Subject: PDP-8 Frequently Asked Questions (posted every other month)
From: [email-masked] (Douglas W. Jones)
Date: Fri, 8 Oct 1993 08:08:08 GMT
Newsgroups: alt.sys.pdp8,alt.answers,news.answers

Archive-name: dec-faq/pdp8
Last-modified: Oct 1, 1993

Frequently Asked Questions about the DEC PDP-8 computer.

	By Douglas Jones, [email-masked]
	(with help from many folks)

The most recent version of this file is available by anonymous FTP from:

	rtfm.mit.edu:/pub/usenet/alt.sys.pdp8
	sunsite.unc.edu:/pub/academic/computer-science/history/pdp-8/doc

Contents:

	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?

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
	wanted to call the 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", plug-in circuit boards
	with a few transistorized 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 of this
	machine.  The venture capitalist's insistance on avoiding the term
	computer was based on the steriotype that computers were big and
	expensive and needed a computer center and a large staff; by using
	the term Programmable Data Processor, or PDP, DEC avoided these
	stereotypes by entirely avoiding the term "computer"; thus, for
	over a decade, all digital computers sold by DEC were called PDPs.
	(In early DEC documentation, 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 computer leasing plan.  IBM and all the
	other larger 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.
	Many 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:

	MODEL  DATE  PRICE     BITS  COMMENTS
	=====  ====  ========  ====  =====
	PDP-1  1960  $120,000  18    DEC's first computer
	PDP-2            NA    24    Never built?
	PDP-3            NA    36    One was built by a customer, not by DEC.
	PDP-4  1962   $60,000  18    Predecessor of the PDP-7.
	PDP-5  1963   $27,000  12    The ancestor of the PDP-8.
	PDP-6  1964  $300,000  36    A big computer; 23 built, most for MIT.
	PDP-7  1965   $72,000  18    Widely used for real-time control.
	PDP-8  1965   $18,500  12    The smallest and least expensive PDP.
	PDP-9  1966   $35,000  18    An upgrade of the PDP-7.
	PDP-10 1967  $110,000  36    A PDP-6 successor, great for timesharing.
	PDP-11 1970   $10,800  16    DEC's first and only 16 bit computer.
	PDP-12 1969   $27,900  12    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    A TTL upgrade of the PDP-9.
	PDP-16 1972      NA    8/16  A register-transfer module system.

	Corrections and additions to this list are welcome!  The prices
	given are for minimal systems in the year the machine was first
	introduced.  The bits column 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 began as the Virtual
	Address eXtension of the never-produced PDP-11/78.

	It is worth mentioning that it is generally accepted that the Data
	General Nova (see photo, Computers and Automation, Nov. 1968,
	page 48) was originally developed as the PDP-X, a 16-bit
	multi-register version of the PDP-8.  A prototype PDP-X was built
	at DEC before the design was rejected.  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 know as the PDP-11.
	(A less common version of this story is that the reason that DEC
	never produced a PDP-13 was because the number 13 was assigned to
	what became the Nova; this is unlikely because the PDP-X prototype
	came before the PDP-11.)  Both DEC and Data General are quiet about
	these stories.

	Today, all of the PDP machines are in DEC's corporate past, with the
	exception of the PDP-11 family of minicomputers and microprocessors.

	Of course, occasionally, some lab builds a machine out of DEC
	hardware and calls 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.

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 in early 1968.
	(See Interdata ad, Computers and Automation, May 1968, page 10).
	The PDP-8 was largely upward compatable 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 no hardware byte structure, a 4K minimum memory configuration,
	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
	and later, 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 architecture lacked the notion of current page addressing,
	and the result is that, for examples that don't involve indexing,
	PDP-8 code is generally as tight as the code on Cray's machines.

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:
		 _ _ _ _ _ _ _ _ _ _ _ _
		|1|1|0|_|_|_|_|_|_|_|_|_|
		|     |           |     |
		|     |   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 left to
	right order.  Prior to the PDP-8/E, there were severe restrictions
	on the interpretation of the op field.

	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):
		 _ _ _ _ _ _ _ _ _ _ _ _
		|1|1|0|_|_|_|_|_|_|_|_|_|
		      |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 it with KRB.
		 _ _ _ _ _ _ _ _ _ _ _ _
		|1|1|0|_|_|_|_|_|_|_|_|_|
		      |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 use TPC to print 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 transfers.

	Some CPU functions are accessed only by IOT instructions.  For
	example, interrupt enable and disable are IOT instructions:
		 _ _ _ _ _ _ _ _ _ _ _ _
		|1|1|0|_|_|_|_|_|_|_|_|_|
		      |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 was requested when any device raised its flag.  The
	console master clear switch would reset all flags and disable
	interrupts.  Effectively, an interrupt is a JMS instruction to
	location zero, with the side effect of disabling interrupts.  The
	interrupt service routine would test 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 23K 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 instructions:
	         _ _ _ _ _ _ _ _ _ _ _ _
	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
	                       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
	                       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, 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 operations.

What does PDP-8 assembly language look like?

	There are many different assemblers for the PDP-8, but most use
	a compatable 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, the instruction to clear both AC and the link bit.  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 feature should be avoided!).

	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 an optional mechanism 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 generally discouraged.  Furthermore, the widely
	used PAL8 assembler interprets "(3)+1" as being the same as "(3+1)".

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

		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 included 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)

		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 replace
	functions performed by pseudo-operations on many other assemblers.
	In addition, PAL8, the most widely used PDP-8 assembler supports
	the following pseudo-operations:

		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
		PAGE 	 	/ Advance location counter to next page
		FIELD	N	/ Assemble into extended memory field N
		TEXT	STRING	/ Pack STRING 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

	Conditonally assembled code must be enclosed in angle brackets.
	The conditionally assembled code may extend over multiple lines.

What character sets does the PDP-8 support?

	With its 12 bit word, the PDP-8 is somewhat awkward in its support
	for modern 7 and 8 bit character sets.  Nonetheless, from the
	beginning, PDP-8 software has generally assumed that text 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 interpreted in terms
	of a tab-stop every 8 spaces.

	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.

	Internally, PDP-8 programmers are free to use other character sets,
	but the TEXT pseudo-operation strongly 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-standardized 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.

	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
			ccccCCCC

	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 scheme.

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.
	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.

	MODEL	DATES	SALES   COST	TECHNOLOGY	REMARKS

	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
	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?	 $7,390	TTL MSI	Omnibus
	PDP-8/F	72-78?	>10K?	<$7K	TTL MSI Omnibus Based on 8/E CPU
	PDP-8/M	72-78?	>10K?	<$7K	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		<$10K	Intersil IM6100
	Dm I    80-84			Harris 6120
	Dm II	82-86		 $1,435	Harris 6120
	Dm III	84-90		 $2,695	Harris 6120
	Dm III+	85-90			Harris 6120

	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 compatable microprocessor based VT78 and DECmate (Dm in the
	above table) machines.  DEC also used the Intersil IM6100 and
	Harris 6120 microprocessors in many peripheral controllers for
	the PDP-11 and PDP-15.  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.

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

	MODEL		DATE	MAKER, NOTES

	MP-12		6?	Fabritek
	TPA		68?	Hungarian, a PDP-8/L clone, ran FOKAL
	Saratov		?	Russian, another PDP-8/L clone
	Voronezh	?	Russian, another PDP-8/? clone
	SPEAR u-LINC	?	SPEAR, Inc, Waltham Mass.
	DCC-112		70-71	Digital Computer Controls
	DCC-112H	71	Digital Computer Controls
	6100 Sampler	7?	Intersil, their IM6100 promotional kit
	Intercept I	7?	Intersil, based on IM6100
	Intercept Jr	7?	Intersil, based on IM6100
	PCM-12		7?	Pacific CyberMetrix, based on Intercept bus
	PCM-12A		7?	Pacific CyberMetrix, fixed to clock at 4MHz
	SBC-8		84-88	CESI, Based on IM6120, SCSI bus

What about the LINC/8 and PDP-12?

	Wesley Clark, then at Lincoln Labs, developed the LINC, or
	Laboratory INstrumentation Computer, as a personal laboratory
	computer in the early 1960's.  He developed it in response to
	the needs of Mary Brazier, a neurophysiologist at MIT who needed
	better laboratory tools.  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).

	When Lincoln Labs decided that the LINC did not fit their mission,
	a group at the the National Institute of Health funded an
	experiment to see if the LINC would be a productive tool in the
	life sciences.  As a result of this project, 12 LINCs were
	built and debugged, each by its eventual user.

	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.  These tapes could be carelessly pocketed or dropped on
	the floor without fear of data loss, and they allowed random
	access to data blocks.  DEC improved on this idea slightly to
	make their DECtape format, and 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
	motivives 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.

	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
	software, but because they included instructions for switching
	between modes, a third 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.

	LAP, the Linc Assembly Program, was the dominant assembler used
	on the LINC.  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.

Where can I get a PDP-8 today?

	The CESI machine may still be on the market, for a high price, but
	generally, you can't buy a new PDP-8 anymore.  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 on the order of 10,000 OMNIBUS
	based machines per year, with perhaps 2000 per year going to other
	customers.

	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.

	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.  Many students who have used this
	book were required to build working PDP-8 systems as lab projects.

Where can I get PDP-8 documentation?

	Part II of this FAQ cites the key documents published by DEC
	describing each model of the PDP-8.  These 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.

	Maintenance manuals are harder to find, but more valuable.  If you
	need one, you usually need to find someone willing to photocopy
	one of the few surviving copies.  Fortunately, 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.

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.  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.

	The DECtape Library System was an early DECtape oriented save and
	restore system that allowed a reel of tape to hold a directory of
	named files that could be loaded and run on a 4K system.
	Eventually, this supported a very limited tape-based text editor
	for on-line program development.  This did not use the DECtape's
	block addressable character; the software was based on minimal
	ports of the paper-tape based software described above.

	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).

	MS/8 or the R-L Monitor System, was developed starting in 1966
	and 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.

	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 compatably on
	most PDP-8 machines including DECmates, excepting only the PDP-8/S
	and PDP-5.  P?S/8 is still being developed!

	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.  OS/78 was developed from OS/8 to
	support the DECmate I, and OS/278 was developed for the later
	DECmate machines.  These have unnecessary incompatabilities with
	earlier versions of OS/8 and with pre-Omnibus machines.  There are
	also stories that DEC included code in either OS/8 or one of its
	predecessors to make it incompatable with the DCC-112.

	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
	compatable with that used on the PDP-10.

	TSS/8 was developed in 1968 as a timesharing system.  It required
	a minimum of 12K words of memory and a swapping device.  It was
	the standard operating system on the EduSystem 50 which was sold
	to many small colleges and large public school systems.  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.

	Other timesharing systems developed for the PDP-8 include MULTI-8,
	ETOS, MULTOS, and OMNI-8; some of these required nonstandard
	memory management hardware.  By the mid 1970's, some of these were
	true virtual machine operating systems in the same spirit as IBM's
	VM-370; they typically supported some version of OS/8 running on a
	32K virtual PDP-8 assigned to each user.  Some could support
	different user operating systems on each virtual machine, others
	supported addressing of more than 4K for data, but limited code to
	field zero of a process's virtual memory.

	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 CAPS-11.

	WPS was DEC's word processing system, developed on the 8/E and
	widely used on the 1980's vintage machines with a special WPS
	keycaps replacing the standard keycaps on the keyboard.  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.

	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.

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 produce absolute object code and there are
	versions of PAL for minimally configured machines, although these
	have sever symbol table limitations.

	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, and MACREL was
	developed in (unfulfilled) anticipation of similar use.  MACREL was
	heavily used by the DECmate group at DEC.

	There was also RALF, the relocatable assembler supporting RTPS
	FORTRAN, and FLAP, an absolute assembler 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
	from 1964 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.

	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).

	FOCAL, an interpretive language comparable to BASIC, was available
	on all models of the family, including the PDP-5 and PDP-8/S.
	Varsions of FOCAL run under PS/8, P?S/8 and other systems.

	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, versions for OS/8 and TSS/8, an 8K stand-alone
	time-sharing version, and others.

	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.

	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.

	At least two LISP interpreters were written for the PDP-8; one
	runs in 4K, the other can use up to 16K.

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

	TECO, the text editor, is available, and is also a general purpose
	language, and someone is working on a PDP-8 C.  The story of TECO
	on the PDP-8 is convoluted.  Russ Ham implemented TECO under his
	OS8 (without a slash) system.  This version of TECO was pirated by
	the Oregon Museum of Science and Industry (OMSI), where the system
	was ported to PS/8.  Richard Lary and Stan Rabinowitz made it
	more compatible with other versions of TECO, and the result of
	work is the version distributed by DECUS.  RT-11 TECO for the
	PDP-11 is a port of this code.

Where can I get PDP-8 software?

	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 is available on-line
	by anonymous FTP at acfcluster.nyu.edu in subdirectory DECUS.
	To quote the README file from the current 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 [email-masked].

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

	  ftp.telebit.com:/pub/pdp8
	  ftp.update.uu.se:/pub/pdp8
	  sunsite.unc.edu:/pub/academic/computer-science/history/pdp-8.

	The latter archive also maintains an archive of traffic in
	alt.sys.pdp8 in the directory ...pdp8/usenet.

Where can I get additional information?

	The file WHAT-IS-A-PDP8, by Charles Lasner contains considerable
	additional information; this file is included in the telebit.com
	archive cited above.  This file gives details of every PDP-8 model
	including the small quirks and incompatabilities that (to be
	generous) allow software to determine which model it is running on.
	These quirks also make it all too easy for careless programmers to
	write almost portable software with very obscure bugs.

	The mailing list [email-masked] reaches a number of PDP-8
	owners and users, not all of whom have USENET feeds.  The USENET
	newsgroup alt.sys.pdp8 needs to be gatewayed to this mailing list.

	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 (but difficult to read).

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 IBM PC/AT.

	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.

	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 techniques.

Who's Who?

	C. Gordon Bell is generally credited with the original design of
	the PDP-8.  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.

	Ben Gurley designed most of the big 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.

	Ken Olson ran DEC from the beginning.

	Ed Yourdon, who later became well known as a programming methodology
	guru, hacked up the PAL III assembler for the -8, based on PAL II.

	Charles Lasner developed P?S/8, and he is widely known as the grand
	old man of the movement to preserve these historic machines.

	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.

	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, and he has written a PAL-like cross assembler in C.