PDP-11

The PDP-11 is a series of minicomputers introduced in 1969 [1] by the Digital Equipment Corporation (DEC), in production by them from 1970-1990. Their life-time spanned a period of momentous changes in the computer world: when they were first introduced integrated circuits had just been introduced (small ones, containing only a few simple gates); and for a little less than half their life, core memory was still the standard main memory technology. By the end, the now-ubiquitous microprocessors and dynamic RAM had completely taken over.

Front panel of the KA11, the first PDP-11 model

It was the machine which made UNIX, which now seems to have taken over the world (in the form of Linux) widely known and popular.

The machine word size was 16 bits, and it was a general register architecture. Although it was not the first to feature the latter, its wide distribution (in 1980, it was the world's best-selling computer) and its novel addressing modes (below) have helped influence almost all later machines to follow that path.

(For more information about the architecture, including its innovative and much-copied stack-oriented addressing modes, see here.)

PDP-11's came in two groups: those which used the UNIBUS for a bus, and the later ones which used the QBUS. Eventually DEC stopped producing UNIBUS PDP-11's (the last were the PDP-11/44 and PDP-11/24); later 'UNIBUS' machines (the PDP-11/84 and PDP-11/94) actually contained QBUS processors with a QBUS<->UNIBUS adapter board.

Towards the end of its life, there were several microprocessor implementations of the PDP-11, as chips. After DEC discontinued production of PDP-11's, the line was sold to Mentec, who produced a few newer models.

PDP-11 Models and notes

Model	Introduced	Bus Type	Addressing	Split I & D	Notes	Speed (VUPS)
11/20	1969[1]	UNIBUS	16-bit	no
11/05	1972	UNIBUS	16-bit	no	OEM model
11/10[*]	1972	UNIBUS	16-bit	no
11/15	1972	UNIBUS	16-bit	no	OEM model
11/35	1973	UNIBUS	18-bit	no	OEM model
11/40	1973	UNIBUS	18-bit	no
11/45	1973	UNIBUS	18-bit	yes	core memory
11/50	1975	UNIBUS	18-bit	yes	MOS memory
11/55	1976	UNIBUS	18-bit	yes	fast bipolar memory
11/70	1975	UNIBUS	22-bit	yes		0.6
11/03	1975	QBUS	16-bit	no	first QBUS model	0.05
11/04	1976	UNIBUS	16-bit	no		0.11
11/34	1976	UNIBUS	18-bit	no		0.21
11/60	1977	UNIBUS	18-bit	no	writable control store
11/23	1979	QBUS	18-bit or 22-bit	no	first F-11	0.12
11/24	1979	UNIBUS	22-bit	no	only UNIBUS model to use F-11 chip	0.18
11/44	1979	UNIBUS	22-bit	yes	last non-LSI PDP-11	0.42
11/23+	1981 Nov	QBUS	22-bit	no		0.18
11/73	1983	QBUS	22-bit	yes	first J-11 machine, 15MHz, integrated FPU, also first PMI PDP-11	0.45
11/53	1984	QBUS	22-bit	yes	S-box or standard QBUS, integrated FPU, 768KiW memory	0.29
11/83	1988	QBUS	22-bit	yes	J-11 at 18MHz, integrated FPU	0.72
11/84	1988	UNIBUS	22-bit	yes	J-11 at 18MHz, integrated FPU	0.72
11/93	1990	QBUS	22-bit	yes	J-11 at 18MHz, integrated FPU, 2MiW onboard memory	1.0
11/94	1990	UNIBUS	22-bit	yes	J-11 at 18MHz, integrated FPU, 2MiW onboard memory	1.0

[*]The name PDP-11/10 was recycled by DEC from an earlier KA11 CPU-based 11/10 from 1969, or at least it existed in advertisements[1]

Operating Systems

There were a number of well-known operating systems for the PDP-11. Many were produced by DEC themselves, but several were produced by third-parties. Often DEC would purchase or re-brand such an OS and re-sell it as their own product; for example, UNIX sold as Ultrix by DEC.

UNIX was not originally a PDP-11 system (it was born on the PDP-7), but it moved to the PDP-11 shortly after it was first created, and spread widely there (since PDP-11's were common machines, due to their relatively low cost and good engineering), with the real explosion of use starting with UNIX Sixth Edition.

UNIX

• UNIX First Edition
• UNIX Second Edition
• UNIX Third Edition
• UNIX Fourth Edition
• UNIX Fifth Edition
• UNIX Sixth Edition
• Unix Seventh Edition

These are the original Bell Laboratories releases of Unix; the first 4 were only internal to Bell, the Fifth saw limited distribution outside it, and the Sixth took over the world.

• LSX
• MINI-UNIX

For PDP-11's without memory management.

• System III

This was the first shipping Unix distro by AT&T. It only supported the PDP-11 and VAX computers.

• 2.9 BSD

This version was a port of the 4.3 BSD feature set to the PDP-11. Although considered an impossible goal by many, it accomplished this by using overlays for portions of the kernel, and to allow for user programs larger than 64KB.

• 2.10 BSD
• 2.11 BSD

This last version is still supported, and if one really felt the need to load a Unix for use on a PDP-11 this would be the best fit. It has support for TCP/IP, large memory space and is the best UNIX experience one can get going to get on a 16-bit mini.

DEC OS's

• DOS-11
• RSX-11
• RSTS/E - a descendant of RSTS-11
• RT-11
• IAS

Technically also an OS is XXDP, which is an 'overseer' for running the PDP-11 diagnostics produced/provided by DEC.

Real-time OS's

PDP-11's were very popular in real-time and data networking uses; a number of OS's were developed for such environments:

• ANTS
• ELF
• MOS - an ELF descendant
• MERT - another Bell system
• MINITS - network appliance framework

Miscellaneous

• SITS - "Small ITS" for running Logo
• Camexec - hosted the Camex typesetting system
• Trantor - written for the MIT Applied Mathematics Department

Simulators

There a number of good simulators for the PDP-11:

• SIMH
• Ersatz-11

The latter is less well known than the ubiquitous SIMH, but is very fast; simulated systems running on Ersatz-11 are usually faster than real PDP-11's.

New Emulator Hardware

• UniBone/QBone

See also

• PDP-11 architecture
• PDP-11 Memory Management
• MicroNotes
• UNIBUS memories
• QBUS memories
• UNIBUS peripherals
• QBUS peripherals
• UNIBUS boot ROMs
• QBUS boot ROMs
• H95xx-H96xx Series of DEC Cabinets and Accessories
• List of VAX models with PDP-11 compatibility mode
• PDP11GUI

Further reading

• PDP-11 Family Differences, MicroNote #053

External links

• PDP11 - Documentation at Bitsavers
  • memos - Interesting internal DEC memos about the PDP-11
    • How Good is the PDP-11? - interesting early comparison with competitors
• PDP-11 Handbook (1969)
• PDP-11 Variations on a Theme - marketing brochure from DEC
• A new architecture for mini-computers - The DEC PDP-11 - the AFIPS paper which introduced the PDP-11
• PDP-11 Recognition - Illustrated guide to identifying PDP-11 models
• The PDP-11 archives - miscellaneous software, links
• The PDP-11 archives at metalab.unc.edu
  • PDP-11 hardware - large collection of odds and ends
• PDP-11
  • The Pre-Life of a Legend is another Legend
  • How the PDP-11 Was Born (according to Larry McGowan)
• PDP-11 history and links to materials
• Henk's DIGITAL computer room - an amazing collection of PDP-11's
• A brief tour of the PDP-11, the most influential minicomputer of all time - has some minor errors, but generally good
• Proposal for PDP-11 I/O Architecture
• What Have we Learned From the PDP-11? - a retrospective on a group of papers by Gordon Bell
• Computer Structures: What Have we Learned From the PDP-11?

References

1. PDP-11/20 Price List