PROGRAM
To load the object tape, place the tape in the reader title end first, and depress START with
17770 in the ADDRESS switches.

The RIM Loader must be in memory. The FF Loader is

normally punched at the beginning of the object tape in binary.
loader-loader punched in Readin Mode for the RIM Loader.

It is preceded by a 6-instruction

If the object tape has no loader

(NOINPUT was used), the FF Loader must be in memory in addition to the RIM Loader.

(Read

in any object tape with the loader punched on it.) The FF Loader occupies registers 17600 to
17761. While the FF Loader is being read in, the loader-loader is stored from 17572 to 17577.
When an object tape is being loaded normally, the RIM Loader reads in the loader-loader which
in turn reads in the FF Loader.

The FF Loader then loads the user's object program {punched

in FF Binary).

If START followed by an address was used to terminate the program, it is ex-

ecuted immediately.

If PAUSE was used followed by an address, depressing CONTINUE causes

execution to begin.

Halts During Loading
A checksum is computed as each block is read from tape and compared with the checksum read
from the block heading.

If these differ, the loader halts, displaying (in the ACCUMULATOR)

a word whose 0 bits are those wh ich differ between the computed and the read checksum.

repeated loadings cause the same difference to appear in the AC I ights, the object tape is
probably faulty and should be reassembled.
may be the difficulty.

If the difference varies, the computer or reader

In any case, depressing CONTINUE causes the loader to ignore the

checksum seen.

APPENDIX 1
PSEU DO-INSTRUCTION

There are certain symbols which, when used in a program, are commands directly affecting the
assembly process without appearing in the output.

These pseudo-instructions have no other

effect upon assembly and are ignored when form ing storage words.

These symbols may not be

used as address tags or variable names.

RADIX CONTROL
DECIMAL
OCTAL

All numbers not imbedded in symbols can be interpreted as decimal
or octal, respectively. The initial mode is octal (see page 7).

TABLE FORMA liON
SHIFT

Causes the word preceding SHIFT to be rotated left nine binary positions and masked with 777000, leaving the right half blank. SHIFT
is useful in forming double entry tables, tables with one value stored
in the leftmost nine bits and another value in the rightmost nine bits.

SYMBOL TABLE CONTROL
EXPUNGE

Removes all symbols (except pseudo-instructions) from the Assembler's
symbol table.

FIX

Resets the symbol table so that all currently defined symbols are part
of the permanent symbol table. (This instruction overrides the ACS
option to restore at assembl y time.)

VARIABLE CONTROL
VARIABLES

Places all currently defined variables at addresses beginning with the
address indicated by the location counter before processing of the program continues.

BAR N

Allows N registers for each variable containing the character $ (see
page 10).

PUNCH CONTROL
PUNDEF SYMB 1,
SYMB2, SYMB3

Punches definitions of the I isted symbols in Assembler format.

SYMBOLS

Punches ensuing symbols in Assembler format.

NOSYMBOLS

Stops the punching of Assembler symbols and restores the mode
set by ACS concern ing DDT symbols.

PUNCH A

Punches the value of the expression A in binary at this time.

See pages 25-26 for a more detailed explanation.

END OF PROGRAM
NOINPUT

Suppresses punching of the loader--punches a JMP 17600, the
starting address of the FF Loader.

START A

Upon loading, causes the program to start at register A.

PAUSE A

Upon loading, causes the computer to halt. When CONTINUE
is pressed, the program will start at A. (See page 17.)

TEXT HANDLING
The following pseudo-instructions pack character codes (to be output upon execution of the
program) into storage words in the computer.
CHARRA
CHAR LA
CHAR MA

Code values for single characters (represented here by A) are
assembled into the right, left, or middle six bits of the word
following to the character mode below.

FLEX ABC

Code values for three characters (represented here by ABC) are
assembled into a single register from left to right according to the
character mode below.

TEXT /THIS IS
TEXT ./

A string of characters of any length is assembled, three to a word,
into successive registers according to the character mode below.
The string is terminated by the second occurrence of the delimiting
character chosen by the user. / has been chosen here. In order
to separate the string from other data following it, a termination
code determined by the character mode is inserted automatically
after the last character code of the string. If FIODEC characters

are used, double-punch characters--center dot, period (:), center
dot, comma 0) and vertical stroke, capital S ($)--may not be used
as del imiters. Note the space after the instruction TEXT.

CHARACTER MODE CONTROL
The mode control pseudo-instructions specify the character code to be used when evaluating
the instructions TEXT, FLEX, and CHAR.
TELETYPE

Initially, the mode is Teletype.

All characters are to be converted to their respective 6-bit packed
Baudot codes. In order to print on line on the Teletype Model 33KSR
or 28KSR using Output Package (Digital 7-10-0), it is necessary
to restrict the characters used in these pseudo-instructions to those
listed in Appendix 4 in the column titled Baudot. The termination
code in TELETYPE mode is the code 00.
The 6-bit code consists of a 5-bit Baudot character (most significant bits), see PDP-7 Manual, F-75P, and a case bit (least significant bit) which is 1 if the character is in upper case in this
code and 0 if the character is in lower case. Tab is converted
to enough spaces to space to the next tab stop. Tab stops are
internally set to every ten spaces.

ANELEX

Indicates that all character translations are to DEC high-speed
pri nter code. (See PD P-7 Manua I, F-7 5P). The term i nation
code in ANELEX mode is 00.

FIODEC

Indicates that all character translations are to FIODEC code. The
termination code in FIODEC mode is 13 (stop code). The conversion
from ASCII to flODEC code is not always 1 to 1 since case shifts
do not generate codes in ASCII whereas in FIODEC they appear
as the first character of the stri ng of characters to wh ich they appl y.
TlCTOC (Digital 7-11-10) provides for printing FIODEC codes.
on the Teletype.

Codes for the Teletype characters used in the example below are:
Character

Code

....... A code is 30 (11 000 in binary);
appending a for lower case yields the binary
code 110 000 or 60 in octal.

Examples of use of characters input pseudo- instructions:
Symbol ic

Result

CHARRA
CHAR MB
CHAR 1j1
FLEX A. B
TEXT .ABC.

000060
004600
330000
601746
604634
000000
000064
760017

CHAR RA+4
LAW CHAR R.

/NOTICE 00 TERMINATION CODE ADDED

APPENDIX 2
PERMANENT SYMBOLS

MODIFYING PERMANENT SYMBOL TABLE

It is often desirable to make modifications to the permanent symbol table of the PDP-7 Assembler.
This may be accompl ished in the following way:
1. Assemble a symbol ic tape which has the following format:
TITLE
NOINPUT
SYMBOLS
{body}
NOSYMBOLS
START
where the body consists of all symbols with definitions as parameter assignments that the user wishes to add to permanent symbol table.
2. Splice this on the end of a binary tape of the Assembler, cutting the Assembler
just before the last block on tape {the termination block, two binary words},
and cutting the other tape after the first binary word following the title.
If the user wishes to delete any permanent symbols, the entire permanent
symbol table must be removed and replaced with a table containing only
those symbols desired.
To cause symbols which are already defined to be punched on the symbol tape
being prepared, define the symbol equal to itself.

For example, to cause the

symbols LAC and SZA to be punched on the new binary symbol table tape, write
in the body of the symbol ic tape
LAC = LAC
SZA = SZA
To remove the entire symbol table, cut the Assembler tape after the block which
consists of one binary word (SKP), which should be before the next to the last
block on tape.

BASIC SYMBOLS
DAC
JMS
DZM
LAC
XOR
ADD
TAD
XCT
ISZ
AND
SAD
JMP
lOT
aPR
CAL
LAW
LAM
I

NOP
CLA
CLl
CMA
CML
CLC
CCL
RAL
RAR
RTl
RTR
RCR
RCl
OAS
LAS
LAT
HLT
SPA
SMA
SPl
SMl
SZA
SNA
SKP
SZl
SNl
GlK
STL

040000
100000
140000
200000
240000
300000
340000
400000
440000
500000
540000
600000
700000
740000
0
760000
777777
020000
740000
750000
744000
740001
740002
750001
744002
740010
740020
742010
742020
744020
744010
740004
750004
750004
740040
741100
740100
741400
740400
740200
741200
741000
741400
740400
750010
744002

740040

Interrupt
IOF
ION
ITON
CAF

700002
700042
700062
703302

I/O States
IORS
SKP7

700314
703341
Clock
-

CLSF
CLOF
CLON

700001
700004
700044

Perforated Tape Reader
RSF
RSA
RSB
RRB
RCF

700101
700104
700144
700112
700102
Tape Punch

PSF
PLS
PCF
PSA
PSB

700201
700206
700202
700204
700244
Keyboard

KSF
KRB

700301
700312
Teleprinter

TSF
TLS
TCF
TTS

700401
700406
700402
703301

Display 30D
DXL
DXS
DYl
DYS

700506
700546
700606
700646
42

DLB
DXC
DYC

700706
700502
700602

Light Pen Type 370
DSF
DCF

700501
700601

EXTENDED SYMBOLS
Type 57A Mag Tape
MTS
MTC
MCD
MNC
MRC
MRD
MTRS
MCEF
MEEF
MIEF
MCWF
MEWF
MIWF
MSEF
MSWF
MSCR
MSUR
MCC
MCA
MWC
MRCA
MDEF
MDWF

707006
707106
707042
707152
707244
707204
/
707314
707322
707342
707362
707222
707242
707262
707301
707201
707001
707101
707401
707405
707402
707414
707302
707414

Card Punch
CPSF
CPSE
(PLR
(PCF

706401
706444
706406
706442

Symbol Generator Type 33
GPL
GPR
GLF
GSF
GCL
GSP

701002
701042
701004
701001
700641
701034

Card Reader

Line Printer
LPSF
LPCF
LPLD
LPSE
LSSF
LSCF
LSLS
LPB-1
LPB-2
LPB-3
PRI
PAS

706501
706502
706542
706506
706601
706602
706606
706;:04
706524
706544
706604
706624

DECtape
MMRD
MMWR
MMSE
MMLC
MMRS
MMDF
MMBF
MMEF

707512
707504
707644
707604
707612
707501
707601
707541

Extended Arithmetic
Element Type 177
EAE
LRS
LRSS
LLS
LLSS
ALS
ALSS
NORM
NORMS
MUL
MULS
DIY
DIYS

CRSF

706701

IDIV
IDIVS

CRSA
CRSB
CRRB

706704
706744
706712

FRDIY
FRDIVS
LACQ

640000
640500
660500
640600
660600
640700
660700
640444
660444
653122
657122
640323
644323
653323
657323
650323
654323
641002

LACS
CLQ
ABS
GSM
OSC
OMQ
CMQ

641001
650000
644000
664000
640001
640002
640004

Automatic Priority
Interrupt Type 172
CAC
ASC
DSC
EPI
DPI
ISC
DBR

705501
705502
705604
700004
700044
705504
705601

Precision Incremental
Display Type 340
IDLA
IDSE
IDSI
IDSP
IDRS
IDRD
IDRA
IDRC
IDCF

700606
700501
700601
700701
700504
700614
700512
700712
700704

Memory Extension
Control Type 148
SEM
EEM
LEM
EMIR

707701
707702
707704
707742

Serial Drum Type 24
DRLR
DRLW
DRSS
DRCS
DRCS
DRSF

706006
706046
706106
706106
706204
706101

DRSN
DRCF

706201
706102

Multiplexer Control Type 139
ADSM
ADIM

701103
701201

A-to-D Converter Type 138B
ADSC
ADRB
ADSF

701304
701312
701301

APPENDIX 3
THE FORTRAN ASSEMBLY SYSTEM

The FORTRAN Assembler is a modified version of the PDP-7 Assembler.

The FORTRAN As-

sembler produces a relocatable object program unless absolute address assignments are used.
Relocatable programs are loaded by the Linking Loader consecutively from location 22.

The

loader also joins programs by supplying definitions for symbols which are referenced in one
program and defined in another. Any program written for the PDP-7 Assembler can be assembled by the FORTRAN Assembler.

The differences present in the FORTRAN Assembler are:

1. The addition of pseudo-instructions to define symbols used by the loader
to I ink relocatable programs to each other.

These pseudo-instructions are

EXTERNAL, INTERNAL, and L1BFRM.
2.

Error printouts associated with these three pseudo-instructions have been

included.
3.

The object programs produced by the FORTRAN Assembler are relocatable;

the programs are loaded into an area of memory determined by the position of
other programs at load time.
4.

DDT cannot be used with relocatable programs since symbol definitions

are not establ ished until loading.
5. START and PAUSE can be used as previously described.

However, execution

of a program assembled and loaded by the FORTRAN system can only be accom-

pi ished by placing the starting address (usually 22) in the ADS and depressing
START.
NOTE: To avoid improper loading, all absolute parameter assignments shou Id precede any references to them in the program.

The following definitions are used in this appendix:
main program

A program which is terminated by START or
PAUSE and may contain only EXTERNAL lists.

subroutine

A program which contains I NTERNAL symbols
and is terminated by START or PAUSE. An
EXTERNAL list can be included also.

I ibrary routine

A program in I ibrary format; that is, a subroutine
terminated by a L1BFRM list.

The Iinking pseudo-instruction is a convenient method for a program to call subroutines and
I ibrary routines.

Library routines may be a group of often-used functions or subroutines wh ich

are assembled in library format (LlBFRM used). After loading a main program and subroutines,
any number of I ibrary format routines can be read in. Only those routines called by previous
programs will be loaded.

By placing 5 in the address switches and depressing START, the user

can obtain the locations of I NTERNAL symbols and check to see that all referenced routines
have been loaded (see Loading A Relocatable Program). Again, the loader handles the I ink-

•

ing of programs so that assembly can take place separately and symbol punches are not required.
Since programs are packed consecutively, memory space is also conserved by using relocatably
assembled object programs.
The major disadvantage of relocatable assembly is that DDT cannot interpret the symbols produced.

Methods of surmounting this problem will be discussed under Debugging A Relocatable

Program.

THE LINKING PSEUDO-INSTRUCTIONS

EXTERNAL SYM 1, SYM2, ••••
An external I ist of subroutine or I ibrary routine symbols must precede any reference to the symbols. Symbols are not allowed to appear inmore than one EXTERNAL list.

The list is in the same

format as a PUNDEF list (see page 26). Of the three linking pseudo-instructions, only EXTERNAL
can be used in a main program.

INTERNAL SYM1
Internal is used only in subroutines and library routines. It is followed by one symbol and immediately precedes the comma definition of that symbol. I NTERNAL forms the other half of the
link established by EXTERNAL. Using INTERNAL also causes the automatic punching of a
secondary entry to the linking Loader. This loader must already be in memory when a subroutine
is loaded. In general, the linking Loader is punched on main programs only; it is always suppressed
if INTERNAL occurs.

LlBFRM SYM1, SYM2, •...
Any program with INTERNAL symbols may be assembled in library format by replacing (or
preceding) START or PAUSE with LlBFRM. When a LlBFRM list is encountered, the FORTRAN
Assembler punches out a Iibrary format tape immediately with no operator action required.
Symbols are not printed. library format is essentially the format generated by using INTERNAL,
but preceded by a heading block containing all INTERNAL symbols as specified in the LlBFRM
list.
Another way to obtain a library format assembly is to prepare a tape consisting of a dummy title
and a LlBFRM list. Assembl ing the subroutine and L1BFRM tape as a 2-tape program produces
a I ibrary format tape.
Example:
Five programs are to be assembled re locatobl y:
1. A MAIN PROGRAM containing
EXTERNAL SR1X, SR2X, LR1X, LR2X
JMS SR1X
JMS LR2X
JMS LR1X
CAL SR2X

START GO

2. A subroutine SR2X containing
INTERNAL SR2X
SR2X,
EXTERNAL SR1X, LR2X
JMS SR1X
JMS lR2X
START
3. A subroutine SR1X containing
internal SR1X
SR1X,
START
4. A library routine lR1X containing
INTERNAL lR1X
LR1X
EXTERNAL lR2X
JMS LR2X
lIBFRM lR1X
5. A library routine lR2X containing
INTERNAL lR2X
lR2X,

lIBFRM LR2X
After assembly the main program must be loaded first, placing the linking loader in memory.
The subroutines can be loaded in any order following the main porgram.
An EXTERNAL call for a symbol must have been encountered by the loader before the library
routine containing that symbol as an INTERNAL symbol will be loaded. Consequently to avoid
repeated readins of routines, the library routines called by subroutines or other library routines
should follow them in the loading sequence.

When an INTERNAL symbol is encountered by the Linking Loader, its definition is saved.
definition then replaces the memory reference in an EXTERNAL call.

The

Thus, the main program

and associated routines are joined together when loaded by the Linking Loader.

FORTRAN ASSEMBLER ERROR PRINTOUTS
The error printouts produced by the FORTRAN Assembler are identical to those produced by the
PDP-7 Assembler with the addition of:
SYS

Internal symbol previously defined or incremented in this definition.

IFZ

More than one symbol in an internal symbol definition.

IFY

Internal symbol not defined by comma.

I FX

Externa I symbol referenced before the externa I declaration occurred or
external symbol already defined.

I FQ

Illega I format in I ibrary list.

Illegal term punctuation in I ibrary list.

RELOCA TABLE OUTPUT
When loaded by the Linking Loader, relocatable programs are placed in consecutive memory
locations beginning at 22.

This is also the first address of the main' program.

It must be loaded

first since the loader is punched only on its tape.
Absolute program segments or routines are placed in memory where specified by their absolute
address assignment. Care should be taken to insure that relocatable programs and absolute programs are not loaded over each other.

To facil itate the detection of o~erlays, at the end of

any stage of the loading process, the loader will display in the ACCUMULATOR lights the
va lue of the next sequential memory location following the reg ister just loaded.
In the loading procedure, constants are stored following the last location in the last program
loaded.

If an absolute segment is loaded last, all the constants wi II be stored following that

segment.

If this is undesirable, the absolute segment could be preceded by a symbolic address

tag assignment and followed by the tag used as an address assignment (slash). Constants would
then be stored following the location of the tag.

DEBUGGING A RELOCATABLE PROGRAM
A relocatable program cannot be debugged with DDT.

If problems are encountered, the best

solution is to assemble the main program and called routines together absolutely using the PDP-7
Assembler.

DDT can be used to debug the absolute program; then assemble the corrected pro-

gram relocatably.
No changes need be made in the relocatable source program to do this.
ignores the three linking pseudo-instructions.

The PDP-7 Assembler

However if a tape has been prepared using

LlBFRM, it should be followed with a START. When read by the FORTRAN Assembler, the
START is not encountered; when read by the PDP-7 Assembler, the LlBFRM is ignored and the
START interpreted as usual.

If this format is used, the routine is compatible with either

Assembler.

LOADING THE RELOCATABLE OBJECT PROGRAM
1.

Load the main program, with the Linking Loader punched on the tape,

through RIM (ADS=17770, depress START).
2.

Load subroutines through RIM which are called by other programs.

3. Load library routines by depressing START with ADS=6. Any number may
be assembled together and read in from one tape.
are loaded into memory.

Only those routines called

If a I ibrary routine calls other I ibrary routines, it

should precede them in the loading sequence.

4. To obtain a printout of the locations of subroutine and I ibrary routine symbols
and to find if all called routines have been loaded, depress START with ADS=5.

If a routine has been called but not loaded, its symbol is printed preceded by a
minus sign. The address of the first reference to this symbol is also printed.

further routines are needed, they should be loaded as in Steps 2 and 3 above.
5. To execute the relocatable program in memory, depress START with ADS=22.

APPENDIX 4

CHARACTER SETS

t
t
t

ASCII

FIODEC

A-Z

0-9

t
t

Teletype (Baudot)

$
%

(

)

- (overbar)
(underbar)

$
no equivalent

(
)
x
+

(
)
n.e.
n.e.

t
t
t

t =

>
?

: (center dot, period)
; (center dot, comma)

;

<
=
>

n.e.
n.e.
n.e.

-..

[

\
J

t
~

t~tab

t +line-feed
t ~ carriage return

space

rub-out
blank
form feed

]

tab
(included in
~ ~ carriage return

space
n.e.
n.e.
stop code

t designates basic character set
51

n.e.
n.e.
n.e.
n.e.
n.e.
n.e.
bell
~
line-feed
~
) carriage return
space
n.e.
n.e.
n.e.