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DEC-D8-SBAB-D

December 1967

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IDENTIFICATION
Product Code:

DEC-08-SBAB-D

Product Name:

PDP-8 Disc System Builder

Date Created:

October 27, 1967

Maintainer:

Software Services Group

DEC -08- SBAB- D

ABSTRACT
This document describes a fast convenient keyboard oriented monitor which will enable the
user to efficiently control the flow of programs through his PDP-8. The system is modular and openended allowing the user to build the software components required in his environment. The user may
specify the system device (disc or DECtape), the amount of core, number of disc units available and
the number, name and size of his resident system programs.
This is a basic package to which DEC will add FORTRAN, Assembler, Editor, Peripheral
Interchange Program, DDT, and perhaps other programs, as they become available. The system described here includes all features of the monitor and allows the user full access to the disc as a program
storage medium.
The System Monitor is designed to facilitate user/system communication.

By typing appro-

priate commands to the monitor the user may call the loader from the systems device, save a core image
on the systems device, or call a saved file into core from the systems device.

PRELIMINARY REQUIREMENTS
PDP-8 (4K)
3 cycle Data Break
DF32 Disc (or TC01 DEC tape)

SYSTEM GENERATION

3.1

Building the Monitor
To load a monitor system on an empty disc or DECtape, perform the following sequence of

operations.
a.

Load RIM Loader

Load Binary Loader

Load BUILD using the Binary Loader

Disable write protect on the desired device; DISC or DTA8

Start at 2008

Answer the questions (underlined) asked by the BUILD Program, as shown below,

to fit your hardware system.
*TYPE SIZE OF CORE (IN K) •
*4~

*HIGH SPEED PAPER TAPE? ~
*YES ~

DEC-08-SBAB-D
~DISC OR TAPE? ..
!DISC ~
!TYPE NUMBER OF DISC UNITS •
!1.
(MAXIMUM 1 MINUTE DELAY)

!.
~.

You will get either:
1• ~
2.

meaning the monitor and loader are correct or
WRITE ERROR.

If in a disc environment, you get the latter; start over at step a (you may have

hardware problems). If you have DECtape, try a new DECtape at step d or try writing timing and
mark tracks on the DECtape and start over. The builder will ask other questions as necessary allowing
you to tailor your system to the hardware.
3.2

Building the System Library
a.

Read the LOADER description (section 4.1) and CALL/SAVE description (section 4.2).

Type in appropriate commands as indicated in Loader description. By using the

starting address of 76008 (IST=7600"), control is returned to the monitor after load and each program
may be saved. An example of loader use follows •
• LOAD.
*IN-~: j

*OPT -2
*ST= 7600_

tttt

READ ONE TAPE FROM READER
LOADER FOUND DEVICE R: IS VALID
LOAD IN 2 PASS MODE (PROGRAM REQUIRES
USE OF CORE ABOVE 70008)
JUMP TO 7600 AFTER LOAD IS FINISHED

II ~~PE

< CONTROL> P TO JUMP TO START ADDRESS.
LOAD TAPE FOR PASS 2, TYPE < CONTROL> P•
. "'. NO OF PASS 1 TYPE < CONTROL> P
LOADER IS WAITING FOR YOU TO LOAD A TAPE AND
TYPE < CONTROL> P

Save the core image as described under SAVE/CALL in this document. Note the

exclamation point (!) after the name. This means system program as opposed to the normal colon (:) of
a user program save. (See Append ix A).
d.

Repeat process from step b until you have stored all the desired systems programs.

DEC-08-SBAB-D

OPERATION OF THE DISC SYSTEM

4.1

Loader

4.1 .1

General - The loader is a disappearing program allowing paper tape or disc input. It loads

in one or two passes and makes all of core available to the user, except the page reserved for the
monitor head (7600-7777). The user specifies a command list to the loader and the entry point. After
a successful load the loader disappears and enters at the specified starting address.
4.1.2

Operations - From the monitor, type LOAD and wait for *IN - . The loader uses either

high or low speed paper tape as specified at build time by the answer to HIGH SPEED PAPER TAPE?
If you specified high speed paper tape either T: or R: is forced to the high speed reader; if you said
no, both devices are forced to low speed. This applies only to the loader.
The loader will load up to five separate tapes. This is done by typing the appropriate
command string in response to *IN - . For example,
"*IN -

R:,R:,R: ~ "

causes the loader to expect three separate tapes to be loaded. If the two pass option is specified,
each of the three tapes must be read in twice. The loader acknowledges each valid input with a
"<cr><LF>t" .
The next typeout is "*OPT - " which expects a I-character answer 1 or 2. This argument
is the number of passes required. The one pass loader will not load programs in field 0 above 7000.
The loader disturbs core from 6000 to 7577. The two pass loader wi II load programs in all of field 0
except the cells described below.
Core Size

Cells Excepted

7600-7777

8-32K

7574-7777

The two pass loader requires the input tapes be passed through the reader twice. In either
case higher fields will be loaded. In the command string, each field, delimited by commas, contains
device of one binary program and this command string will be identical for one pass or two pass load
of the same program.
The loader asks "*ST=" and expects a one to five digit answer describing, in octal, the
starting address. The highest digit of a five digit number describes the memory extension involved; a
four digit or less string implies field O. For example,
"ST= 30200~ "
means jump to field 3, address 200.

DEC-08-SBAB-D
The loader then types" t" and waits for a tape to be inserted in the reader. When the tape is
positioned, type <CONTROL>P. The tape is read-in and the loader again types" t " and waits. The low
speed reader version of the loader halts; press Continue to resume the requested operation. Continue the
above operation until you have matched the number of tapes loaded with the input request. The typing of
one more <CONTROL>P causes the loader to enter at the specified starting address.

Note that two

<CONTROL>P's are required at the end of pass 1 of a two pass load. In two pass mode, tapes are loaded
in the following order:

Tape 1, Tape 2, Tape 3 •.. , then Tape 1, Tape 2, Tape 3 ....

4.1.3

Errors - An illegal checksum causes the loader to type "?" and return to the monitor.

4.2

CALL/SAVE

4.2.1

SAVE - Core may be saved on the disc in blocks of 128 10 words.

From 1 to 31 (decimal)

blocks may be placed on the disc and the blocks may be contiguous or non-contiguous as specified by
the user.
The syntax is as follows:
.SAVEuXXXX{ ~ }yyyyy

-zzzzz, WWWWW;UUUUU ~

ENTRY POINT

ONE PAGE SAVE

'-----MULTIPLE PAGE SAVE
: USER PROGRAM
SYSTEM PROGRAM

'-----PROGRAM NAME
Any combination of multiple page SAVE and/or single page SAVE requests may be requested,
up to 32 (decimal) requests. Beginning address of a multiple page request must be smaller than the
ending address. All addresses must be in the same field. All addresses are in octal and are one through
five digits where the most significant digit of five is the memory extension. For example,
• SAVE

CORE:

0,400-6177; 20 ~

saves a core image called CORE, as page 0, and all cells from 400 through 6177 with an entry point
of 20. Extended memory may be saved through the use of a five digit number. This most significant
digit describes the field to be accessed and no check is made for validity in your system. If you save
in field 7 you must have memory to match the request. The last non-zero extended field digit entered
is remembered. The following three examples perform in exactly the same way.
. SAVE
. SAVE
. SAVE

Faa: 10000 - 17777; 10200
FOO:30000 - 67777; 10200
FOO: 10000 - 7777; 200

DEC-08-SBAB-D

Only one field may be saved at a time, and an entry point of zero causes a return to monitor
at call time.
4.2.2

CALL - Core may be loaded from the disc by name in the following ways:
.CALL XXXX ~
or

.XXXX

The former is used if a user program is desired. The latter is used if a system program is desired (see " ~ " and" :" in SAVE). At the successful completion of the "call", control is transferred to
the entry point specified at save time. Only the core specified at save time is disturbed.
5.

DETAILS OF OPERATION AND STORAGE

5.1

Core Configuration
The system resides in the top page of field O. The entry point to the system I/O routine is

7642. The loader, SAVE and CALL can access extended memory. Two modes are used in the system;
monitor mode, and user mode. Monitor mode is present when tC is typed at system program time or
when the monitor is started. The"." means monitor mode; the "*,, means user mode. (See Figure 1).
Monitor Time

User Time

System 10 +
Monitor Head

Monitor
Body

!
USER

,1"

1
Figure 1

Core Storage Configuration

OEC-08-SBAB-O

The operation of the system after it is bui It and stored on the disc is described below.
Appendix B shows the bootstrap which must be toggled in to start the system, if the machine core is
c Ieared or destroyed.
At monitor and SP time the keyboard listens to a tC and returns to the head of the monitor
(top page of field 0). The system is completely open ended, allowing flexibility in system content at
the user installation. The system is built easily on the user's machine from a tutorial system builder
program.
Monitor start is at location 7600 of field O. At monitor start the code from 7200 to 7577 is
saved in the first two scratch blocks on the systems device, and the monitor (block 1 and 2) is read into
these cells. Control is transferred to the monitor which types a carriage return, line feed and a".".
A "." at the start of any line informs the user that he is communicating with the monitor. A <CONTROL>
C(tC), when typed to either the monitor or loader causes an immediate transfer of control to 7600. A
rubout typed to the monitor causes a monitor restart without destroying the contents of the scratch blocks
on the systems device.
Core images may be saved by typing: SAVE NAME: n 1 -"2' n3 ; n4 }, where NAME is any
one to four character program name. The dash between n 1 and n 2 indicates a contiguous save from the
page containing address n 1 through the page containing n2 . Address n3 is comma delimited and indicates a single page save. The entry point n4 is delineated by a semicolon.
A list showing the required pages is constructed from the information typed by the operator
and a block requirement count is kept. When the carriage return is typed (allowing the save to begin)
a directory name (ON) search is initiated. If a file of the name specified is found, it is replaced by
the present file; if not, a new file is created. A storage availability search finds sufficient available
blocks to satisfy the requirement count generated as the page list was built. Their numbers are stored
on a corresponding block list. These blocks are then filled with the contents of the pages to be saved
unti I the typed request has been satisfied. When the save is completed control returns to the start of
the monitor (7600).
A save file may be recovered by typing "CALL NAME ~". The file is loaded back into core
and control is transferred to the entry point specified during the save. If the file was designated a
systems program at save time it will be recovered by typing only NAME ~. When a program is called,
a ON search is initiated to find the name specified. Associated with the name is the entry point, and
file protection and extension information. If the ON entry is found to be a save file of the proper mode
(systems or non-systems), ca II ing proceeds. If not, the call is term inated and control is returned to the
monitor. When the call is completed (i.e., all of the saved code is back in core) control is transferred
to the entry point specified at save time. The time for a program to be saved or called is less then the
time to type the request.

DEC -08- SBAB- D

5.2

Mass Storage Configuration

5.2.1

File Structure - A relatively sophisticated file structure is implemented to allow for automatic

retrieval of storage for use by the system and a requirement for a single DECtape system.

(See Figure 2.)

Two types of blocks are present - directory name (DN), and storage allocation map (SAM) blocks. The
former are described in Figure 3, the latter in Figure 4. All blocks are linked with the end of chain
line - O. The top of the SAM block chain is addressed in all DN blocks. The SAM block is the item
wh ich allows automatic storage retrieval.

Each DN entry has a SAM number between 1-778 , A zero

means unused. A SAM block has a record of 256 10 pages of mass storage (Figure 4). The first page of
any numbered program is found by searching SAM blocks until an entry with matching number is found.
To add to a file, find a SAM block entry equal to 0, insert the correct SAM number and adjust the
storage block linking. To delete a block, the DN entry is cleared and the SAM block entries (equal to
the DN number) are zeroed out.

DIRECTORY (BAC K-UP)
MONITOR
LOADER
FORTRAN
EDITOR
DDT
OTS
PIP
ASSEMBLER
16K
DIRECTORY (NORMAL)

USER

SCRATCH

Figure 2

256

Mass Storage Configuration

DEC-08-SBAB-D

0
0

FIRST SAM BLOCK = # = NUMBER

PROG. TYPE
00 Source
01 Binary
10 Fort. Binary
11 Save

START

ADDRESS

ENTRY

POINT

Extended Memory Bits

+-4 CHARACTER NAME (SIX BIT)

NAME BLOCK ENTRY

I I I'T'I
25 ENTRYS/BLOC K

1 = System Program

1.0

LINK

Figure 3

Directory Name Block

DEC-08-SBAB-D

SAM BLOCK LAYOUT

SAM BLOCK IMAGE

Block 200
Block 201
Block 202
Block 203

I
I

Block 0
Block 1
Block 2
Block 3
Block 4

I
I
I
I

I
I

I
I
I

I
I
I
I

I
Block 377

,I"

Block 177

LINK

Link to SAM Block 2 wI
Block 400 - 577

Figure 4

Storage Allocation Map

DEC-08-SBAB-D

5.2.2

Disc Configuration - The Disc configuration is shown in Figure 5B. The scratch area for

system overlay is located in the last two pages of disc storage. Additional ON and SAM blocks are
added, one each per 32K of disc.
5.2.3

DECtape Configuration - The DECtape configuration (Figure 5A) is logically equivalent to

the disc system but has the SAM and ON blocks ordered differently for efficiency.
BLOCK
HEAD OF MONITOR

HEAD OF MONITOR

MONITOR

SAM

SCRATCH

SYSTEMS

SCRATCH

SAM

16K

SYSTEMS
ON
SAM

USER

SCRATCH
SCRATCH
LAST

SCRATCH

DEC TAPE

DISC

(A)

(B)

Figure 5 Mass Storage Configuration

1 per 32K
additional disc

DEC-08-SBAB-D

5.3

Error Messages
a.

The monitor protects the user from himself to the extent that it recognizes incorrect

syntax in input command strings. Any errors discovered are remembered and at the time the user types
a carriage return the monitor signals them with a "?".
b.

The command decoder notifies the user of the following errors, by typing the key

letter shown opposite the offending condition.
No such inputs
Illegal input characters or
miscellaneous error
condition
Directory full
System I/O fa i lure
Excessive inputs or outputs

?
D
S
E

The local errors are described in the write-ups for each system program.

Monitor time read/write errors cause a halt to occur. Persistence of this condition

means a hardware failure, as the system I/O routine tries to read and write three times before it gives
up.
6

SPECIAL REQUIREMENTS INSTRUCTIONS OR FORMATS

6.1

Command Strings
The command string structure for the PDP-8 Disc System software enables the user to easily

communicate with the system. The punctuation of the input is as described below.
= mon itor mode

= system program mode
= more devices in list
= separator between device and file name

tC = <CONTROL> C Request to return to monitor
~ = carriage return

tP
Rubout
6.2

= <CONTROL> P.

Proceed with next operation.

= <RO> Go back to start of input string.

Device Names
DX
S
R
T

- DECtape if disc and DECtape are on system [ X= unit number 0-7] *
- System (disc or DECtape)
- High-speed paper tape
- Teletype (low-speed paper tape)

* Not implemented
11

DEC -08- SBAB- 0

The command decoder used by all system programs asks one to three questions:
a.

OUTPUT in the form II *OUT _" .

INPUT in the form n*IN-".

OPTIONS or SWITCHES in the form "*OPT _".

Each system program may chose to use any or all of these options when it enters the command
decoder. The user may type in one output device, one option (any character, which is transmitted in
ASCII to the using program) and a string of one or more input devices.
Each system program using the command decoder must tell it the type of file being dealt
with, i.e., Binary, Fort. Bin., or ASCII.
Some examples of command strings are shown below.
*IN - S:SAM ~
*IN -

S:F002,R: ~

*OUT - D7:BAR ~
*OPT - M ~

*TYPE SIZE OF CORE (IN K)
*4
*HIGH SPEED PAPER TAPE?
*YES
*DISC OR TAPE?
*DISC
*TYPE NUMBER OF DISC UNITS
*1

*
• LOAD
*IN-R:

Typical System Builder Output

*OPT-2
ST= 7600

Figure 6

tttt
• SAVE FORT! 0-7577;4511
• LOAD
*IN-R:
t
*OPT-1
ST= 7600

tt
• SAVE PAL! 0-3400;241
• LOAD
*IN-R:

t
*OPT -2
ST= 7600
tttt
• SAVE DDT! 5000-7577;5400
12

DEC-08-SBAB-D

APPENDIX A
SAVE SYNTAX FOR STANDARD PDP-8 SYSTEM PROGRAMS

• SAVE FORT! 0 - 7577; 4511
• SAVE PAL!

0 - 3400; 241

• SAVE DDT~

5000 - 7577; 5400

• SAVE LODT! 1000 - 1577; 1000
• SAVE HODT! 7000 - 7577; 7000
. SAVE MAC8~ 0 - 7577; 2041*
• SAVE FOTS! 0 - 6200; 200

* LOAD 200, SET SWITCHES + RUN.

DEC -08- SBAB- D

APPENDIX B
BOOTSTRAP lOADERS
A.

DISC BOOTSTRAP
0200
0201
0202
0203
0204
0205
0206
0207
0210
0211
0212
0213
0214
0215

/7600
/6615
/1215
/3613
/1212
/3614
/6603
/6622
/5207
/5600
/7576
/7750
/7751
/7577

/ClA
/DEAl
/TAD 215
/DCA I 213
/TAD 212
/DCA I 214
/DMAR
/DFSC
/JMP 207
/JMP I 200

/WC
/CA

DECT APE BOOTSTRAP
(See TCOI System Bootstrap)

DEC-08-SBAB-D
APPENDIX C
CALLING SEQUENCE (SYSTEM ROUTINE)
A

CALLING SEQUENCE
JMS I SYSIO

/ CELL" SYSIO" POINTS TO I/O

FUNCT

/ SEE PARA. II

BLOCK

/ BLOCK TO BE ACCESSED

CORE

/ LOW ORDER CORE ADDRESS

LINK

/ FILLED BY WRITE, USED BY READ

ERROR

/ ERROR RETURNS HERE
/ NORMAL RETURN HERE

FUNCTION CELL
BITS 0-1

UNUSED

BITS 2

=0, NORMAL; = 1, RETURN AT END OF
READ/WRITE TO (ADDRESS + 1 IN
ERROR RETURN.

BITS 3-5

UNIT NO. (IF DECTAPE)

BITS 6-8

MEMORY FIE LD

BITS 9-11

FUNCTION; READ = 3, WRITE = 5.