Archive-name: dec-faq/pdp8
Last-modified: May. 12, 1993

Frequently Asked Questions about the DEC PDP-8 computer.

 By Douglas Jones, jones@cs.uiowa.edu
 (with help from many folks)

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 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?

 For over a decade, all programmable digital computers sold by
 Digital Equipment Corporation were sold as Programmed Data
 Processors (PDPs) instead of computers.  I have DEC documentation
 that actually calls them "PDPs", so this is not improper usage.
 DEC's first computer, the PDP-1, had a selling price of only
 $120,000 at a time when competing machines were selling for over
 $1,000,000.  Everyone (the government and DEC's stockholders
 included) knew that computers were big and expensive and needed
 a computer center and a large staff; DEC chose to avoid dealing
 with these stereotypes by entirely avoiding the term "computer".

 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; the PDP-5 may also have been built on this
 basis.  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                  36    One was built by a customer, not by DEC.
 PDP-4  1962            18    Predecessor of the PDP-7.
 PDP-5  1963   $27,000  12    The ancestor of the PDP-8.
 PDP-6  1964            36    A big computer; 23 built, most for MIT.
 PDP-7  1965  ~$60,000  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            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 above are the prices for minimal systems in the year the
 machine was first introduced.  The bits column indicates the word
 size.  It's worth noting that the DEC PDP-10 became the
 DECSYSTEM-20 as a result of marketing considerations, and DEC's
 VAX series of computers began as the Virtual Address eXtension of
 the never-produced PDP-11/78.

 It is worth mentioning that there are persistant rumors that the
 the Data General Nova was originally developed as the PDP-X, a
 16-bit multi-register version of the PDP-8.  A prototype PDP-X
 was apparently 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 possibly also Gordon Bell, who was at C-MU at the
 time) evaluated the competing designs and rejected both in favor
 of the design now known as the PDP-11.  (A less common variant on
 this story is that the reason that DEC never produced a PDP-13
 was because this number was assigned to what became the Nova;
 this is unlikely because the PDP-X prototype predated the PDP-11.)
 Both DEC and Data General don't talk publically 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.
 

What is a PDP-8?

 The PDP-8 family of minicomputers were built by Digital Equipment
 corporation between 1965 and 1990 (if you include the PDP-5, the
 starting date should be 1963).  These machines were characterized
 by a 12 bit word, with no hardware byte structure, a 4K minimum
 memory configuration, and a simple but powerful instruction set.
 By 1970, the PDP-8 was the best selling computer in the world, and
 many models of the PDP-8 set new records as the least expensive
 computer on the market.  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 (who later was chief architect of the PDP-11 and
 who, as Vice President, oversaw the development of the VAX) says
 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, sold starting around 1959, certainly was a very similar 12
 bit architecture, and the peripheral processors of Cray's first
 supercomputer, the CDC 6600, are also familiar to PDP-8
 programmers.

 Note that the CDC 160 and CDC 6600 peripheral processors 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 frequently
 just as effective as the code on the more complex CDC 12-bit
 minicomputers.


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 for indirect addressing.  These locations are called the
 auto-index registers (despite the fact that they are in memory),
 and 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 subroutine calling sequence involves putting the return
 address in relative word zero of the subroutine, with execution
 starting with relative word one.  Return from subroutine is done
 with an indirect jump through the return address.  Subroutines
 frequently increment their return addresses to index through
 inline parameter lists or to provide return codes by conditionally
 skipping the next instruction.

 The IOT instruction has the following form:
   _ _ _ _ _ _ _ _ _ _ _ _
  |1|1|0|_|_|_|_|_|_|t|c|s|
  |     |           |     |
  |     |   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 as follows:  If the s bit is set, the
 instruction causes a skip if the device is ready, if the c bit is
 set, the device ready status is reset and, for some devices, AC is
 also cleared, and if the t bit is set, data is either ored with AC
 or output from AC to the device.  Prior to the PDP-8/E, there were
 severe restrictions on the interpretation of the t, c and s bits.

 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, as
 are instructions controlling the optional memory management
 unit that is needed to address more than 4K words.

 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?

 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 some 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.

 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, PAL 8, has trouble when unary
 operators are mixed with multiplication or division.
 
 Generally, identifiers are not limited in length, but only the
 first 6 characters are significant.  All numeric constants are
 in octal, unless a DECIMAL pseudo-op has been used to change number
 base (change back with the OCTAL pseudo-op).

 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.


What different PDP-8 models were made?

 The total sales figure for the PDP-8 family is estimated at over
 300,000 machines.  Over 8500 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 63-65   Transistor Limited compatibility
 PDP-8 65-68  >1000 $18,500 Transistor Table-top or rack
 LINC-8 66-69 153 $38,500 Transistor Rack only
 PDP-8/S 66-70? >1000? $10,000 Transistor  Incompatable, slow!
 PDP-8/I 68-70? >2000? $12,800 TTL  Pedistal or rack
 PDP-8/L 68-70? >2000?  $8,500 TTL  Scaled down 8/I (1)
 PDP-12 69-71 3500 $27,900 TTL  Followup to LINC-8
 PDP-8/E 70-78 >10K?  $7,390 TTL MSI Omnibus Table-top or rack
 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? <$7K TTL LSI Omnibus New CPU or 8/E CPU
 VT78 (2)78-80 > ? Microprocessor  Intersil IM6100
 Dm I (3)80-84   Microprocessor Harris 6120
 Dm II 82-86   $1,435 Microprocessor Harris 6120
 Dm III 84-90   $2,695 Microprocessor
 Dm III+ 85-90   Microprocessor

 Notes   (1) Memory upgrade to 32K words was eventually sold.
  (2) The VT78 was also known as the DECstation 78.
  (3) Dm stands for DECmate.

 When possible, the costs given in the above table are for a minimal
 system in the first year of production; for most PDP-8 systems,
 such a system would have 4K of main memory, a console teletype,
 and the minimal software needed to use the machine (FOCAL, BASIC,
 or a paper-tape based assembler).  Additional information on costs
 and production is needed!

 The above list does not include many PDP-8 variants sold by DEC
 to meet the needs of various special users.  For example, the
 Industrial-8 was really just a PDP-8/E with a different nameplate
 and color scheme.  Burger King had thousands of PDP-8/M based
 point-of-sale systems with no standard peripherals.  In addition,
 DEC made many peripheral controllers for the PDP-11 and PDP-15
 that used IM6100 and 6120 microprocessors from Intersil and Harris.

 The last years of the PDP-8 family were dominated by the DECmate
 machines.  DEC sold these primarily as word processing systems,
 and in the end, they chose to obscure the ability of the DECmate
 systems to run any software other than WPS, DEC's word processing
 system.

 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, possibly a DEC PDP-8/L in drag
 DCC-112  70-71 Digital Computer Controls
 DCC-112H 71 Digital Computer Controls
 6100 Sampler 7? Intersil, their IM6100 promotional kit
 Intercept 7? Intersil, based on IM6100
 Intercept Jr 7? Intersil, based on IM6100
 PCM-12  7? Pacific CyberMetrix, based on Intercept bus
 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.

 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 built using DEC's first family of logic modules,
 and 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.

 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.  This was not a general purpose dual processor, in the
 sense of allowing both machines to execute in parallel, but
 rather, a machine with the hardware of both but restrictions
 that effectively prevented more than one from running at a time.

 The sales 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 developed as the LINC-8/I, but it was sold as the
 PDP-12; thousands were sold.  Both the LINC-8 and the PDP-12 had
 impressive consoles, with full sets of lights and switches for
 the registers of each processor.

 These machines 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 instruction sets.

 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.

 LAP-6 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.
 Reuters bought the tail end of the Omnibus based production run.

 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.

 It is worth noting that the PDP-8, when it was introduced in 1965,
 was about as fast as was practical with the logic technology used
 at the time; only by using tricks like memory interleaving or
 pipelining could the machine have been made much faster.

 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?

 The 1973 Introduction to Programming was probably DEC's definitive
 manual for this family, but it is 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 in
 the trash.

 Maintenance manuals are harder to find, but more valuable.
 Generally, you'll need to find someone who's 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, 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, a BASIC only system submitted to DECUS and later sold
 by DEC as part of its EduSystem marketing program.

 P?S/8, developed starting in 1971 from an MS/8 foundation.  Runs
 on minimal PDP-8 configurations, supports device semi-independant
 I/O and a file system on a random-access device, including DECtape.
 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.

 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 could support 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 required
 OS/8 as the user system and only allowed code to execute from
 virtual 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 that was widely used on the
 1980's vintage machines with a special WPS keycaps replacing the
 standard keycaps on the keyboard.  This was written in the 1970's,
 and was the primary system used on the DECmate 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 OS/8 compatable,
 and a few applications can run under either COS or OS/8.


What programming languages are supported on the PDP-8

 The PAL family of assembly languages are as close to a standard
 assembly language as can be found for the PDP-8.  These produce
 absolute object code and versions of PAL will run on minimally
 configured machines (but with a small symbol table).  Assembly
 of large programs frequently requires far more memory for symbol
 table management.

 MACRO-8 was DEC's first macro assembly language for the PDP-8, but
 it was never used outside the paper-tape environment except under
 the OS8 operating system.  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.

 BASIC was also available, and was widely used on PDP-8 systems
 sold under the EduSystem marketing program.  A paper-tape version
 was available that ran in 4K, there were versions that ran under
 OS/8 and TSS/8, there was an 8K stand-alone time-sharing version,
 and there were many 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.

 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 PAL-8 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 INFORMATION@DECUS.ORG.

 There is a young but growing FTPable archive of PDP-8 software at
 ftp.telebit.com in directory /pub/pdp8.  Another archive that
 contains considerable PDP-8 related material, along with material
 related to other DEC computers, is at sunsite.unc.edu in directory
 /pub/academic/computer-science/history/.


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 FTPable
 archive cited above.  This file gives details of every model
 of the PDP-8, 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 pdp8-lovers@ai.mit.edu reaches a number of PDP-8
 owners and users, not all of whom have USENET feeds.  The USENET
 newsgroup alt.sys.pdp8 is fairly new, but someday, the newsgroup
 and mailing list should be gatewayed to each other.

 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
 complete (and difficult to read).  Considering Bell's role in the
 design of the PDP-8 and the history of DEC, the description in
 this book should be accurate!


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 have much practical use.

 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, are building 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 seems to have been 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 also recommended 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.

 Ben Gurley designed most of the big DEC machines, starting with
 the PDP-1.

 Ken Olson ran DEC from the beginning.

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