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Various bits on this site are from Douglas W. Jones FAQ's The PDP-8 family of minicomputers were built by Digital Equipment Corporation (DEC) between 1965 and 1990. It was the follow on to the PDP-5 computer with some instruction set changes. By late 1973, the PDP-8 family was the best selling computer in the world. Download Acer support drivers by identifying your device first by entering your device serial number, SNID. Laptops Desktops Tablets Monitors Smartphones Projectors Mixed Reality Headsets. (Windows operating systems only) Or select your device: Category. Download drivers to communicate with NI products or third-party instruments. 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.

Contents

What is a PDP?

In 1957, Ken Olsen and Harlan Anderson founded Digital EquipmentCorporation (DEC), capitalized at $100,000, and 70% owned by AmericanResearch and Development Corporation. Olsen and Anderson had designedmajor parts of the AN/FSQ-7, the TX-0 and the TX-2 computers atLincoln Labs. They wanted to call their company Digital ComputerCorporation, but the venture capitalists insisted that they avoid theterm Computer and hold off on building computers.

With facilities in an old woolen mill in Maynard Massachusetts, DEC'sfirst product was a line of transistorized digital 'systems modules'based on the modules used in building TX-2 at Lincoln Labs; thesewere plug-in circuit boards with a few logic gates per board. Startingin 1960, DEC finally began to sell computers (the formal acceptance ofthe first PDP-1 by BBN is reported in Computers and Automation, April1961, 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 whenother computers sold for over $1,000,000. (A good photo of a PDP-1 isprinted in Computers and Automation, Dec. 1961, page 27). DEC quotedprices as low as $85,000 for minimal models. The venture capitalist'sinsistance on avoiding the term computer was based on the stereotypethat computers were big and expensive, needing a computer center and alarge staff; by using the term Programmable Data Processor, or PDP, DECavoided this stereotype. For over a decade, all digital computers soldby 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 computerswithout a leasing plan. IBM, Burroughs, CDC and other computermanufacturers leased most of their machines, and many machines werenever offered for outright sale. DEC's cash sales approach led to thegrowth of third party computer leasing companies such as DELOS, aspinoff of BB&N.

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

* 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! As of 2002, William Vickers still had some of the original PDP-3 design documents.

** Includes DECsystem 20.

*** Number difficult to imagine counting, since these were, to asignificant extent, kit machines that were built by the customeras embedded systems.

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

Max Dietrich recalls that at least one (and possibly more) defectivePDP-9 computer was tossed from the DEC's old woolen mill building inMaynard Massachusetts into the millpond. If there is a scuba diverin the Maynard area, we might be able to verify this story.

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

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

In any case, when DeCastro, Burkhardt and Soggee founded Data General,Ken Olsen at DEC was very angry, claiming for a long time that theNova design was stolen; Olsen only approved the PDP-11 architectureafter Data General launched the Nova.Gordon Bell and others concluded that theNova design was sufficiently original that a lawsuit was unwarranted,but the feud between DeCastro and Olsen lasted until after Ken Olsenleft DEC. It is more correct to say that the Nova is a reaction to thePDP-X than to say that it is based on the PDP-X. I am indebted toJim Campbell, retired VP at Data General, for some of the details ofthis story, with additional comments Gordon Bell and Luis Villalobos.

Today, all of the PDP machines are in DEC's corporate past. ThePDP-11 family survived for the longest time, and as of 2013, a fair numberof the later machines may still be in service as embedded controllers.DEC discontinued PDP-11 sales on Sept. 30, 1996. There were a few machinesknown as PDPs that were not built by DEC; generally, these were built fromDEC hardware as one-off efforts and called it a PDP with a new number.For example, the Australian Atomic Energy Commissiononce upgraded a PDP-7 by adding a PDP-15 on the side; they called theresult a PDP-22. There is also a story about the PDP-2 1/2, built byEd Rawson of the American Science Institute out of surplus modules thatwere originally used in the prototype PDP-2.

In 1998, Compaq purchased DEC, and DEC's corporate identity was slowlysubmerged. DEC's most interesting product at the timewas the Alpha RISC architecture, a 64-bit architecture developed toreplace the VAX. Compaq worked to phase this out, sellingthe rights to the Alpha CPU to Intel in 2001. When HP bought Compaq in 2002,they agreed to continue development of Alpha software until 2004 and theycontinued selling Alpha systems to support the existing customer base untilApril 2007.

What is a PDP-8?

The PDP-8 family of minicomputers were built by Digital EquipmentCorporation between 1965 and 1990, although it is worth noting that theterm minicomputer first came into prominence after the machine wasintroduced. The first use of the term appears to have been made bythe head of DEC's operations in England, John Leng. He sent back asales report that started: 'Here is the latest minicomputer activityin 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 fromthere; the first computer explicitly sold as a minicomputer, though,was made by by Interdata (See the Interdata ad in Computers andAutomation, May 1968, page 10).

The PDP-8 was largely upward compatible with the PDP-5, a machine thatwas unveiled on August 11, 1963 at WESCON, and the inspiration for thatmachine came from two earlier machines, the LINC and the CDC 160.Both of these machines were characterized by a 12 bit word with little or nohardware byte structure, typically 4K words of memory, and simple butpowerful instruction sets. The LINC was particularly important becauseit was built with DEC parts.

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

C. Gordon Bell has said that the basic ideas behind the PDP-5 and -8 werenot really original with him. He gives credit to Seymour Cray (of CDCand 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 ofCDC 160 systems sold as stand-alone machines, a derivative 12 bitarchitecture was used for the I/O processors on Cray's first greatsupercomputer, the CDC 6600.

Note that Cray's 12 bit machines had 6 basic addressing modes withvariable length instruction words and other features that were far fromthe simple elegance of the PDP-8. Despite its many modes, the CDC 160architecture lacked the notion of current page addressing, it had nounconditional jump instruction, and the I/O instructions all blockedthe CPU until I/O complete. As a result, the PDP-8 is both far moreflexible and it supports much tighter programming styles.

Wesley Clark's LINK architecture was particularly important becauseit was made from DEC modules, thereby demonstrating the price-performancepoint that a 12-bit machine could achieve using DEC's technology. Theoperating system developed for the LINC, with its LINCtape-based filesystem and graphics display, makes it an excellent candidate for thehonor of first personal computer or first workstation.

What is the PDP-8 instruction set?

The PDP-8 word size is 12 bits, and the basic memory is 4K words. Theminimal 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 .

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

Instruction words are organized as follows:

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

The basic instructions are:

• 000 - AND - and operand with AC.
• 001 - TAD - add operand to (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 nextinstruction in sequence. The ISZ is commonly used to increment a loopcounter and skip if done, and it is also used as an general incrementinstruction, either followed by a no-op or in contexts where it is knownthat the result will never be zero.

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

The IOT instruction has the following form:

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

As an example of the use of IOT instructions, consider the consoleterminal interface. On early PDP-8 systems, this was always assumed tobe an ASR 33 teletype, complete with low-speed paper tape reader andpunch. It was addressed as devices 03 (the keyboard/reader) and 04(the teleprinter/punch):

The keyboard flag is set by the arrival of a character. The KCCinstruction clears both the flag and the accumulator. KRS ors the 8 bitinput data with the low order 8 bits of AC. The commonly used KRBinstruction is the or of KCC and KRS. To await one byte of input, useKSF to poll the flag, then read the byte with KRB.

The teleprinter flag is set by the completion of the TPC operation (asa result, on startup, many applications output a null in order to getthings going). TCF clears the flag, and TPC outputs the low order 8bits of the accumulator. The commonly used TLS instruction is the orof 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 blockdevices such as disk or tape. The term 'data break' was, for years,DEC's preferred term for cycle-stealing direct-memory-access datatransfers.

Some CPU functions are accessed only by IOT instructions. For example,interrupt enable and disable are IOT instructions:

An interrupt is requested when any device raised its flag. The consolemaster clear switch resets all flags and disables interrupts. Ineffect, an interrupt is a JMS instruction to location zero, with theside effect of disabling interrupts. The interrupt service routineis expected to test the device flags and perform the operations neededto reset them, and then return using ION immediately before the indirectreturn JMP. The effect of ION is delayed so that interrupts are notenabled until after the JMP.

The instructions controlling the optional memory management unit arealso IOT instructions. This unit allows the program to address up to32K of main memory by adding a 3 bit extension to the memory address.Two extensions are available, one for instruction fetch and directaddressing, the other for indirect addressing.

A wide variety of operations are available through the OPR microcodedinstructions:

In general, the above operations can be combined by oring the bitpatterns for the desired operations into a single instruction. If noneof the bits are set, the result is the NOP instruction. When theseoperations are combined, they operate top to bottom in the order shownabove. The exception to this is that IAC cannot be combined with therotate operations on some models, and attempts to combine rotateoperations 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 theaccumulator, while previous models took this to mean something like'shift neither left nor right 2 bits').

The above operations may be combined by oring them together, except thatthere are two distinct incompatible groups of skip instructions. Whencombined, SMA, SZA and SNL, skip if one or the other of the indicatedconditions are true (logical or), while SPA, SNA and SZL skip if all ofthe indicated conditions are true (logical and). When combined, theseoperate top to bottom in the order shown; thus, the accumulator may betested and then cleared. Setting the halt bit in a skip instruction isa 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 leastsignificant bit) deals with the optional extended arithmetic element toallow such things as hardware multiply and divide, 24 bit shiftoperations, and normalize. These operations involve an additional dataregister, MQ or multiplier quotient, and a small step count register.On the PDP-8/E and successors, MQ and the instructions for loading andstoring it were always present, even when the EAE was absent, and theEAE was extended to provide a useful variety of 24 bit arithmeticoperations.

What does PDP-8 assembly language look like?

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

Note that labels are terminated by a comma, and comments are separatedfrom the code by a slash. There are no fixed fields or columnrestrictions. The 'CLA CLL' instruction on the first line is an exampleof the microcoding of two of the Group 1 operate instructions. CLAalone has the code 7200 (octal), while CLL has the code 7100; combiningthese as 'CLA CLL' produces 7300. As a general rule, except when memoryreference instructions are involved, the assembler simply ors togetherthe 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 noexplicit notation to distinguish between page zero and current pageaddresses. Instead, the assembler is expected to note the page holdingthe operand and automatically generate the appropriate mode. If theoperand is neither in the current page nor page zero, some assemblerswill raise an error, others will automatically generate an indirectpointer to the off-page operand; this should be avoided because it onlyworks for directly addressed off-page operands, and only when the memorymanagement unit is not being used to address a data field other than thecurrent instruction field.

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

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

Assemblers that support this automatically fill the end of each pagewith constants defined in this way that have been accumulated during theassembly of that page. Note that the variants '(3' and '[5' (with noclosing parentheses) are usually allowed but the use of this sloppy formis discouraged. Furthermore, the widely used PAL8 assembler interpretsthe unlikely operand '(3)+1' as being the same as '(3+1)'.

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

Other operators allowed include and (&), or (!), multiply (^) and divide(%), as well as a unary sign (+ or -). Unfortunately, one of the mostwidely used assemblers, PAL8, has trouble when unary operators are mixedwith multiplication or division.

Generally, only the first 6 characters of identifiers are significantand numeric constants are evaluated in octal.

Other assembly language features are illustrated below:

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

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

Conditonally assembled code must be enclosed in angle brackets. Theenclosed code may extend over multiple lines and, because differentassemblers treat comments within conditionals differently, the closingbracket should not be in a comment and any brackets in comments shouldbe balanced.

What character sets does the PDP-8 support?

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

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

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

Internally, PDP-8 programmers are free to use other character sets, butthe 'X notation provided by the assembler encourages use of 7 bit ASCIIwith the 8th bit set to 1, and the TEXT pseudo-operation encourages the6 bit character set called 'stripped ASCII'. To map from upper-case-onlyASCII to stripped ASCII, each 8 bit character is anded with octal 77 andthen packed 2 characters per word, left to right. Many programs use asemi-standard scheme for packing mixed upper and lower case into 6 bitTEXT form; this uses ^ to flip from upper to lower case or lower toupper case, % to encode CR-LF pairs, and @ (octal 00) to mark end ofstring. Note that this scheme makes no provision for encoding the %,^ and @ characters, nor does it allow control characters other than theCR-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 anynonzero filler to a word boundary, and end of file is 0000.

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

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

What different PDP-8 models were made?

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

Additional information is available inpart two of this FAQ, where allknown models of the PDP-8, along with variants, alternate marketingnames, and other peculiarities are given.

The last years of the PDP-8 family were dominated by the PDP-8 compatiblemicroprocessor based VT78 and DECmate workstations. The Intersil 6100,also known as the CMOS-8 chip, was developed in 1976; GE later acquiredIntersil. DEC also used the followup Harris 6120 microprocessors(Introduced 1981) in many peripheral controllers for the PDP-11 andPDP-15 as well as in the DECmate series of systems. While all of theearlier PDP-8 systems were open architecture systems, the DECmates hadclosed architectures with an integrated console terminals and limitedperipheral options. It is interesting to note that the Harris 6120 wasa 10Mhz chip and some chips could be clocked at 15Mhz; furthermore, the6120 was essentially based on gate array technology.

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

More information on the Hungarian TPA series, built by KFKI (the CentralResearch Institute for Physics), was provided by Varga Akos Endre,hamster@telnet.hu; information on and photos of these machines arecurrently available from:

The original machine in this series, the TPA1001, was built from thedescription in DEC's Small Computer Handbook. Only after the series wasin production, when a machine was exhibited in Ljubljana, Yugoslavia, wasfull DEC compatability demonstrated, when a DEC user booted the TPA machinefrom a DEC paper tape. By the end of the TPA production run, around 900PDP-8 compatable machines had been built. Given the Soviet era centralplanning for the computer industries in eastern europe, it is quite possiblethat the Electrotechnica and Electronica models listed above may have beenTPA 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 namePDP used by DEC! There was a decree that computer development in Hungary wasto cease, with all computers to be purchased from the USSR. In response, thepeople at KFKI ceased developing computers and began developing 'Stored ProgramAnalyzers' or, the acronym for which is TPA in Hungarian.

According to Thomas Lopez at theUniversidad de las Ciencias Informátics in Havana,the Cuban CID-201, -201A and -201B minicomputerswere built from the description in DEC's Introduction to Programming.The first prototype, implemented with DTL IC logic, was operational inApril 1970. Given that DEC never did a DTL implementation of the PDP-8,the Cuban hardware qualifies as an original reimplementation of thearchitecture. The Cuban efforts were written up inDatamation in December, 1973.In the mid to late 1970s, there was some interchange between the Cuban andHungarian groups. By that time, both Cubans and Hungarians were at work onPDP-11 clones.

The CESI (Computer Extension Systems, Inc.) machine had 128K words of localRAM 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-sliceprocessor chips were used to build the 12-bit ALU and data paths, controlledby an 80-bit microword.

What about the LINC/8 and PDP-12?

Wesley Clark and Charles Molnar, then at Lincoln Labs, built the LINC, orLaboratory INstrumentation Computer, as a personal laboratory computer,finishing the first in March 1962. The machine was developed in responseto the needs of Mary Brazier, a neurophysiologist at MIT who needed betterlaboratory 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, inJanuary 1963, the project moved to MIT, and then in 1964, to WashingtonUniversity in St Louis. The National Institute of Health funded theproject as an experiment to see if coumputers would be a productive toolin the life sciences. By the end of 1963, 20 LINCs had been built anddebugged, many by their eventual users.

Over 24 LINC systems were built by customers before late 1964 when DECbegan selling a commercial version (see Computers and Automation, Nov.1964, page 43). By the time DEC introduced the LINC-8, 43 LINC systemshad been installed (see Computers and Automation, Mar. 1966, page 34).In total, 50 LINC systems were built, 21 by DEC, 29 by customers (seeDigital at Work, page 52). A photo of the last LINC in production useis available from http://www.mit.edu:8001/people/ijs/epl/LINC.htmlWesley Clark wrote a history of the LINC, 'The LINC was Early andSmall', 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 SystemModules, 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 wellsuited for general purpose computation, but the common peripheralsneeded for lab work such as analog-to-digital and digital-to-analogconverters were all bundled into the LINC system. Users judged it tobe 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 drivedeveloped for the TX-2 at Lincoln Labs. LINCtapes could be carelesslypocketed or dropped on the floor without fear of data loss, and theyallowed random access to data blocks. Stockebrand improved on this ideaslightly after he came to DEC, where the improved idea was calledDECtape; DECtape was widely used with all DEC computers made in thelate 1960's and early 1970's.

The motives behind the development of LINCtape were the same motivesthat led IBM to develop the floppy disk almost a decade later, and infact, DECtape survived as a widely used medium until DEC introduced theRX01 8 inch floppy disk drive around 1975, and even after this, DECtapewas 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. Thesuccess of the LINC-8 led DEC to re-engineer the machine using TTLlogic in the late 1960's; the new version was originally to be calledthe LINC-8/I, but it was sold as the PDP-12. Both the LINC-8 and thePDP-12 had impressive consoles, with separate sets of lights andswitches for the LINC and PDP-8 halves.

The success of the LINC-8 also led to the development of a clone, theSPEAR micro-LINC. This machine used Motorola MECL integrated circuitsand 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 primitiveinterrupt structure of the original LINC architecture; on the LINC-8,all interrupts were handled by the PDP-8 side of the hardware. Becausethe LINC-8 and PDP-12 had instructions for switching between modes, anew body of software was developed that required both modes.

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

Various versions of LAP, the Linc Assembly Program, were the dominantassemblers used on the LINC; the original version of LAP was a crossassembler written on the TX-2. WISAL (WISconson Assembly Language) orLAP6-W was the version of this assembler that survived to run on thePDP-12. Curiously, this includes a PDP-8 assembler written in LINC code.

LAP6-DIAL (Display Interactive Assembly Language) evolved from this onthe PDP-12 to became the dominant operating system for the PDP-12. The8K version of this is DIAL MS (Mass Storage), even if it has only twoLINCtape drives. These were eventually displaced by the OS/8 variantknown as OS/12.

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 CESImay still make their clone, for a high price, but you can't buy a newDEC PDP-8. There are quite a few PDP-8 machines to be found in oddplaces on the used equipment market. They were widely incorporated intoproducts such as computer controlled machine tools, X-ray diffractionmachines, and other industrial and lab equipment. Many of them weresold under the EduSystem marketing program to public schools anduniversities, and others were used to control laboratory instrumentation.After about 1976, Reuters bought as many as 10,000 OMNIBUS basedmachines per year, with perhaps 2000 per year going to other customers.

Through the 1980's and 1990's, PDP-8 hardware was frequently discardedor sold for scrap, and many collectors were able to obtain CPU's,peripherals and occasional complete systems in exchange for the effortrequired to haul them away. This may be changing! In early 1999, aPDP-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, butfar more than the scrap value of such a system! Owners of original DEChardware will likely need maintenance and test supplies. DouglasElectronics still makes extender boards and breadboards in DEC format,see:

As of 2000, there were still a modest number of PDP-8 systems inproduction use, mostly PDP-8/A systems. These were supported by ashrinking number of commercial suppliers and maintenance contractors.For example, Michael Coffey (mdynac@netscape.net) advertised theavailability of spare parts and maintencance documents for Omnibussystems.

By the year 2000, a number of PDP-8 parts and systems had changed handson E-bay, and this continues in 2013. Sadly, many systems are soldpiecemeal, with parts such as core memory being auctioned off in a way thatmakes it likely that they will be framed and hung on den and office wallsinstead of being used to preserve a vanishing breed of computer.The market isspotty, but looking back over the ebay.com sales history is a good wayto 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 areindistinguishable from the real machine from a software prespective,and on a modern high-speed RISC platform, these frequently outperformthe 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 theNova and other DEC machines as well as the IBM 1401.

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

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

The Spare Time Gizmos designed theFP6120 adecade or more ago. This was a PDP-8 clone based on the Harris 6120 chip, andthe plans are on-line; with the SBC6120 front planel, this hasthe look and feel of a PDP8-E, while drawing a small fraction of the powerand occupying 1/20th the volume. It even included a laptop-pc hard-drive anda compact-flash card serving for mass storage. In early 2013, they putthe SBC6120 up onKickstarter and got 30 backers at $599 each, allowing for a new(albeit short) production run.

ThePiDP-8/Iis a Raspberry Pi behind arecreation of the PDP-8/I front panel. A modified version of SimH runs on theRaspberry, using the front panel to accurately and completely emulate a 32KPDP-8/I with a 10 MB RK08 hard drive.

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

'Modern VLSI Design - A system approch' by Wayne Wolf (1994 Prentice-Hall)also uses the PDP-8 as a by Mark Gordon Arnold (Prentice Hall).

It is worth noting that there were a sufficient number of PDP-8 systemsstill operational that in 2000, some companies still made peripherals.For example, Storage Computer(formerly www.storage.com)made RK05compatable semiconductor 'disk drives' that can be directly connected tothe Omnibus RK05 controllers of the PDP-8/E,F,M and A.

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

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

Where can I get PDP-8 documentation?

The key documents published by DEC describing each model of the PDP-8are all out of print, and DEC was in the habit of printing much oftheir documentation on newsprint with paperback bindings, which is tosay, surviving copies tend to be yellow and brittle. DEC distributedhuge numbers of catalogs and programming handbooks in this inexpensivepaperback 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 booksbeing thrown out.

Douglas Jones has made a small number of bound photocopies of DEC's1973 introduction to programming, perhaps the definitive introductionto the PDP-8, and the other early DEC handbooks need similar treatmentbefore 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 maintenancemanual, and prints of various boards have been scanned in and madeavailable on the web at:

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

Much more material was available from:

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

What operating systems were written for the PDP-8?

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

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

The 4K Disk Monitor System provided slightly better facilities. Thissupported on-line program development and it worked with any device thatsupported 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 byRichard F. Lary; it was submitted to DECUS in 1970. This was a diskoriented system, faster than the above, with tricks to make it runquickly on DECtape based systems.

POLY BASIC was a BASIC only system submitted to DECUS and later sold byDEC as part of its EduSystem marketing program. PAL source code forEduSystem 25 Basic is available from:

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

Richard F. Lary developed a system called the Fully Upward CompatibleKeyboard Monitor; and between a Wednesday and the following Friday, aprototype was up and running from DECtape. The original intention ofthis project was to build a programming environment for the PDP-8 thatlooked like TOPS-10 on the PDP-10.As the acronym was distinctly impolite, when the product was released ayear later, it became Programming System/8 (or PS/8). Comments in thesource code called it the THE *BLEEP* MONITOR.The *bleep* monitor was renamed OS/8 in 1971 becauseEli Glaser (a salesman from Long Island) said he could sell more systemswith an operating system than with a programming system, and because, byrenaming the system, DEC could increase the price despite Nixon'swage-price freeze.

OS/8, developed in parallel with P?S/8, became the main PDP-8 programmingenvironment sold by DEC. The minimum configuration required was 8K wordsand a random-access device to hold the system. For some devices, OS/8requires 12K. There are a large number of OS/8 versions that are notquite portable across various subsets of the PDP-8 family. RX01 images ofOS/8 Version 3Q are available, with DEC's free non-commercial use licence,from:

The second site above also includes an incomplete but useful RK05 imageof 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 thisdistribution, support for Omnibus machines was no longer important. OS/78V4 was developed for the DECmate I, and the name OS/278 used for theversions released with later DECmate machines. These have unnecessaryincompatabilities with earlier versions of OS/8. OS/278 and relatedmaterial is available from DECUS as catalog item 800941, or from:

A growing collection of OS/8 documentation, including the OS/8 softwaresupport 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, anextended arithmetic unit, and DECtape hardware. Unlike most PDP-8operating systems, it uses a directory structure on DECtape compatiblewith that used on the PDP-10.

The timesharing system TSS-8 was developed by Don Witcraft and John Everettat DEC, starting in late 1967, and with the first beta sites up and runningin the fall of 1968. This was based on a protection architecture proposed byAdrian 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 a24K word machine, it could give good support for 17 users. It wasthe standard operating system on the EduSystem 50 which was sold to manysmall colleges and large public school systems. The first installation wasat Lexington High School in Massachusetts, and the second was at NorthernArizona University. Each user gets a virtual 4K PDP-8; many of theutilities users ran on these virtual machines were only slightly modifiedversions of utilities from the Disk Monitor System or paper-tapeenvironments. 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 andAlgol were available.

Significant parts of TSS-8 have been found, but at this time, nobody hasmanaged to recover a complete working system. Much of the available TSS/8code could formerly 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 operatingsystem in the spirit of IBM's VM/370, and a special board was requiredto optionally trap JMP and JMS instructions; this was enabled after anemulated CIF instruction so that the actual change of instruction fieldcould be emulated when the JMP or JMS was attempted. After leaving Quodataand founding Network-Systems Design in 1976, Dempsey went on to developOMNI-8, first installed at Ripon College; initially it was priced at $4900,several hundred copies were sold. The OMNI-8 operating system supportedthe enlarged PDP-8 address space of the CESI (Computer Extension SystemsInc) memory cards, and when CESI began making PDP-8 clones, OMNI-8 wasextended to support asymmetric multiprocessors (one CPU handled the I/O).The end of OMNI-8 development came around 1990. Dumps of the ETOS kerneland drivers survive in various places, and Jim Dempsey still has the fullsource for OMNI-8.

Other timesharing systems developed for the PDP-8 include MULTI-8and 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 fileformat on cassette is compatible with a PDP-11 based system calledCAPS-11.

RTS/8 was a real-time system developed by DEC, developed from an earliersystem, SRT8, dating back to at least 1974. Curiously, even the lastversions of RTS/8 continued to support paper-tape and DECtape. RTS/8 alsooffered a virtual PDP-8 for background processing, unlike ETOS, this didnot require special hardware; instead, software emulation was used to retaincontrol of the machine between the CIF instruction and a followingJMP or JMS. Source code for most of the versions of RTS and SRT isavailable from:

WPS was DEC's word processing system, developed for the 8/E with a VT50terminal with special WPS keycaps replacing the standard keycaps, andwidely used on the 1980's vintage machines. It was heavily promoted onthe VT-78, and when the DECmates came out, DEC began to suppress knowledgethat DECmates could run anything else. WPS-11 was a curious distributedsystem 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 andDECmate III under the catalog entry DM0114.

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

SCPSYS, developed by D. C. Amoss prior to 1974 at Clemson University, isa system that copies most of the features of LAP (the LINC AssemblyProgram) for a pure PDP-8 based system. A DECtape of this system hasrecently 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 supportingassembly and text editing on a 4K machine.

What programming languages are supported on the PDP-8

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

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

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

RALF, the relocatable assembler supporting RTPS FORTRAN is also includedin OS/8 standard distributions. FLAP, an absolute assembler, was derivedfrom RALF. Both SABR and RALF/FALP are assemblers that handle theirintended applications but have quirky and incompatible syntax.

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

RTPS FORTRAN required 8K and a floating point processor; it had real-timeextensions and was a full implementation of FORTRAN IV (also known asANSI FORTRAN 66). OS/8 F4 is RTPS FORTRAN stripped of the requirementfor hardware floating point (if the hardware is missing, it usessoftware emulation). Versions of FORTRAN is available from

FOCAL, an interpretive language comparable to BASIC, was available onall models of the family, including the PDP-5 and PDP-8/S. Versions ofFOCAL run under OS/8, P?S/8 and other systems, and there were many specialpurpose overlays for FOCAL developed by DEC and by various users. DEC'slater FOCAL releases for the PDP-8 included code to deliberately introducesubtle bugs when run on a DCC 112 computer! Various versions of FOCALare 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 EduSystemmarketing program. A paper-tape version was available that ran in 4Kand was compatible with disk based systems, versions distributed withOS/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 was 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 torun under the COS operating system.

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

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

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

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

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

TECO, the text editor, was included in the standard OS/8 distributions andis a general purpose language (the Emacs editor began as a set of TECOmacros!). The story of TECO on the PDP-8 is convoluted. Russ Hammimplemented TECO under his OS8 (without a slash) system, and then gavea 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 thebeginning of what became Oregon Software, later famous for OMSI Pascal.

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

DECUS also lists the PAGE8 language (catalog numbers 800936), the VISTAeditor (catalog number 800938), and the ICE text editor (catalog number800939).

Where can I get PDP-8 software?

DEC was bought by Compaq which was then bought by HP, which is still makingcomputers. Unfortunately, they've largely forgotten about thePDP-8. This URL still works, but redirects to HP as of late 2013.

DECUS, the DEC User Society, is still alive and well, billing themselvesas 'a Compaq user's group.'The old DECUS library catalog, however, is no-longer on-line at decus.org:

Significant parts of the DECUS library are on line at:

Unfortunately, this appears to be an unmaintained archive, and it isnot clear that older DECUS media have been transcribed onto the modernservers.

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

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

The latter archive also maintains an archive of traffic in alt.sys.pdp8in the directory ..pdp8/usenet and an archive of traffic in thepdp8-lovers mailing list in ../pdp8/pdp8-lovers.

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

Where can I get additional information?

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

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

The mailing list PDP8-Lovers@dbit.com reaches a number of PDP-8owners and users. This list does not accept postings fromnonsubscribers; to subscribe, send mail to PDP8-Lovers-request@dbit.com

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The pdp8-lovers mailing list has previously been hosted by onelist.com,mc.lcs.mit.edu, ai.mit.edu, zach1.tiac.net, egroups.com and yahoo.com.The archives for these older lists should be available on dbit.com, butcurrently, the best archive is at ftp://zach.dyndns.org/pub/pdp8-lovers.

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

What use is a PDP-8 today?

What use is a Model T today? Collectors of both come in the same basicclasses. First, there are antiquarians who keep an old one in thegarage, 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 stillworks, but they don't make much practical use of it.

PDP-8 systems maintained by antiquarians are frequently in beautifulshape. Antiquarians worry about dust, chipped paint, and missingswitches, and they establish newsgroups and mailing lists to help themlocate 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 originalfrom the outside, or keeping the old mechanism and putting it to usesthat were never intended. Some PDP-8 owners, for example, have builtPDP-8 systems with modern SCSI disk interfaces! There is seriousinterest in some quarters in constructing an omnibus board that wouldsupport an IDE disk of the variety that was mass-produced for theIBM PC/AT.

Last, there are the old folks who still use their old machines for theirintended purposes long after any sane economic analysis would recommendsuch 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-8machines are simple enough that end users can maintain and repair themindefinitely. All you need to keep a vintage -8 running are a stockof inexpensive silicon diodes and a stock of 2N3639B or better,2N3640 transistors.

Unlike most modern personal computers, PDP-8 systems were routinely soldwith 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 farbetter documented and far more open.

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

Finally, the PDP-8 is such a minimal machine that it is an excellentintroduction to how computers really work. Over the years, many studentshave 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 veryappropriate environment for learning operating system programmingtechniques.

Who's Who?

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

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C. Gordon Bell

is generally credited with the original design of thePDP-8 (as well as designing the PDP-4, 5 and 6). He was also involvedwith recommending what became the PDP-11 when that design was competingwith the PDP-X (the design that led to Data General's Nova).As vice president of research, he oversaw the development of the DEC VAXfamily. (On Apr. 3, 2012, Bell sent E-mail declaring this FAQ'quite accurate.')

Alan Kotok

worked with Bell in working up the original specificationsof the PDP-8.

Edson DeCastro

was a key man in the design of the PDP-5 through thePDP-8/L and the never-marketed 16-bit PDP-X. He left DEC to foundfounded Data General to build the Nova, a machine that could be considereda reaction to the PDP-X.

Ben Gurley

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

Warren K. Smith

was manager of applications engineering for Flip-Chipmodules between 1969 and 1975. Much of the M-series TTL module familydates from this period.

Saul B. Dinman

was product line manager for the module product line from1966 to 1969; he designed the PDP-8/S and built the engineering prototype,largely in his spare time. Most of his time was devoted to the K-seriesof industrial automation modules. Later, he founded GRI Computer Corp,where he designed the GRI 909 16-bit minicomputer.

Ken Olsen

ran DEC from the beginning.

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A number of others have played a part, some DEC employees and some workingfor customers or competitors:

Jozsef Lukacs and Janos Bogdany

designed the Hungarian TPA1001implementation of the PDP-8 instruction set.

Laszlo Szonyi and Pal Karadi

designed the Hungarian TPA/i clone of the PDP-8.

Ed Yourdon

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

Richard Merrill

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

Luis Villalobos

Consulted for DEC from 1962 to 1966. Did design reviews on thelogic and wrote diagnostics for many early DEC processors, starting withthe PDP-1.

Richard F. Lary

developed the RL Monitor System, and then went on todevelop OS/8, with help from Ed Friedman and another programmer namedPaul, under the management of Chuck Conley.

Charles Lasner

developed P?S/8, and he is widely known as a leader inthe movement to preserve these historic machines. He created thealt.sys.pdp8 newsgroup on Usenet.

George Thissell

oversaw the development of OS/8 FORTRAN-IV, with DennyPavlock as part of the team.

Wesley Clark

developed the 9-bit FX-1 and thenthe 12-bit LINC while working at Lincoln Labs; the latter wasthe first 12 bit minicomputer built with DEC parts, and served asan important inspiration for the PDP-5.

Mary Allen Wilkes Clark

developed the early LAP programs for the LINC.

Don Witcraft

wrote the TSS-8 scheduler, command decoder and UUOhandler, after working on the first swapping monitor for the PDP-10.

John Everett

worked with Roger Pyle on the the PDP-8 Disk Monitor System,and adapted PAL-III to make PAL-D for DMS. This was the first disk-basedversion of PAL.After this, he wrote the disk handler, file system, TTY handlerand 680-I service routine for TSS-8. (He also contributed correctionsto this FAQ).

Roger Pyle

wrote the PDP-8 Disk Monitor System with John Everett.

Bob Bowering

author of MACRO (the assembler for the -6 and -10), wrote a crossassembler called PAL-X, which ran on the PDP-10 and producedPDP-8 binary code. Its first use was for the development of TSS-8, butit was a 'spare time' unfunded project.

Jimmy Dykes

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

Max Dietrich

was hired by DEC in 1968 to write the PDP-X FORTRAN compiler,after 2-years as a PDP-8 programmer at the University of Iowa.His career at DEC spanned the PDP-8 through PDP-15, before heleft to join Tandem as a developer of their Non-Stop operating system.

Douglas W. Jones

wrote this FAQ, but prior to the summer of 1992, he'dnever used a PDP-8. He has also written a report on how to photocopyand archivally bind ailing paperback books such as DEC's handbook series, aPAL-like cross assembler in C, and a UNIX-based PDP-8 emulator.

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Document history

This document had its origins in a USENET FAQ that was postedperiodically in the 1990s, ending in 2001. Here is the headingon the last version distributed over USENET:

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Clickable links in this document have been verified to be useful asof late 2013. URLs that don't respond to clicking are either dead orunchecked. Maintenance of this web page is on a sporadic basis, at best.