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PDP-8 Frequently Asked Questions (posted every other month)

Answers to common questions about antique DEC PDP-8 computers. Those posting to alt.sys.pdp8 should read this.
Archive-name: dec-faq/pdp8
Last-modified: Apr 8, 2001

Frequently Asked Questions about the DEC PDP-8 computer.

	By Douglas Jones,
	(with help from many folks)

Reasonably current versions of this file is available by anonymous FTP from:

Reasonably current automatic translations of this document to HTML format
for the World Wide Web are available from:

An obsolete version of this file is available on the Walnut Creek USENET
FAQ CDROM; another version will be published as part of the FAQbook by
Pamela Greene et al.

This posting conforms to RFC1153 USENET digest format (with exceptions due
to the fact that it is not really a digest).


	What is a PDP?
	What is a PDP-8?
	What is the PDP-8 instruction set?
	What does PDP-8 assembly language look like?
	What character sets does the PDP-8 support?
	What different PDP-8 models were made?
	What about the LINC-8 and PDP-12?
	Where can I get a PDP-8 today?
	Where can I get PDP-8 documentation?
	What operating systems were written for the PDP-8?
	What programming languages were supported on the PDP-8?
	Where can I get PDP-8 software?
	Where can I get additional information?
	What use is a PDP-8 today?
	Who's Who?


Subject: What is a PDP?

In 1957, Ken Olson and Harlan Anderson founded Digital Equipment
Corporation (DEC), capitalized at $100,000, and 70% owned by American
Research and Development Corporation.  Olson and Anderson had designed
major parts of the AN/FSQ-7, the TX-0 and the TX-2 computers at
Lincoln Labs.  They wanted to call their company Digital Computer
Corporation, but the venture capitalists insisted that they avoid the
term Computer and hold off on building computers.

With facilities in an old woolen mill in Maynard Massachusetts, DEC's
first product was a line of transistorized digital "systems modules"
based on the modules used in building TX-2 at Lincoln Labs; these
were plug-in circuit boards with a few logic gates per board.  Starting
in 1960, DEC finally began to sell computers (the formal acceptance of
the first PDP-1 by BBN is reported in Computers and Automation, April
1961, page 8B).  Soon after this, there were enough users that DECUS,
the Digital Equipment Computer User's Society was founded.

DEC's first computer, the PDP-1, sold for only $120,000 at a time when
other computers sold for over $1,000,000.  (A good photo of a PDP-1 is
printed in Computers and Automation, Dec. 1961, page 27).  DEC quoted
prices as low as $85,000 for minimal models.  The venture capitalist's
insistance on avoiding the term computer was based on the stereotype
that computers were big and expensive, needing a computer center and a
large staff; by using the term Programmable Data Processor, or PDP, DEC
avoided this stereotype.  For over a decade, all digital computers sold
by DEC were called PDPs.  (In early DEC documentation, the plural form
"PDPs" is used as a generic term for all DEC computers.)

In the early 1960's, DEC was the only manufacturer of large computers
without a leasing plan.  IBM, Burroughs, CDC and other computer
manufacturers leased most of their machines, and many machines were
never offered for outright sale.  DEC's cash sales approach led to the
growth of third party computer leasing companies such as DELOS, a
spinoff of BB&N.

DEC built a number of different computers under the PDP label, with a
huge range of price and performance.  The largest of these are fully
worthy of large computer centers with big support staffs.  Some early
DEC computers were not really built by DEC.  With the PDP-3 and LINC,
for example, customers built the machines using DEC parts and
facilities.  Here is the list of PDP computers:

    =====  ====  ======== ==== ====== ========
    PDP-1  1960  $120,000 18       50 DEC's first computer
    PDP-2            NA   24        - Never built?  Prototype only?
    PDP-3            NA   36          One built by a customer*, not by DEC.
    PDP-4  1962   $60,000 18       45 Predecessor of the PDP-7.
    PDP-5  1963   $27,000 12    1,000 The ancestor of the PDP-8.
    PDP-6  1964  $300,000 36       23 A big computer; 23 built, most for MIT.
    PDP-7  1965   $72,000 18      120 Widely used for real-time control.
    PDP-8  1965   $18,500 12  ~50,000 The smallest and least expensive PDP.
    PDP-9  1966   $35,000 18      445 An upgrade of the PDP-7.
    PDP-10 1967  $110,000 36   **~700 A PDP-6 followup, great for timesharing.
    PDP-11 1970   $10,800 16 >600,000 DEC's first and only 16 bit computer.
    PDP-12 1969   $27,900 12      725 A PDP-8 relative.
    PDP-13           NA             - Bad luck, there was no such machine.
    PDP-14                        *** A ROM-based programmable controller.
    PDP-15 1970   $16,500 18      790 A TTL upgrade of the PDP-9.
    PDP-16 1972      NA   8/16      ? A register-transfer module system.

*  Scientific Engineering Institute of Waltham MA.  SEI was aledgedly
     founded in 1956 by the CIA to study the effects of microwaves (radar)
     on the human brain.  If so, the PDP-3 may have been used as an
     instrumentation computer.  More info on the CIA connection and the
     use of the PDP-3 would be nice!
** Includes DECsystem 20.

Corrections and additions to this list are welcome!  The prices given
are for minimal systems in the year the machine was first introduced.
Most of the production run numbers come from "Computer Engineering" by
Bell, Mudge and McNamara, 1978, or from Computers and Automation's
computer census figures published regularly throughout the 1960's.
The bits column in the table indicates the word size.  Note that the
DEC PDP-10 became the DECSYSTEM-20 as a result of marketing
considerations, and DEC's VAX series of machines began as the Virtual
Address eXtension of the never-produced PDP-11/78.

It is worth mentioning that it is widely (but somewhat incorrectly)
accepted that the Data General Nova (see photo, Computers and
Automation, Nov. 1968, page 48) grew out of the PDP-X, a 16-bit
multi-register version of the PDP-8 designed by Edson DeCastro, Henry
Burkhardt and Dick Soggee.  (DeCastro was one of DEC's key design
engineers; his name appears on many of the blueprints for machines
from the PDP-5 up through the PDP-8/L).

A prototype PDP-X was built at DEC; this and a competing 16-bit design
were apparently submitted to Harold McFarland at Carnegie-Mellon
University for evaluation; McFarland (and perhaps Gordon Bell, who was
at C-MU at the time) evaluated the competing designs and rejected both
in favor of what we now know as the PDP-11.  (I was at Carnegie-Mellon
at the time, and McFarland gave a guest lecture in a class I attended
telling part of this story.)  Some speculate, incorrectly, that Bell
rejected the Nova design because the competing proposal used the
register-transfer notation he had introduced in "Bell and Newell,
Computer Structures -- Readings and Examples".  An alternate and equally
unfounded story is that the reason DEC never produced a PDP-13 was
because the number 13 had been assigned to what became the Nova.

In any case, when DeCastro, Burkhardt and Soggee founded Data General,
Ken Olson at DEC was very angry, claiming for a long time that the
Nova design was stolen.  Gordon Bell and others concluded that the
Nova design was sufficiently original that a lawsuit was unwarranted,
but the feud between DeCastro and Olson lasted until after Ken Olson
left DEC.  It is more correct to say that the Nova is a reaction to the
PDP-X than to say that it is based on the PDP-X.  I am indebted to
Jim Campbell, retired VP at Data General, for some of the details of
this story.

Today, all of the PDP machines are in DEC's corporate past, except the
PDP-11 family, which survives as a line of microcomputers; DEC has
promised to discontinue PDP-11 sales on Sept. 30, 1996.  Occasionally,
some lab has built a machine out of DEC hardware and called it a PDP
with a new number.  For example, the Australian Atomic Energy Commission
once upgraded a PDP-7 by adding a PDP-15 on the side; they called the
result a PDP-22.  There is also a story about the PDP-2 1/2, built by
Ed Rawson of the American Science Institute out of surplus modules that
were originally used in the prototype PDP-2.

In 1998, Compaq purchased DEC, and it is unclear how long DEC will retain
any semblance of its original identity as a division of a larger company.


Subject: What is a PDP-8?

The PDP-8 family of minicomputers were built by Digital Equipment
Corporation between 1965 and 1990, although it is worth noting that the
term minicomputer first came into prominence after the machine was
introduced.  The first use of the term appears to have been made by
the head of DEC's operations in England, John Leng.  He sent back a
sales report that started: "Here is the latest minicomputer activity
in the land of miniskirts as I drive around in my [Austin] Mini Minor."
The term quickly became part of DEC's internal jargon and spread from
there; the first computer explicitly sold as a minicomputer, though,
was made by by Interdata (See the Interdata ad in Computers and
Automation, May 1968, page 10).

The PDP-8 was largely upward compatible with the PDP-5, a machine that
was unveiled on August 11, 1963 at WESCON, and the inspiration for that
machine came from two earlier machines, the LINC and the CDC 160.  All
of these machines were characterized by a 12 bit word with little or no
hardware byte structure, typically 4K words of memory, and simple but
powerful instruction sets.

Although some people consider the CDC 160 the first minicomputer, the
PDP-8 was the definitive minicomputer.  By late 1973, the PDP-8 family
was the best selling computer in the world, and it is likely that it was
only displaced from this honor by the Apple II (which was displaced by
the IBM PC).  Most models of the PDP-8 set new records as the least
expensive computer on the market at the time of their introduction.
The PDP-8 has been described as the model-T of the computer industry
because it was the first computer to be mass produced at a cost that
just about anyone could afford.

C. Gordon Bell has said that the basic idea of the PDP-8 was not really
original with him.  He gives credit to Seymour Cray (of CDC and later
Cray) for the idea of a single-accumulator 12 bit minicomputer.  Cray's
CDC 160 family (see CACM, march 1961, photo on page 244, text on page
246) was such a machine, and in addition to the hundreds of CDC 160
systems sold as stand-alone machines, a derivative 12 bit architecture
was used for the I/O processors on Cray's first great supercomputer,
the CDC 6600.

Note that Cray's 12 bit machines had 6 basic addressing modes with
variable length instruction words and other features that were far from
the simple elegance of the PDP-8.  Despite its many modes, the CDC 160
architecture lacked the notion of current page addressing, it had no
unconditional jump instruction, and the I/O instructions all blocked
the CPU until I/O complete.  As a result, the PDP-8 is both far more
flexible and it supports much tighter programming styles.


Subject: What is the PDP-8 instruction set?

The PDP-8 word size is 12 bits, and the basic memory is 4K words.  The
minimal CPU contained the following registers:

	PC - the program counter, 12 bits.
	AC - the accumulator, 12 bits.
	L  - the link, 1 bit, commonly prefixed to AC as <L,AC>.

It is worth noting that many operations such as procedure linkage and
indexing, which are usually thought of as involving registers, are done
with memory on the PDP-8 family.

Instruction words are organized as follows:
	 _ _ _ _ _ _ _ _ _ _ _ _
	|     | | |             |
	|  op |i|z|    addr     |

	op   - the opcode.
	i    - the indirect bit (0 = direct, 1 = indirect).
	z    - the page bit (0 = page zero, 1 = current page).
	addr - the word in page.

The top 5 bits of the 12 bit program counter give the current page, and
memory addressing is also complicated by the fact that absolute memory
locations 8 through 15 are incremented prior to use when used as indirect
addresses.  These locations are called auto-index registers (despite the
fact that they are in memory); they allow the formulation of very tightly
coded array operations.

The basic instructions are:

	000 - AND - and operand with AC.
	001 - TAD - add operand to <L,AC> (a 13 bit value).
	010 - ISZ - increment operand and skip if result is zero.
	011 - DCA - deposit AC in memory and clear AC.
	100 - JMS - jump to subroutine.
	101 - JMP - jump.
	110 - IOT - input/output transfer.
	111 - OPR - microcoded operations.

The ISZ and other skip instructions conditionally skip the next
instruction in sequence.  The ISZ is commonly used to increment a loop
counter and skip if done, and it is also used as an general increment
instruction, either followed by a no-op or in contexts where it is known
that the result will never be zero.

The JMS instruction stores the return address in relative word zero of
the subroutine, with execution starting with relative word one.
Subroutine return is done with an indirect JMP through the return
address.  Subroutines commonly increment their return addresses to index
through inline parameter lists or to perform conditional skips over
instructions following the call.

The IOT instruction has the following form:
	 _ _ _ _ _ _ _ _ _ _ _ _
	|     |           |     |
	|     |   device  | op  |

The IOT instruction specifies one of up to 8 operations on one of 64
devices.  Typically (but not universally), each bit of the op field
evokes an operation, and these can be microcoded in right to left
order.  Prior to the PDP-8/E, there were severe restrictions on the
interpretation of the op field that resulted from the fact that the
operation was delivered as a sequence of IOP pulses, each on a separate
line of the I/O bus.  Each line was typically used to evoke a different
device function, so essentially, the operation 000 was always a no-op
because it evoked no functions, and the code 111 evoked all three
functions in series.

As an example of the use of IOT instructions, consider the console
terminal interface.  On early PDP-8 systems, this was always assumed to
be an ASR 33 teletype, complete with low-speed paper tape reader and
punch.  It was addressed as devices 03 (the keyboard/reader) and 04
(the teleprinter/punch):
	 _ _ _ _ _ _ _ _ _ _ _ _
	      |0 0 0 0 1 1|0 0 1  - KSF - keyboard skip if flag
	      |0 0 0 0 1 1|0 1 0  - KCC - keyboard clear flag
	      |0 0 0 0 1 1|1 0 0  - KRS - keyboard read static

The keyboard flag is set by the arrival of a character.  The KCC
instruction clears both the flag and the accumulator.  KRS ors the 8 bit
input data with the low order 8 bits of AC.  The commonly used KRB
instruction is the or of KCC and KRS.  To await one byte of input, use
KSF to poll the flag, then read the byte with KRB.
	 _ _ _ _ _ _ _ _ _ _ _ _
	      |0 0 0 1 0 0|0 0 1  - TSF - teleprinter skip if flag
	      |0 0 0 1 0 0|0 1 0  - TCF - teleprinter clear flag
	      |0 0 0 1 0 0|1 0 0  - TPC - teleprinter print static

The teleprinter flag is set by the completion of the TPC operation (as
a result, on startup, many applications output a null in order to get
things going).  TCF clears the flag, and TPC outputs the low order 8
bits of the accumulator.  The commonly used TLS instruction is the or
of TCF and TPC.  To output a character, first use TSF to poll the flag,
then write the character with TLS.

IOT instructions may be used to initiate data break transfers from block
devices such as disk or tape.  The term "data break" was, for years,
DEC's preferred term for cycle-stealing direct-memory-access data

Some CPU functions are accessed only by IOT instructions.  For example,
interrupt enable and disable are IOT instructions:
	 _ _ _ _ _ _ _ _ _ _ _ _
	      |0 0 0 0 0 0|0 0 1  - ION - interrupts turn on
	      |0 0 0 0 0 0|0 1 0  - IOF - interrupts turn off

An interrupt is requested when any device raised its flag.  The console
master clear switch resets all flags and disables interrupts.  In
effect, an interrupt is a JMS instruction to location zero, with the
side effect of disabling interrupts.  The interrupt service routine
is expected to test the device flags and perform the operations needed
to reset them, and then return using ION immediately before the indirect
return JMP.  The effect of ION is delayed so that interrupts are not
enabled until after the JMP.

The instructions controlling the optional memory management unit are
also IOT instructions.  This unit allows the program to address up to
32K of main memory by adding a 3 bit extension to the memory address.
Two extensions are available, one for instruction fetch and direct
addressing, the other for indirect addressing.

A wide variety of operations are available through the OPR microcoded
         _ _ _ _ _ _ _ _ _ _ _ _
Group 1 |1|1|1|0|_|_|_|_|_|_|_|_|
	         1                - CLA - clear AC
	           1              - CLL - clear the L bit
                     1            - CMA - ones complement AC
                       1          - CML - complement L bit
                               1  - IAC - increment <L,AC>
                         1 0 0    - RAR - rotate <L,AC> right
                         0 1 0    - RAL - rotate <L,AC> left
	                 1 0 1    - RTR - rotate <L,AC> right twice
	                 0 1 1    - RTL - rotate <L,AC> left twice

In general, the above operations can be combined by oring the bit
patterns for the desired operations into a single instruction.  If none
of the bits are set, the result is the NOP instruction.  When these
operations are combined, they operate top to bottom in the order shown
above.  The exception to this is that IAC cannot be combined with the
rotate operations on some models, and attempts to combine rotate
operations have different effects from one model to another (for example,
on the PDP-8/E, the rotate code 001 means swap 6 bit bytes in the
accumulator, while previous models took this to mean something like
"shift neither left nor right 2 bits").
         _ _ _ _ _ _ _ _ _ _ _ _
Group 2 |1|1|1|1|_|_|_|_|_|_|_|0|
                   1     0        - SMA - skip on AC < 0  \
                     1   0        - SZA - skip on AC = 0   > or group
                       1 0        - SNL - skip on L /= 0  /
                   0 0 0 1        - SKP - skip unconditionally
                   1     1        - SPA - skip on AC >= 0 \
                     1   1        - SNA - skip on AC /= 0  > and group
                       1 1        - SZL - skip on L = 0   /
                 1                - CLA - clear AC
                           1      - OSR - or switches with AC
                             1    - HLT - halt

The above operations may be combined by oring them together, except that
there are two distinct incompatible groups of skip instructions.  When
combined, SMA, SZA and SNL, skip if one or the other of the indicated
conditions are true (logical or), while SPA, SNA and SZL skip if all of
the indicated conditions are true (logical and).  When combined, these
operate top to bottom in the order shown; thus, the accumulator may be
tested and then cleared.  Setting the halt bit in a skip instruction is
a crude but useful way to set a breakpoint for front-panel debugging.
If none of the bits are set, the result is an alternative form of no-op.

A third group of operate microinstructions (with a 1 in the least
significant bit) deals with the optional extended arithmetic element to
allow such things as hardware multiply and divide, 24 bit shift
operations, and normalize.  These operations involve an additional data
register, MQ or multiplier quotient, and a small step count register.
On the PDP-8/E and successors, MQ and the instructions for loading and
storing it were always present, even when the EAE was absent, and the
EAE was extended to provide a useful variety of 24 bit arithmetic


Subject: What does PDP-8 assembly language look like?

There are many different assemblers for the PDP-8, but most use a
compatible basic syntax; here is an example:

	START,	CLA CLL		/ Clear everything
		TAD	X	/ Load X
		AND I	Y	/ And with the value pointed to by Y
		DCA	X	/ Store in X
		HLT		/ Halt

	X,	1 		/ A variable
	Y,	7 		/ A pointer

Note that labels are terminated by a comma, and comments are separated
from the code by a slash.  There are no fixed fields or column
restrictions.  The "CLA CLL" instruction on the first line is an example
of the microcoding of two of the Group 1 operate instructions.  CLA
alone has the code 7200 (octal), while CLL has the code 7100; combining
these as "CLA CLL" produces 7300.  As a general rule, except when memory
reference instructions are involved, the assembler simply ors together
the values of all blank separated fields between the label and comment.
Indirection is indicated by the special symbol I in the operand field,
as in the third line of the example.  The typical PDP-8 assembler has no
explicit notation to distinguish between page zero and current page
addresses.  Instead, the assembler is expected to note the page holding
the operand and automatically generate the appropriate mode.  If the
operand is neither in the current page nor page zero, some assemblers
will raise an error, others will automatically generate an indirect
pointer to the off-page operand; this should be avoided because it only
works for directly addressed off-page operands, and only when the memory
management unit is not being used to address a data field other than the
current instruction field.

Note, in the final two lines of the example, that there is no "define
constant" pseudo-operation.  Instead, where a constant is to be
assembled into memory, the constant takes the place of the op-code field.

The PDP-8 has no immediate addressing mode, but most assemblers provide
a notation to allow the programmer to ignore this lack:

		TAD	(3)	/ add 3, from memory on the current page.
		TAD	[5]	/ add 5, from memory on page zero.
		JMP I	(LAB)	/ jump indirect through the address of LAB.

Assemblers that support this automatically fill the end of each page
with constants defined in this way that have been accumulated during the
assembly of that page.  Note that the variants "(3" and "[5" (with no
closing parentheses) are usually allowed but the use of this sloppy form
is discouraged.  Furthermore, the widely used PAL8 assembler interprets
the unlikely operand "(3)+1" as being the same as "(3+1)".

Arithmetic is allowed in operand fields and constant definitions, with
expressions evaluated in strict left-to-right order, as:

		TAD	X+1	/ add the contents of the location after X.
		TAD	(X-1)	/ add the address of the location before X.

Other operators allowed include and (&), or (!), multiply (^) and divide
(%), as well as a unary sign (+ or -).  Unfortunately, one of the most
widely used assemblers, PAL8, has trouble when unary operators are mixed
with multiplication or division.
Generally, only the first 6 characters of identifiers are significant
and numeric constants are evaluated in octal.

Other assembly language features are illustrated below:

	/ Comments may stand on lines by themselves
				/ Blank lines are allowed

		*200		/ Set the assembly origin to 200 (octal)

	NL0002=	CLA CLL CML RTL	/ Define new opcode NL0002.

		NL0002		/ Use new opcode (load 0002 in AC)
		JMP	.-1	/ Jump to the previous instruction

	X1=	10		/ Define X1 (an auto-index register address)
	LETA=	"A		/ Define LETA as 000011000001 (ASCII A)

		TAD I	X1	/ Use autoindex register 1

		IAC; RAL	/ Multiple instructions on one line

		$		/ End of assembly

The assembly file ends with a line containing a $ (dollar sign) not in
a comment field.

The $, * and =  syntax used by most PDP-8 assemblers replaces functions
performed by pseudo-operations on many other assemblers.  In addition,
PAL8, the most widely used PDP-8 assembler supports the following

		DECIMAL		/ Interpret numeric constants in base 10
		OCTAL		/ Interpret numeric constants in base 8
		EJECT		/ Force a page eject in the listing
		XLIST		/ Toggle listing
		XLIST	N	/ Turn on listing if N=0, off if N=1
		PAGE 	 	/ Advance location counter to next page
		PAGE 	N	/ Set location counter start of page N
		FIELD	N	/ Assemble into extended memory field N
		TEXT	"STR"	/ Pack STR into consecutive 6 bit bytes
		ZBLOCK	N	/ Allocate N words, initialized to zero
		IFDEF	S <C>	/ Assemble C if symbol S is defined
		IFNDEF	S <C>	/ Assemble C if symbol S is not defined
		IFZERO	E <C>	/ Assemble C if expression E is zero
		IFNZRO	E <C>	/ Assemble C if expression E is not zero
		FIXMRI  OP= VAL	/ Define OP as memory reference instruction

Conditonally assembled code must be enclosed in angle brackets.  The
enclosed code may extend over multiple lines and, because different
assemblers treat comments within conditionals differently, the closing
bracket should not be in a comment and any brackets in comments should
be balanced.


Subject: What character sets does the PDP-8 support?

From the beginning, PDP-8 software has generally assumed that textual
I/O would be in 7 bit ASCII.  Most early PDP-8 systems used teletypes
as console terminals; as sold by DEC, these were configured for mark
parity, so most older software assumes 7 bit ASCII, upper case only,
with the 8th bit set to 1.  On output, lines are generally terminated
with both CR and LF; on input, CR is typically (but not always) the
line terminator and LF is typically ignored.  In addition, the tab
character (HT) is generally allowed, but software support output of text
containing tabs varies.

One difficulty with much PDP-8 software is that it bypasses the device
handlers provided by the operating system and goes directly to the
device.  This results in very irregular device support, so that, for
example, control-S and control-Q work to start and stop output under
OS/8, but the OS/8 PAL assembler ignores them when reporting errors.

Most of the better engineered PDP-8 software tends to fold upper and
lower case on input, and it ignores the setting of the 8th bit.  Older
PDP-8 software will generally fail when presented with lower case
textual input (this includes essentially all OS/8 products prior to
OS/278 V1).

Internally, PDP-8 programmers are free to use other character sets, but
the "X notation provided by the assembler encourages use of 7 bit ASCII
with the 8th bit set to 1, and the TEXT pseudo-operation encourages the
6 bit character set called "stripped ASCII".  To map from upper-case-only
ASCII to stripped ASCII, each 8 bit character is anded with octal 77 and
then packed 2 characters per word, left to right.  Many programs use a
semi-standard scheme for packing mixed upper and lower case into 6 bit
TEXT form; this uses ^ to flip from upper to lower case or lower to
upper case, % to encode CR-LF pairs, and @ (octal 00) to mark end of
string.  Note that this scheme makes no provision for encoding the %,
^ and @ characters, nor does it allow control characters other than the
CR-LF pair.

The P?S/8 operating system supports a similar 6 bit text file format,
where upper and lower case are folded together, tabs are stored as _
(underline), end-of-line is represented by 00, padded with any
nonzero filler to a word boundary, and end of file is 0000.

Files under the widely used OS/8 system consist of sequences of 256 word
blocks.  When used for text, each block holds 384 bytes, packed 3 bytes
per pair of words as follows:

		aaaaaaaa		ccccaaaaaaaa
		bbbbbbbb		CCCCbbbbbbbb

Control Z is used as an end of file marker.  Because most of the PDP-8
system software was originally developed for paper tape, binary object
code is typically stored in paper-tape image form using the above packing


Subject: What different PDP-8 models were made?

The total sales figure for the PDP-8 family is estimated at over 300,000
machines.  Over 7000 of these were sold prior to 1970, and 30,000 were
sold by 1976.  During the PDP-8 production run, a number of models were
made, as listed in the following table.  Of these, the PDP-8/E is generally
considered to be the definitive machine.  If the PDP-8 is considered to
be the Model T of the computer industry, perhaps the PDP-8/E should be
considered to be the industry's Model A.


    PDP-5	63-67	 116		Transistor
    PDP-8	65-69 	1450	$18,500	Transistor
    LINC-8	66-69	 142	$38,500	Transistor
    PDP-8/S	66-70	1024	$10,000	Transistor	Very slow
    PDP-8/I	68-71 	3698	$12,800	TTL
    PDP-8/L	68-71 	3902	 $8,500	TTL		Scaled down 8/I
    PDP-12	69-73?	3500?	$27,900	TTL		Followup to LINC-8
    PDP-8/E	70-78	>10K?	 $6,500	TTL MSI	Omnibus
    PDP-8/F	72-78?	>10K?	<$5K	TTL MSI Omnibus	Based on 8/E CPU
    PDP-8/M	72-78?	>10K?	<$5K	TTL MSI Omnibus	OEM version of 8/F
    PDP-8/A	75-84?	>10K?	 $1,317	TTL LSI Omnibus	New CPU or 8/E CPU
    VT78	78-80		 $7,995	Intersil 6100	Workstation
    DECmate I	80-84			Harris 6120	Workstation
    DECmate II	82-86		 $1,435	Harris 6120	Workstation
    DECmate III	84-90		 $2,695	Harris 6120	Workstation
    DECmate III+85-90			Harris 6120	Workstation

Additional information is available in part two of this FAQ, where all
known models of the PDP-8, along with variants, alternate marketing
names, and other peculiarities are given.

The last years of the PDP-8 family were dominated by the PDP-8 compatible
microprocessor based VT78 and DECmate workstations.  The Intersil 6100,
also known as the CMOS-8 chip, was developed in 1976; GE later acquired
Intersil.  DEC also used the followup Harris 6120 microprocessors
(Introduced 1981) in many peripheral controllers for the PDP-11 and
PDP-15 as well as in the DECmate series of systems.  While all of the
earlier PDP-8 systems were open architecture systems, the DECmates had
closed architectures with an integrated console terminals and limited
peripheral options.  It is interesting to note that the Harris 6120 was
a 10Mhz chip and some chips could be clocked at 15Mhz; furthermore, the
6120 was essentially based on gate array technology.

The following PDP-8 compatible or semi-compatible machines were made and
sold by others; very little is known about many of these:


	TPA1001		 69	Hungarian, KFKI product, transistorized.
	TPA1001/i	 71	Hungarian, KFKI, IC version of 1001.
	TPA/i		 71	Hungarian, KFKI, renamed TPA1001/i
	TPA/l		 7? 	Hungarian, KFKI, enhanced TPA/i.
	TPAl/128H	 7? 	Hungarian, KFKI, TPA/l with 128K memory.
	TPA/s		 7?	Hungarian, KFKI, based on Intersil CPU chip.
	TPA Quadro	 8?	Hungarian, KFKI, comparable to a DECmate.
        Electronica-100  ?	Russian, discrete transistor technology.
        Electronica-100I ?	Russian, probably a PDP-8/I clone.
	Electrotechnica-100I ?	Yugoslavian, PDP-8/I?  Possibly same as above.
	Saratov-2	 ?	Russian, built like a PDP-8/M but bulkier.
	SPEAR u-LINC 100 ?	SPEAR, Inc, Waltham Mass (a LINC clone!)
	SPEAR u-LINC 300 ?	SPEAR, Inc, Waltham Mass (a LINC clone!)
	DCC-112		 70	Digital Computer Controls, PDP-8/L clone.
	DCC-112H	 71	Digital Computer Controls
	MPS-1		 74	Fabritek, PDP-8/L clone
	MP-12		 74	(is this just different numbering of above?)
	6100 Sampler	 76?	Intersil, their IM6100 promotional kit
	Intercept I	 7?	Intersil, based on IM6100
	Intercept Jr	 7?	Intersil, based on IM6100
	TLF MINI-12      77     Based on IM6100, in an elegant package.
	PCM-12		 7?	Pacific CyberMetrix, based on Intercept bus
	PCM-12A		 77	Pacific CyberMetrix, fixed to clock at 4MHz
	SBC-8		 84-88	CESI, Based on IM6120? SCSI bus

More information on the Hungarian TPA series, built by KFKI (the Central
Research Institute for Physics), was provided by Varga Akos Endre,; information on and photos of these machines are
currently available from:

The original machine in this series, the TPA1001, was built from the
description in DEC's Small Computer Handbook.  Only after the series was
in production, when a machine was exhibited in Ljubljana, Yugoslavia, was
full DEC compatability demonstrated, when a DEC user booted the TPA machine
from a DEC paper tape.  By the end of the TPA production run, around 900
PDP-8 compatable machines had been built.  Given the Soviet era central
planning for the computer industries in eastern europe, it is quite possible
that the Electrotechnica and Electronica models listed above may have been
TPA machines packaged for use in the USSR and other Soviet Block countries.

It is amusing to note that the name TPA is very similar in origin to the name
PDP used by DEC!  There was a decree that computer development in Hungary was
to cease, with all computers to be purchased from the USSR.  In response, the
people at KFKI ceased developing computers and began developing "Stored Program
Analyzers" or, the acronym for which is TPA in Hungarian.

The CESI (Computer Extension Systems, Inc.) machine had 128K words of local
RAM on each processor card and allowed up to 4 processor cards per OMNIBUS,
along with 128K words of global shared memory.  3 AMD 2901 bit-slice
processor chips were used to build the 12-bit ALU and data paths, controlled
by an 80-bit microword.


Subject: What about the LINC/8 and PDP-12?

Wesley Clark and Charles Molnar, then at Lincoln Labs, built the LINC, or
Laboratory INstrumentation Computer, as a personal laboratory computer,
finishing the first in March 1962.  The machine was developed in response
to the needs of Mary Brazier, a neurophysiologist at MIT who needed better
laboratory tools, and it was a followup to the Average Response Computer,
an 18 bit special purpose machine built in 1958 for the same purpose.
When Lincoln Labs decided that the LINC did not fit their mission, in
January 1963, the project moved to MIT, and then in 1964, to Washington
University in St Louis.  The National Institute of Health funded the
project as an experiment to see if coumputers would be a productive tool
in the life sciences.  By the end of 1963, 20 LINCs had been built and
debugged, many by their eventual users.

Over 24 LINC systems were built by customers before late 1964 when DEC
began selling a commercial version (see Computers and Automation, Nov.
1964, page 43).  By the time DEC introduced the LINC-8, 43 LINC systems
had been installed (see Computers and Automation, Mar. 1966, page 34).
In total, 50 LINC systems were built, 21 by DEC, 29 by customers (see
Digital at Work, page 52).  A photo of the last LINC in production use
is available from
Wesley Clark wrote a history of the LINC, "The LINC was Early and
Small", published in "A History of Personal Workstations," ACM Press,
1988, page 347.

The LINC was the first 12 bit minicomputer built using DEC hardware.
Like the PDP-5 and other early DEC computers, it was built with System
Modules, DEC's first family of logic modules.  Along with the CDC 160,
it paved the way for the PDP-5 and PDP-8.

When compared with the PDP-8, the LINC instruction set was not as well
suited for general purpose computation, but the common peripherals
needed for lab work such as analog-to-digital and digital-to-analog
converters were all bundled into the LINC system.  Users judged it to
be a superb laboratory instrument.

One of the major innovations introduced with the LINC was the LINCtape,
designed by Tom Stockebrand scaled down from an experimental tape drive
developed for the TX-2 at Lincoln Labs.  LINCtapes could be carelessly
pocketed or dropped on the floor without fear of data loss, and they
allowed random access to data blocks.  Stockebrand improved on this idea
slightly after he came to DEC, where the improved idea was called
DECtape; DECtape was widely used with all DEC computers made in the
late 1960's and early 1970's.

The motives behind the development of LINCtape were the same motives
that led IBM to develop the floppy disk almost a decade later, and in
fact, DECtape survived as a widely used medium until DEC introduced the
RX01 8 inch floppy disk drive around 1975, and even after this, DECtape
was only slowly phased out.

Within a year of the introduction of the PDP-8, DEC released the LINC-8,
a machine that combined a PDP-8 with a LINC in one package.  The
success of the LINC-8 led DEC to re-engineer the machine using TTL
logic in the late 1960's; the new version was originally to be called
the LINC-8/I, but it was sold as the PDP-12.  Both the LINC-8 and the
PDP-12 had impressive consoles, with separate sets of lights and
switches for the LINC and PDP-8 halves.

The success of the LINC-8 also led to the development of a clone, the
SPEAR micro-LINC.  This machine used Motorola MECL integrated circuits
and was available for delivery in (June 1965? this date must be wrong!).

The LINC-8 and PDP-12 could run essentially any PDP-8 or LINC program,
with the exception of the few programs that relied on the primitive
interrupt structure of the original LINC architecture; on the LINC-8,
all interrupts were handled by the PDP-8 side of the hardware.  Because
the LINC-8 and PDP-12 had instructions for switching between modes, a
new body of software was developed that required both modes.

One feature of LINC and LINC-8 software is the common use of the graphic
display for input-output.  These machines were some of the first to
include such a display as a standard component, and many programs used
the knobs on the analog to digital converter to move a cursor on the
display in the way we now use a mouse.

Various versions of LAP, the Linc Assembly Program, were the dominant
assemblers used on the LINC; the original version of LAP was a cross
assembler written on the TX-2.  WISAL (WISconson Assembly Language) or
LAP6-W was the version of this assembler that survived to run on the
PDP-12.  Curiously, this includes a PDP-8 assembler written in LINC code.

LAP6-DIAL (Display Interactive Assembly Language) evolved from this on
the PDP-12 to became the dominant operating system for the PDP-12.  The
8K version of this is DIAL MS (Mass Storage), even if it has only two
LINCtape drives.  These were eventually displaced by the OS/8 variant
known as OS/12.


Subject: Where can I get a PDP-8 today?

The IM6100 chip may still be available (Electronic Expediters, (818)781-1910
(in North America) listed them at US$23.50 each as of 10/1994), and CESI
may still make their clone, for a high price, but you can't buy a new
DEC PDP-8.  There are quite a few PDP-8 machines to be found in odd
places on the used equipment market.  They were widely incorporated into
products such as computer controlled machine tools, X-ray diffraction
machines, and other industrial and lab equipment.  Many of them were
sold under the EduSystem marketing program to public schools and
universities, and others were used to control laboratory instrumentation.
After about 1976, Reuters bought as many as 10,000 OMNIBUS based
machines per year, with perhaps 2000 per year going to other customers.

Through the 1980's and 1990's, PDP-8 hardware was frequently discarded
or sold for scrap, and many collectors were able to obtain CPU's,
peripherals and occasional complete systems in exchange for the effort
required to haul them away.  This may be changing!  In early 1999, a
PDP-8 system in unknown condition was sold at auction through eBay for
$1526.00; this is far less than the new cost of such a machine, but
far more than the scrap value of such a system!  Owners of original DEC
hardware will likely need maintenance and test supplies.  Douglas
Electronics still makes extender boards and breadboards in DEC format,

As of 2000, there are still a modest number of PDP-8 systems in
production use, mostly PDP-8/A systems.  These are supported by a
shrinking number of commercial suppliers and maintenance contractors.
For example, Michael Coffey <> has advertised the
availability of spare parts and maintencance documents for Omnibus

By the year 2000, a number of PDP-8 parts and systems have changed hands
on E-bay.  Sadly, many systems are sold piecemeal, with parts such as
core memory destined to hang on den and office walls.  The market is
spotty, but looking back over the sales history is a good way
to get an idea of what parts might be worth.

If you can't get real hardware, you can get emulators.  Over the years,
many PDP-8 emulators have been written; the best of these are
indistinguishable from the real machine from a software prespective,
and on a modern high-speed RISC platform, these frequently outperform
the hardware they are emulating.  An emulator is available from DECUS,
catalog number RB0128; This and other emulators are available from:

The final collection of emulators listed above includes emulators for the
Nova and other DEC machines as well as the IBM 1401.

Bernhard Baehr's emulator for the Apple Mac, complete with an emulated front
panel and a fair amount of software is available from:

The Spare Time Gizmos emulator for Windows doesn't have the elaborate
front panel interface, but it appears to be reasonably complete and has a
very realistic teletype interface window.

Finally, you can always build your own.  The textbook "The Art of
Digital Design," second edition, by Franklin Prosser and David Winkel
(Prentice-Hall, 1987, ISBN 0-13-046780-4) uses the design of a PDP-8 as
a running example; development new material based on this book continues,
including an asynch interface chip and, now, several implementations based
on Xilinx FPGAs.  Contact Ingo Cyliax or Caleb Hess (
or for information on the current state of this work.
Other FPGA implementation are being developed by Jon Andrews and David Conroy;

"Modern VLSI Design - A system approch" by Wayne Wolf (1994 Prentice-Hall)
also uses the PDP-8 as a data-path example, as does "Verilog Design Computer
Design" by Mark Gordon Arnold (Prentice Hall).

It is worth noting that there are a sufficient number of PDP-8 systems
still operational that some companies still manufacture peripherals.
For example, Storage Computer  makes RK05
compatable semiconductor "disk drives" that can be directly connected to
the Omnibus RK05 controllers of the PDP-8/E,F,M and A.

Occasionally, someone connects a PDP-8 to the internet.  The most
interesting current venture in that direciton is available at:

This machine, when working, has complete remote control of the front
panel and even the on-off switch from a Java interface, and there's a
web-cam so you can see the real machine as it responds to the remote
front panel operations.


Subject: Where can I get PDP-8 documentation?

The key documents published by DEC describing each model of the PDP-8
are all out of print, and DEC was in the habit of printing much of
their documentation on newsprint with paperback bindings, which is to
say, surviving copies tend to be yellow and brittle.  DEC distributed
huge numbers of catalogs and programming handbooks in this inexpensive
paperback format, and these circulate widely on the second-hand market.
When research laboratories and electronics shops are being cleaned out,
it is still common to find a few dusty, yellowed copies of these books
being thrown out.

Douglas Jones has made a small number of bound photocopies of DEC's
1973 introduction to programming, perhaps the definitive introduction
to the PDP-8, and the other early DEC handbooks need similar treatment
before they all crumble.

Thanks to David Gesswein, a growing collection of PDP-8 documentation,
including the Small Computer Handbook, the PDP-8/e/f/m maintenance
manual, and prints of various boards have been scanned in and made
available on the web at:

Some PDP-8 reference material has been transcribed into Hypertext format
and is available over WWW from:

Much more material is available from:

In general, maintenance manuals are hard to find, but valuable.  If you
need one, you usually need to find someone willing to photocopy one of
the few surviving copies.  DEC has been friendly to collectors, granting
fairly broad letters of permission to reprint obsolete documentation,
and the network makes if fairly easy to find someone who has the
documentation you need and can get copies.  The most difficult to copy
material is the large prints, many of which would be quite useful if
photoreduced, but this is expensive.


Subject: What operating systems were written for the PDP-8?

A punched paper-tape library of stand-alone programs was commonly used
with the smallest (diskless and tapeless) configurations from the
beginning up through the mid 1970's.  This included a paper-tape based
text editor, the PAL III and MACRO-8 assembler, and a FORTRAN compiler,
as well as a library of support routines.  Many paper tapes from this
library survive to the present at various sites!  The minimum
configuration expected by these tapes is a CPU with 4K memory and a
teletype ASR 33 with paper tape reader and punch.  Note that much of this
paper-tape-based software is based on memory-use and I/O conventions that
are incompatible with later disk-based systems.

The DECtape Library System was a DECtape oriented save and restore system
that was available from the start.  The resident portion of this system
occupies only 17 words of memory (7600-7625 octal), and it allowed saving
and restoring absolute core images as named files on a reel of DECtape.
Initially, program development was still done with paper tape, and only
executable memory images were stored on DECtape, but eventually, a limited
DECtape-based text editor was introduced, along with a DECtape based

The 4K Disk Monitor System provided slightly better facilities.  This
supported on-line program development and it worked with any device that
supported 129 word blocks (DECtape, the DF32 disk, or the RF08 disk).
It was quite slow, but it also used very little of the available memory.

MS/8 or the RL Monitor System, was developed starting in 1966 by
Richard F. Lary; it was submitted to DECUS in 1970.  This was a disk
oriented system, faster than the above, with tricks to make it run
quickly on DECtape based systems.

POLY BASIC was a BASIC only system submitted to DECUS and later sold by
DEC as part of its EduSystem marketing program.  EduSystem 25 Basic
is available from:

P?S/8 was developed starting in 1971 from an MS/8 foundation.  It runs
on minimal PDP-8 configurations, supports somewhat device independant
I/O and requires a random-access device for the file system (DECtape is
random-access!).  P?S/8 runs compatibly on most PDP-8 machines including
DECmates, excepting only the PDP-8/S and PDP-5.  P?S/8 is still being

Richard F. Lary developed a system called the Fully Upward Compatible
Keyboard Monitor; and between a Wednesday and the following Friday, a
prototype was up and running from DECtape.  The original intention of
this project was to build a programming environment for the PDP-8 that
looked like TOPS-10 on the PDP-10.  A year later, this was released as
Programming System/8 (or PS/8), and then renamed OS/8 in 1971 because
Eli Glaser (a salesman from Long Island) said he could sell more systems
with an operating system than with a programming system, and because, by
renaming the system, DEC could increase the price despite Nixon's
wage-price freeze.

OS/8, developed in parallel with P?S/8, became the main PDP-8 programming
environment sold by DEC.  The minimum configuration required was 8K words
and a random-access device to hold the system.  For some devices, OS/8
requires 12K.  There are a large number of OS/8 versions that are not
quite portable across various subsets of the PDP-8 family.  RX01 images of
OS/8 Version 3Q are available, with DEC's free non-commercial use licence,

The second site above also includes an incomplete but useful RK05 image
of OS/8 Version 3R.  Parts of the OS/8 source can be found in:

OS/8 V3D was renamed OS/78 (to match the VT78), and in followups to this
distribution, support for Omnibus machines was no longer important.  OS/78
V4 was developed for the DECmate I, and the name OS/278 used for the
versions released with later DECmate machines.  These have unnecessary
incompatabilities with earlier versions of OS/8.  OS/278 and related
material is available from DECUS as catalog item 800941, or from:

A growing collection of OS/8 documentation, including the OS/8 software
support manual on the internals of the system is available on line from:

OS8 (no slash) may still be viable.  It requires 8K of main memory, an
extended arithmetic unit, and DECtape hardware.  Unlike most PDP-8
operating systems, it uses a directory structure on DECtape compatible
with that used on the PDP-10.

The timesharing system TSS-8 was developed by Don Witcraft and John Everett
at DEC, starting in late 1967, and with the first beta sites up and running
in the fall of 1968.  This was based on a protection architecture proposed by
Adrian Van Der Goor, a grad student of Gordon Bell's at Carnegie-Mellon.
It requires a minimum of 12K words of memory and a swapping device; on a
24K word machine, it could give good support for 17 users.  It was
the standard operating system on the EduSystem 50 which was sold to many
small colleges and large public school systems.  The first installation was
at Lexington High School in Massachusetts, and the second was at Northern
Arizona University.  Each user gets a virtual 4K PDP-8; many of the
utilities users ran on these virtual machines were only slightly modified
versions of utilities from the Disk Monitor System or paper-tape
environments.  Internally, TSS8 consists of RMON, the resident monitor, DMON,
the disk monitor (file system), and KMON, the keyboard monitor (command shell).
BASIC was well supported, while restricted (4K) versions of FORTRAN D and
Algol were available.

Significant parts of TSS-8 have been found, but at this time, nobody has
managed to recover a complete working system.  Much of the available TSS/8
code can be found at:

Jim Dempsey, an alum of the OS/8 group at DEC, developed ETOS for Educomp
(later Quodata) for the PDP-8/E; this was a true virtual machine operating
system in the spirit of IBM's VM/370, and a special board was required
to optionally trap JMP and JMS instructions; this was enabled after an
emulated CIF instruction so that the actual change of instruction field
could be emulated when the JMP or JMS was attempted.  After leaving Quodata
and founding Network-Systems Design in 1976, Dempsey went on to develop
OMNI-8, first installed at Ripon College; initially it was priced at $4900,
several hundred copies were sold.  The OMNI-8 operating system supported
the enlarged PDP-8 address space of the CESI (Computer Extension Systems
Inc) memory cards, and when CESI began making PDP-8 clones, OMNI-8 was
extended to support asymmetric multiprocessors (one CPU handled the I/O).
The end of OMNI-8 development came around 1990.  Dumps of the ETOS kernel
and drivers survive in various places, and Jim Dempsey still has the full
source for OMNI-8.

Other timesharing systems developed for the PDP-8 include MULTI-8
and MULTOS.  The source for MULTOS is available from:

CAPS-8 was a cassette based operating system supporting PAL and BASIC.
There are OS/8 utilities to manipulate CAPS-8 cassettes, and the file
format on cassette is compatible with a PDP-11 based system called

RTS/8 was a real-time system developed by DEC, developed from an earlier
system, SRT8, dating back to at least 1974.  Curiously, even the last
versions of RTS/8 continued to support paper-tape and DECtape.  RTS/8 also
offered a virtual PDP-8 for background processing, unlike ETOS, this did
not require special hardware; instead, software emulation was used to retain
control of the machine between the CIF instruction and a following
JMP or JMS.  Source code for most of the versions of RTS and SRT is
available from:

WPS was DEC's word processing system, developed for the 8/E with a VT50
terminal with special WPS keycaps replacing the standard keycaps, and
widely used on the 1980's vintage machines.  It was heavily promoted on
the VT-78, and when the DECmates came out, DEC began to suppress knowledge
that DECmates could run anything else.  WPS-11 was a curious distributed
system using a PDP-11 as a file server for a cluster of VT-78 WPS systems.
DECmate/WPS Version 2.3 is available from DECUS for the DECmate II and
DECmate III under the catalog entry DM0114.

COS-310, DEC's commercial operating system for the PDP-8, supported the
DIBOL language.  COS-310 was a derivative of MS/8 and OS/8, but with a
new text file format.  The file system is almost the same as OS/8, but
dates are recorded differently, and a few applications can even run under
both COS and OS/8.  COS was the last operating system other than WPS
promoted by DEC for the DECmates.

SCPSYS, developed by D. C. Amoss prior to 1974 at Clemson University, is
a system that copies most of the features of LAP (the LINC Assembly
Program) for a pure PDP-8 based system.  A DECtape of this system has
recently come to light, with one known application, Spacewar.

AMOS, an operating system for the PDP-8/E with TD8E DECtape interface,
was a very small system developed in Australia or New Zealand and supporting
assembly and text editing on a 4K machine.


Subject: What programming languages are supported on the PDP-8

The PAL family of assembly languages, particularly PAL III and PAL8 are
as close to a standard assembly language as can be found for the PDP-8;
these are included with all OS/8 distributions.  They produce absolute
object code and there are versions of PAL for minimally configured
machines, although these have severe symbol table limitations.  Cross
assemblers that are somewhat compatable with PAL can be obtained from:

MACRO-8 was DEC's first macro assembly language for the PDP-8, but it
was rarely used outside the paper-tape environment.  MACREL and SABR are
assembly languages that produce relocatable output.  SABR is the final
pass for the ALICS II FORTRAN compiler (developed by ICS); it is included
with the standard OS/8 software distributions.  Source for these is
available from:

MACREL was developed in (unfulfilled) anticipation of similar use.  MACREL
was heavily used by the DECmate group at DEC.  MACREL is available from:

RALF, the relocatable assembler supporting RTPS FORTRAN is also included
in OS/8 standard distributions.  FLAP, an absolute assembler, was derived
from RALF.  Both SABR and RALF/FALP are assemblers that handle their
intended applications but have quirky and incompatible syntax.

A subset of FORTRAN was supported on both the PDP-5 and the original
PDP-8.  Surviving documentation describes a DEC compiler written in 1964
by Larry Portner, nicknamed "Fivetran", and a compiler written by
Information Control Systems from 1968.  The latter, ALICS II FORTRAN,
was originally a paper tape based compiler, but it forms the basis of
the OS/8 8K FORTRAN compiler, and was also adapted to the Disk Monitor
System (the latter version had overlay support that was never carried
forward into more modern systems).

RTPS FORTRAN required 8K and a floating point processor; it had real-time
extensions and was a full implementation of FORTRAN IV (also known as
ANSI FORTRAN 66).  OS/8 F4 is RTPS FORTRAN stripped of the requirement
for hardware floating point (if the hardware is missing, it uses
software emulation).  Versions of FORTRAN is available from

FOCAL, an interpretive language comparable to BASIC, was available on
all models of the family, including the PDP-5 and PDP-8/S.  Versions of
FOCAL run under OS/8, P?S/8 and other systems, and there were many special
purpose overlays for FOCAL developed by DEC and by various users.  DEC's
later FOCAL releases for the PDP-8 included code to deliberately introduce
subtle bugs when run on a DCC 112 computer!  Various versions of FOCAL
are available from:

Many versions of BASIC were also available, from DEC and other sources.
DEC BASIC was widely used on PDP-8 systems sold under the EduSystem
marketing program.  A paper-tape version was available that ran in 4K
and was compatible with disk based systems, versions distributed with
OS/8 and TSS-8, an 8K stand-alone time-sharing version was available,
and there were others.  EduSystem 25 Basic is available from:

The source code for TSS-8 Basic is available from

DIBOL was DEC's attempt at competing with COBOL in the commercial arena.
It was originally implemented under MS/8 but most versions were sold to
run under the COS operating system.

Algol was available from a fairly early date.  One version is available

At least two Pascal compilers were developed for the PDP-8.  One was a
Pascal-S interpreter, written in assembler, the other was a Pascal-P
compiler with a P-code interpreter written in assembler.
A Pascal S interpreter, requiring a 28K PDP-8/E configuration, is available

Another OS/8 Pascal system, the source code for which was rescued by
Larwrence LeMay, is available from:

A LISP interpreters was written for the PDP-8; the original version
ran in 4K (originally written in Germany?); a disassembled and commented
version of this was the basis of expanded versions that eventually
could utilize up to 16K.  One version of LISP is available from:

POLY SNOBOL was a version of SNOBOL that was somewhere between
Griswold's definitions of SNOBOL 3 and SNOBOL 4.

TECO, the text editor, was included in the standard OS/8 distributions and
is a general purpose language (the Emacs editor began as a set of TECO
macros!).  The story of TECO on the PDP-8 is convoluted.  Russ Hamm
implemented TECO under his OS8 (without a slash) system, and then gave
a listing to Don Baccus at the Oregon Museum of Science and Industry
(OMSI) who, along with Barry Smith ported it to PS/8.  This was the
beginning of what became Oregon Software, later famous for OMSI Pascal.

Richard F. Lary and Stan Rabinowitz made OS/8 TECO more compatible with
other versions of TECO, and the result of this work is the version
distributed by DECUS (catalog number 110450 is the manual).  RT-11 TECO
for the PDP-11 is a port of this code.

DECUS also lists the PAGE8 language (catalog numbers 800936), the VISTA
editor (catalog number 800938), and the ICE text editor (catalog number


Subject: Where can I get PDP-8 software?

DEC is still making computers, but they've largely forgotten about the
PDP-8.  The main DEC WWW server is

DECUS, the DEC User Society, is still alive and well, and their submission
form still lists PAL8 and FOCAL as languages in which they accept
submissions!  The DECUS library catalog is available on-line at; www access is through or

To quote the README file from the DECUS on-line catalog, "Items from
older DECUS Library catalogs are still also available (provided their
media can still be read), but machine readable catalog information is
not available for these."  Direct questions by E-mail to

Bob Supnik at DEC has rescued OS/8 from oblivion within DEC and has
managed to get DEC to grant a non-commercial free-use licence for OS/8
to all who wish to use it.  In addition, he has released a demonstration
version of OS/8 for his PDP-8 emulator, available with a copy of the
free-use licence from:

The following anonymous FTP sites also contain publically accessable
archives of PDP-8 software and other information:

The latter archive also maintains an archive of traffic in alt.sys.pdp8
in the directory ...pdp8/usenet and an archive of traffic in the
pdp8-lovers mailing list in .../pdp8/pdp8-lovers.

The archive at Indiana contains source code for many PDP-8 compilers and
interpreters, as well as common utilities and games.


Subject: Where can I get additional information?

The companion faq on PDP-8 models and options contains a detailed production
history of the PDP-8 as well as a high level description of incompatabilities
between models.  This is available from:

A modest attempt at an on-line PDP-8 manual is available from:

The mailing list reaches a number of PDP-8
owners and users.  This list does not accept postings from
nonsubscribers; to subscribe, send mail to

The pdp8-lovers mailing list has previously been hosted by,,,, and
The archives for these older lists should be available on, but
currently, the best archive is at

Many "archival" books have included fairly complete descriptions of the
PDP-8; among them, "Computer Architecture, Readings and Examples" by
Gordon Bell and Allen Newell is among the most accurate and complete,
although notationally difficult (see Chapter 5).  Gordon Bell has put
this on the web at:


Subject: What use is a PDP-8 today?

What use is a Model T today?  Collectors of both come in the same basic
classes.  First, there are antiquarians who keep an old one in the
garage, polished and restored to new condition but hardly ever used.
Once a year, they warm it up and use it, just to prove that it still
works, but they don't make much practical use of it.

PDP-8 systems maintained by antiquarians are frequently in beautiful
shape.  Antiquarians worry about dust, chipped paint, and missing
switches, and they establish newsgroups and mailing lists to help them
locate parts and the advice needed to fix their machines.

In the second class are those who find old machines and soup them up,
replacing major parts to make a hotrod that only looks like the original
from the outside, or keeping the old mechanism and putting it to uses
that were never intended.  Some PDP-8 owners, for example, have built
PDP-8 systems with modern SCSI disk interfaces!  There is serious
interest in some quarters in constructing an omnibus board that would
support an IDE disk of the variety that was mass-produced for the

Last, there are the old folks who still use their old machines for their
intended purposes long after any sane economic analysis would recommend
such use.  If it ain't broke, don't fix it, and if it can be fixed,
why bother replacing it?  Both Model T Fords and the classic PDP-8
machines are simple enough that end users can maintain and repair them
indefinitely.  All you need to keep a vintage -8 running are a stock
of inexpensive silicon diodes and a stock of 2N3639B or better,
2N3640 transistors.

Unlike most modern personal computers, PDP-8 systems were routinely sold
with complete maintenance manuals; these included schematic diagrams,
explanations of not only how to use the devices, but how they are built,
and suggestions to those considering building their own peripherals.
Compared with many so-called "open systems" of today, the PDP-8 was far
better documented and far more open.

Preservation of the PDP-8 has proven to be of immense practical value
in defending against the rising tide of patents in the area of
interactive graphics.  For example, when Sanders Associates sued the
Odyssey division of Magnavox, the key testimony in this suit was Steve
Russell's Spacewar, originally written for the PDP-1 in the fall of 1961.
The fact that documented versions of Spacewar and other computer games
dating back to the early 1960's could still be run on a surviving LINC-8
apparently played an important part in arriving at an out-of-court
settlement that ended, for practical purposes, the Sanders claim to
the technology behind all video games.  It is far easier to prove that
some software technology existed by demonstrating it on original hardware
than by waving a dusty listing in front of someone's face!

Finally, the PDP-8 is such a minimal machine that it is an excellent
introduction to how computers really work.  Over the years, many students
have built complete working PDP-8 systems from scratch as lab projects,
and the I/O environment on a PDP-8 is simple enough that it is a very
appropriate environment for learning operating system programming


Subject: Who's Who?

You can't beat the book Digital at Work (Digital Press, 1992) for short
writeups on the people inside DEC who made the PDP-8!

C. Gordon Bell is generally credited with the original design of the
PDP-8 (as well as designing the PDP-4, 5 and 6).  He was also involved
with recommending what became the PDP-11 when that design was competing
with the design that probably became the NOVA, and as vice president of
research, he oversaw the development of the DEC VAX family.

Alan Kotok worked with Bell in working up the original specifications
of the PDP-8.

Edson DeCastro was a key man in the design of the PDP-5 through the
PDP-8/L, then founded Data General to build the Nova.

Ben Gurley designed the early DEC machines, starting with the
PDP-1.  The actual design work on the -8, however, was done by Ed
deCastro, who later founded Data General to build the Nova.

Warren K. Smith was manager of applications engineering for Flip-Chip
modules between 1969 and 1975.  Much of the M-series TTL module family
dates from this period.

Saul B. Dinman, product line manager for the module product line from
1966 to 1969 designed the PDP-8/S and built the engineering prototype,
largely in his spare time.  Most of his time was devoted to the K-series
of industrial automation modules.  Later, he founded GRI Computer Corp,
where he designed the GRI 909 16-bit minicomputer.

Ken Olson ran DEC from the beginning.

Jozsef Lukacs and Janos Bogdany designed the Hungarian TPA1001
implementation of the PDP-8 instruction set, and Laszlo Szonyi and
Pal Karadi designed the TPA/i.

Ed Yourdon, who later became well known as a programming methodology
guru, helped hack up the PAL III assembler for the -8 from PAL II.

Richard Merrill invented FOCAL and wrote the original (1968) and classic
FOCAL-69 interpreters for the PDP-8.  He also did early translations of
the interpreter to PDP-7/PDP-9 code and perhaps the earliest PDP-11
version.  In addition, he wrote the EDIT-8 paper-tape based text editor
based on the FOCAL built-in text editor.

Richard F. Lary developed the RL Monitor System, and then went on to
develop OS/8, with help from Ed Friedman and another programmer named
Paul, under the management of Chuck Conley.

Charles Lasner developed P?S/8, and he is widely known as a leader in
the movement to preserve these historic machines.  He created the
alt.sys.pdp8 newsgroup.

George Thissell oversaw the development of OS/8 FORTRAN-IV, with Denny
Pavlock as part of the team.

Wesley Clark developed the LINC while working at Lincoln Labs; this was
the first 12 bit minicomputer built with DEC parts.

Mary Allen Wilkes Clark developed the early LAP programs for the LINC.

Don Witcraft wrote the TSS-8 scheduler, command decoder and UUO
handler, after working on the first swapping monitor for the PDP-10.
John Everett wrote the disk handler, file system, TTY handler
and  680-I service routine for TSS-8, after working on the Disk Monitor
System and PAL-D, the first disk-based version of PAL.

Roger Pyle and John Everett wrote the PDP-8 Disk Monitor System, and
John Everett adapted PAL-III to make PAL-D for DMS.  Bob Bowering, author
of MACRO for the -6 and -10, wrote an expanded version, PAL-X, for TSS-8.

Jimmy Dykes was the program manager for Harris in the contract development
of the Harris 6120 microprocessor; he later moved to GE Semiconductor.
Robert M. Smith was involved in the DEC side of this joint venture, after
having designed a number of OMNIBUS interfaces during the 1970's.

Douglas W. Jones wrote this FAQ, but prior to the summer of 1992, he'd
never used a PDP-8.  He has also written a report on how to photocopy
and archivally bind ailing paperback books such as DEC's handouts, a
PAL-like cross assembler in C, and a UNIX-based PDP-8 emulator.


End of PDP-8 Frequently Asked Questions (posted every other month)

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