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PDP-1 COMPUTER 
ELECTRICAL ENGINEERING DEPARTMENT 
M« I» To 
CAMBRIDGE, MASSACHOSEI^S 02139 



PDP-29-1 
INTRODUCTION TO PDP-1 TS SYSTEM PROGRAMS 



November 29, 1965 



INTROOJCTION TO THE P33P-1 TIME SHARING SYSTEM 

This memo was written to give the new user a brief 
introduction to the PDP-1 time sharing system programs a The 
material presented is sufficient information needed to operate 
in the TS system; however, only the essential features of 
the system programs are described- Other useful and time 
saving features are described in the respective program 
raemOc 

The computer has three 4096-word core memories, one is 
used for executive control of the system and the other two are 
available to the users. In addition to these three 4096-word 
core memory, an extra amount of memory in the form of a 
magnetic drum is available* The storage space on the drum 
is broken up into a number of "fields", each of which has 
the capacity to store 4096 18-bit words. This drum memory 
(besides being used for the time-sharing operation) can be 
used to store programs and data when there is not enough 
room in core* It is also used as a permanent storage space 
for "utility" programs which can be used by you* 



*£>' 



To execute a program that you have written for the PDP-i* 
you must do the following things: 

1) Tell the computer, in some way* the sequence of 
machine-language instructions that make up your program, 

2) Have the computer translate or assemble these 
instructions into a sequence of binary words, 

3) Place this binary program into core memory and 
have it executed, and 

4) check you answers to see if your program is working 
correctly* 

The permanently stored utility programs will be of help 
to you in accomplishing the above tasks. One of the programs, 
EXPENSIVE TYPEWRITER, assists you in reading in your machine- 
language program* The POSSIBLE program will assemble your 
machine -language program, producing a binary program* DDT 
is an all-purpose program that will see to it that your 
program is executed. It also allows you to inspect and 
test your binary program for proper operation . 

Each of the three utility programs mentioned above is 
stored (in binary form) on a different field of the magnetic 
drum. Of course, in order to use a program you must first 
place it in core memory . This is done by typing the commands 
shown in Figure 1* 



expensive 
typewriter 



Am* RT. 



N 




TORN CONSOLE SWITCH OFF' 
•TURN CONSOLE SWITCH ON 
TYPE ddt 



ILLEGAL INSTRUCTION 
(HUT) or CALL 
nBUTTON PRESSED or 
BREAK POINT 



POSSIBLE 



Figure !• 
For example , turning the console switch off, then turning it 

on, then typing ddt tJ will bring the program DDT from drum 

to core. Anything typed by you at this point will be 

interpreted as a command to DDT. Typing ( SjLff then OG will 

cause EXPENSIVE TYPESETTER to be brought into core* Anything 

typed at this point will be interpreted as a command to 

EXPENSIVE TYPEWRITER* 

The features of each of the programs will now be described, 



affhe letters typed by you will be underlined in this paper. 
Those tyj>ed by the computer will not be underlined. The 
symbol J is a carriage return. 






>MSXVB TYPEWRITES.;. 

This program is used for the reading in or typing in of 
your machine language program. It is also valuable in modifying 
your machine language program — with it you can do such things 
as changing lines, inserting lines, deleting lines* etc. This 
program has a buffer storage area where a copy of your machine 
language program is kept. When you enter EXPENSIVE TYPEWBITER 
by typing 6£H then OG to DET, this buffer area Is cleared . 
Entering E.T. by typing 6lU then 2£ will not clear this buffer 

E.To has two operating "modes", control mode and text mode. 
In the control mode, characters typed in are commands to 
EXPENSIVE TYPEWRITER,* In the text mode, typed input Is copied 
directly Into the buffer. The only exception to this is the 
character "backspace" which in the text mode has the effect 
of cancelling the previous character. To go from text mode 
to control mode, type a backspace immediately following a 
carriage return or after deleting all characters from a line. 

Certain commands in the control mode put E.T. Into the 
text mode. In the control mode, the typewriter prints 
In red, and In the text mode it prints in black* A lower 
case command is terminated by typing In a carriage return 
or tab I at any time before the terminator is typed the 
command can be cancelled by typing the character middle 
dot (•). 



Commands to EXPENSIVE TYPEWBITER: 

In the examples below, the letter K is used to signify 
the number of a line. (A carriage return denotes the end of 
a line.) For example, the title line of your program will 
be line one, etc. Slash {/) or period {.) may be used for 
the N to signify the last or current line number, respectively* 
I . Input : 

A: append. Enter the text mode; add the following 
typed text onto the end of the previous contents 
of the buffer; the buffer already has a stop 
code at the end. 
Mi: insert. Enter the text mode; insert the 

following typed text immediately before line N. 
That is, the first line of the inserted text 
becomes the new line N. The text is squeezed 
in between the old lines N-l and N; no material 
is lost. 
No: change. Enter the text mode. Line N is entirely 
deleted and the following typed text is Inserted 
in its place; any number of lines may be inserted, 
r: read. The tape in the reader is read in and 
appended to the end of the buffer. 
II, Deletion of Information in the Buffer: 
Nd: delete. Line N is deleted. 
K: kill. The entire buffer is cleared. 



III. Printout; 

#: write * The buffer is printed out (in black)* 
$&: line* Line N is printed out (in red)* 
backspaces prints out the next line 

x : prints out the previous line • 
IV « Punching Tape 

P: punch. This punches the buffer out on paper tape 
Vc Transfer 

N: transfer control to POSSIBIE 
b: transfer control to W2 
Vic Search 

I "": limited- 

s "": unlimited , Search the current page (limited) 
or the entire buffer (unlimited) for the first 
occurrence of the string at the beginning of 
the line. If none is found, ET types "missing" . 
Otherwise the search command is replaced by the 
line number where the string was found. 

Typical Ope n r^ting„Jroceduret 

Probably the first time you use the computer you will 
enter E*T* by typing 6JJJ then 0g> Your program buffer will 
be clear. If you have prepared a FIO-BEC program using the 
off-line f lexowriter, you may read it into the EXPENSIVE 
TYPEWRITER text buffer by typing r. In reading the tape in, 
first, insert your tape with the 5-holed side toward you. 
The input side is the right and the tape feeds from right to 
left. 



-7- 

Turn the reader switch on at the main console by lifting it 
up. Typing r will read the tape into the buffer. After the 
tape is read in, turn the reader switch off by pressing it 
down and remove the tape from the reader. If a tape of your 
program has not been made, typing A, then your program, will 
put your program into the buffer. After correcting any typing 
mistakes using the delete, change, etc. commands, you will be 
ready to enter the POSSIBLE program where your program will 
be assembled. To enter POSSIBLE type a capital N. 

When you have a program in E.T.'s buffer, you can get 
a punched copy by typing the command P. Your tape will 
automatically be punched. After punching, press the tape-feed 
button on the main console to give you a couple of extra 
fanfolds of tape, and tear off your tape. This tape can 
now be used on another attempt to run your program. In this 
way it is not necessary to retype your entire program every 
time you run. You should also get a typewritten copy of the 
machine language program by typing W. Lifting Sense Switch 1 
while the buffer is being typed out by a W will stop the 
typing. 

The S.T. has one all-purpose error signal - ?. If a 
? is typed by E.T. after you have typed something, examine 
what you have done carefully. 



43 



important Reminders: 

1) The meaning of the backspace key to EXPENSIVE TYPEWRITER 
differs depending upon the mode of operation • If E.T. is in 

the control mode, the backspace key means? print out the 
next line (if no previous line has been mentioned, this will 
print out the first line). If E.T. is in the text mode , the 
backspace key has two meanings. If a letter of text has 
been typed by you a backspace erases it. If a backspace 
follows a carriage return typed by you, it means: return 
to the control mode. Do not attempt to type a command 
to EXPENSIVE TYPEWRITER unless it is in the control mode. 

2) After you type in your program and leave EXPENSIVE 
TYPEWRITER, you will probably want to return later to make 
further changes to your program* When entering E*T- from 
DDT the second time, be sure to enter by typing 6lU then gg* 
A Qfi will erase your program entirely. 

3) Remember every command to EXPENSIVE TYPEWRITER must 
be followed by a carriage return before it is executed, 

4) It is a good idea to print out the line you wish 
to modify using the command §1 before and after the change 
Is madee This way you can be sure that you are changing 
the right line and that the change was made correctly. 

5) After making any changes to your machine -language 
program with EXPENSIVE TYPEWRITER you must always reassemble 
your program with POSSIBLE if you want to get a new binary 
program. Therefore, you should leave EXPENSIVE TYPEWRITER 
using the N command to POSSIBIE rather than the b command to 
DDT. 



PQMPffit 

The POSSIBLE program when entered by N will assemble the 

machine-language program which is located in the buffer of 
EXPENSIVE TYPEWRITER* The assembled binary program is placed 
automatically on drum field 1, unless specified that a tape 
is desired* 

Commands to,pos$ffiLg: 
§,: begin assembly 
g,* continue assembly 
& punch binary symbols 
&: transfer to DHP 
k: constants area 

Operating Procedure: 

When you enter POSSIBLE with your machine-language program 
stored in the buffer of EXPENSIVE TYPEWRITER, typing a §, will begin 
pass 1 of the assembly* If an error is discovered during 
assembly an error signal will be printed out in the format 
described below and the assembly process will cease. 

If an error Is found on pass 1, first type c to 
continue pass 1. Then return to EXPENSIVE TYPEWRITER by 
typing J> then 6lU then 2g, in order to correct your machine 
language program . After the corrections have been made, 
return to POSSIBLE with N and try pass 1 again* If pass 1 
assembles correctly, no error message will be printed out* 
In this case, type s to initiate pass 2. Error In assembly 
during pass 2 result exactly as in pass 1 and you should 

proceed as described above- 



ir-seeivs a paper tape punched with your table of symbols by 
typing i;.* Also by typing k the constants storage area will 
be printed out. After your program is assembled by POSSIBLE, 
you will want to return to V>U£ to run it and find errors if 
they exist. Typing b will cause you to leave POSSIBLE and 
return to DHT* 

Upon detecting an error, POSSIBLE will print out the following; 

aaa bbbb ceo dddd eee 

aaa is the three letter code indicating the error, bbbb is 

the octal address at which the error occurred, ccc is the 

symbolic address at which the error occurred, dddd Is the 

name of the last pseudo-instruction encountered. In the 

case of an error caused by a symbol, eee will be the symbol. 

Following is e list of the error indications in POSSIBLE. 

ERROR MEANING 

nca NO CONSTANTS AREA 

The pseudo-op constants Is needed „ 

llf ILLEGAL FORMAT 

lit ILLEGAL TAG 

Tag which is not a single symbol is 
not equal to current location* 
ex,, foo+10, 4 current location 

indt- MULTIPLY DEFINED TAG 

Tag consisting of a single defined synlol 
is not equal to current location. 
Symbol is not redefined. 

usw UNDEFINED SYMBOL 

A symbol which has not been defined in progrss 
is encountered* Symbol is given the value of 



ERROR MEANING 

eld CONSTANTS LOCATION DIFFERENT 

^ e constant pseudo-op appears in different 
location on pass 2* No recovery can be made* 

vld VARIABLES LOCATION DIFFERENT 

Same as eld but for variables* Often 
possible to recover by ignoring this. 

ild ILLEGAL DEFINITION 

Program attempts to redefine pseudo-op 
or previously defined symbol. Redefines 
pseudo-op or symbol if assembly continued « 

see STORAGE CAPACITY EXCEEDED 

Storage of macro definitions, macro arguments * 
repeat ranges, numerical constants (pass X), 
unique constants (pass 2), symbols, or macro 
names has heen filled. No recovery can be made 

pee PUSH DOWN CAPACITY EXCEEDED 

Macro, repeat, or constant nesting Is 
too deep or too complicated arithmetic 
statements are used* No recovery can be made* 

tmc TOO MANY CONSTANTS AND VARIABLES PSEUDO-OPS 

Total number of constants and variables 
pseudo-ops is 20$. No recovery can be madeo 

mdd MUIITIPLY DEFINED DIMENSION 

Symbol representing first location In 
dimension of array is already defined . The 
old symbol definition is retained if 
assembly is continued. 



-12- 



tint TOO MANY TERMINATE PSEUDO-OPS 

There exist more termi nate Instructions 
than define instructions. The terminate 
is ignored if assembly Is continued. 

ids ILLEGAL DIMENSION SIZE 

Specified dimension size is negative . 
Dimension size Is set to zero if assembly 
continued * 

ich ILLEGAL CHARACTER 

Input source has an illegal f lexo code 
or character. Number typed Is the 
illegal character; if the number is in 
the 400»s It is an upper case character. 
Continuing assembly will ignore the 
character. 

DDT: 

This program is a very valuable aid in debugging and testing 
your binary programo It allocs you to examine the contents 
of locations of core* to change the contents of these locations, 
to run your program, and to make use of breakpoints. When 
in DDT, all typing will be performed In black* After a command 
Is typed by you, DDT will type a carriage return to indicate 
that the command was carried out. Therefore, do not type a 
carriage return after a command as you would do in EXPENSIVE 
TYPEWRITER. 



-13- 

Commands to DIfl! 

Control 

2T Typed immediately after returning from POSSIBLE 
assembly. This places your symbols and their 
values into the DDT symbol table. This allows you 
to use symbolic addresses when communicating 
with DDT. 

NUi where N is a number. This takes the program from 
drum field N and places it in core memory so that 
it can be executed. Your binary program has been 
placed on drum field 1. EXPENSIVE TYPEWRITER is on 
absolute drum field 21. MACRO is on absolute drum 
field 22. (Absolute drum fields are Indicated by 
the sign bit being onj thus, absolute field 21 is 
obtained by typing 6lU . ) 
locG s This will cause the computer to begin executing 

the program which is in core starting at location 
loc. loc may be either a symbolic expression or 
an absolute number. 
For an example of the above commands: 

J2 
begG 

will enter your symbol table from POSSIBLE into DDT ? s 
symbol table from POSSIBLE, bring your program from 
drum field 1 into core, and begin executing your 
program at location beg. 



"A* 



Inspecting; and Modifying : 
Examine a Register 

adr/s Types out the contents of register adr c 
"adr" may be a symbolic address or an 
octal number* 

Examples: beg*2/ types out the contents 
of the register 2 positions 
past beg. 55/ types out 
the contents of register 55 • 

Change a Register 

adr/ §23L. : Changes the contents of register 

adr to exp. 

Example: \£, * lifcL examines 

the contents of the register k and 
finds it contains the number 4. This 
is changed to 13 .' 

Set the Mode 

g Sets DDT to type out all words as 

symbolic instructions., 
C Sets DDT to type out all words as 

octal number o 
Examples : §, 

§0/ and k 

C 

20/ 020003 (if k = 3) 









Converts all numeric printouts to decimal 
Resets DGT so that all numeric printouts 
are in the normal octal mode again* 
Examples * C 



20/ 

H 
20/ 



020003 



8195 



020003 



Convert an Expression 

exp « Types out the value of the expression exp 

as an octal number, 
num-** Types out the octal number num as an 

instruction • 
char " Types out the character (s) char as 

concise code. 



Examples : 20/ 



20003 ». and k 

(if k « 3) 



beg = 20 (if beg is the 

symbol for location 20) 
a4Q~ 610420 



-16- 

Examining Sequences of Register 

adr/ ... 1 Opens the register preceding adr and 

types its contents* 

Example; tab/ 13 1 

tab-1/ 21 i 

tab-2/ 

Register tab contained 13, tab-1 

contained 21, and tab-2 contained 0„ 

adr/ . , . (bs ) Opens the contents of the register next 

in sequence to adr and types out. 

(bs) means back- 
space Example: tab/ 13 (bs) 

tab+1 6i 

Register tab+1 contains 61 

adr/ ♦.. (tab) Types out the contents of the 

register addresses by the contents 

of register adr. 

Example: beg/ lac 13 tab 44 

Register 13 contains 44 



-IT- 

It is tempting to load a newly coded program into the 
computer, let it run to completion, and check the final answer 
for correctness. Because of the high probability of one or 
more errors in even short programs and the cumulative effect 
of these errors, this approach is practically worthless 
(except for very 3hort programs )* 

A more satisfactory approach is to run only a small portion 
of the program at a time and check intermediate results. 
This allows one to catch errors before they have a chance to 
make the whole operation unintelligible. The breakpoint 
feature of DIO? permits one to take this approach and check 
out one section of the program at a time. 

A breakpoint is a place in the user's program where 
computation is interrupted and control is transferred to DDT, 
This is accomplished by removing and saving the instruction 
at the breakpoint and inserting in its place a Jump instruction 

to 32DT. 

When the program reaches this Jump, the status of the 
machine (AC, 10, overflow indicator, etc.) is saved and the 
breakpoint location and contents of that lotion are typed out. 



,,A, 



At this point one may examine the intermediate results 
by use of any of the inspection and modification commands 
listed above * If they are satisfactory, one may proceed 
with the next section of the program. If they are in error, 
one may try to correct the errors and re-run the section. 
The DDT breakpoint commands are as follows: 

locB Prepares DDT to insert a breakpoint at 

location loc. The actual insertion occurs 
when a G or X command is given. 
&~ Removes the previous breakpoints. 

P Given after a program has been interrupted 

by a breakpoint* The instruction removed for 
the breakpoint is executed and control is 
returned to the user's program. 
Causes instruction ins to be executed. 



insX 
Examples i 



lupB 
begg 
lup) 

feZ 
P 



add sym 
4 



This example places a breakpoint at location lup and 
transfers control to beg. The content of register lup is 
add sym and location k contains 4 at the break. The breakpoint 
is removed and the program is allowed to proceed. 

dam conX deposits +0 into register con