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

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 microcomputer implementations of the PDP-11, as chips.

It could run a variety of operating systems. Many were produced by DEC themselves, but several were produced by third-parties. 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, with UNIX Sixth Edition. 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.

After DEC discontinued production of PDP-11's, the line was sold to Mentec, who produced a few newer models.

Operating Systems

Unix based Operating Systems

• 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.

• 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 impossible by many, it accomplished this by using overlays for portions of the kernel, and to allow for user programs larger then 64KB.

• 2.10 BSD
• 2.11 BSD

This 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.

Other OS's

• RSX-11
• RSTS/E
• RT-11

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

PDP-11 Models and notes

Model	Introduced	Bus Type	Addressing	Notes	Speed (VUPS)
11/20	1969[1]	UNIBUS	16-bit
11/05	1972	UNIBUS	16-bit	OEM model
11/10[*]	1972	UNIBUS	16-bit
11/15	1972	UNIBUS	16-bit	OEM model
11/35	1973	UNIBUS	18-bit	OEM model
11/40	1973	UNIBUS	18-bit
11/45	1973	UNIBUS	18-bit	core memory
11/50	1975	UNIBUS	18-bit	MOS memory
11/55	1976	UNIBUS	18-bit	fast bipolar memory
11/70	1975	UNIBUS	22-bit		0.6
11/03	1975	QBUS	16-bit	first QBUS model	0.5
11/04	1976	UNIBUS	16-bit		0.11
11/34	1976	UNIBUS	18-bit		0.21
11/60	1977	UNIBUS	18-bit	writable control store
11/23	1979	QBUS	18-bit or 22-bit	first F-11	0.12
11/24	1979	UNIBUS	22-bit	only UNIBUS model to use F-11 chip	0.18
11/44	1979	UNIBUS	22-bit	last non-LSI PDP-11	0.42
11/23+	1981 Nov	QBUS	22-bit		0.18
11/73	1983	QBUS	22-bit	first J-11 machine, 15MHz, integrated FPU, also first PMI PDP-11	0.45
11/53	1984	QBUS	22-bit	S-box or standard QBUS, integrated FPU, 768KiW memory	0.29
11/83	1988	QBUS	22-bit	J-11 at 18MHz, integrated FPU	0.72
11/84	1988	UNIBUS	22-bit	J-11 at 18MHz, integrated FPU	0.72
11/93	1990	QBUS	22-bit	J-11 at 18MHz, integrated FPU, 2MiW onboard memory	1.0
11/94	1990	UNIBUS	22-bit	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]

See also

• PDP-11 architecture
• PDP-11 Memory Management
• UNIBUS memories
• QBUS memories
• UNIBUS boot ROMs
• QBUS boot ROMs
• PDP11GUI

External links

• PDP11 - Documentation at Bitsavers
  • memos - Interesting internal DEC memos about the PDP-11
• PDP-11 Handbook (1969)
• 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
• 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

• PDP-11/20 Price List