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PDP-11/40 with TU56 dual DECtape drive.

The PDP-11 was a series of 16-bit minicomputers sold by Digital Equipment Corp. from 1970[1][2] into the 1990s.[2] Though not explicitly conceived as successor to DEC's PDP-8 computer in the PDP (for Programmed Data Processor) series of computers (both product lines lived in parallel for more than 10 years), the PDP-11 replaced the PDP-8 in many real-time applications. It had several uniquely innovative features, and was easier to program than its predecessors with its use of general registers. It was replaced in the mid-range minicomputer niche by the VAX-11 32-bit extension of the PDP-11.

Smaller personal computers such as the IBM PC, and workstations and servers by vendors such as Sun Microsystems became popular based on standard microprocessors such as the Intel x86 and Motorola 68000, As a PDP-11 based personal computer offering failed in the marketplace, these computers using standard operating systems such as MS-DOS and Unix would eventually evolve up to full 32 bit memory addressing, and replace most proprietary minicomputer and midrange computers including the VAX-11.

Design features of the PDP-11 and its operating systems influenced the design of microprocessors such as the 68000, CP/M and DOS which are still common in current computers and operating systems. The first officially named version of Unix ran on the PDP-11/20 in 1970. The C programming language was written to take advantage of PDP-11 programming features such as byte addressing to rewrite Unix in a high level language.


Unique features of the PDP-11 Series

Instruction set orthogonality

The PDP-11 processor architecture had a mostly orthogonal instruction set. For example, instead of instructions such as load and store, the PDP-11 had a move instruction for which either operand (source and destination) could be memory or register. There were no specific input or output instructions; the PDP-11 used memory-mapped I/O and so the same move instruction was used; orthogonality would even let you move data directly from an input device to an output device. More complex instructions such as add likewise could have memory, register, input, or output as source or destination.

Generally, any operand could apply any of eight addressing modes to eight registers. The addressing modes provided register, immediate, absolute, relative, deferred (indirect), and indexed addressing, and could specify autoincrementation and autodecrementation of a register by one (byte instructions) or two (word instructions). Use of relative addressing let a machine-language program be position-independent.

For these reasons, PDP-11 programmers viewed the assembly language as easy to learn and uniquely elegant.[citation needed]

No dedicated I/O bus

In the most radical departure from earlier computers, the initial models of the PDP-11 had no dedicated bus for input/output; it had only a memory bus called the Unibus. All input and output devices were mapped to memory addresses, so no special I/O instructions were needed.

An input/output device determined the memory addresses to which it would respond and the interrupt priority it would request, and specified its own interrupt vector. This loose framework provided by the processor architecture made it unusually easy to invent new bus devices, including devices to control hardware that had not been contemplated when the processor was designed.

Higher-performance members of the PDP-11 family, starting with the PDP-11/45, departed from the single bus approach. Instead, memory was interfaced by dedicated circuitry and space in the CPU cabinet, while the Unibus continued to be used for I/O only. In the PDP-11/70 this was taken a step further, with the addition of a dedicated interface from disks and tapes, via the Massbus to memory. Use of different buses was not visible to the programmer, however, and the orthogonality of the assembly language was preserved.


The PDP-11 supported hardware interrupts at four priority levels. Interrupts were serviced by software service routines, which could specify whether they themselves could be interrupted (achieving interrupt nesting). The event that caused the interrupt was indicated by the device itself, as it informed the processor of the address of its own service routine.

The article on PDP-11 architecture provides more details on interrupts.

Designed for mass production

Finally, the PDP-11 was designed to be produced in a factory by semiskilled labor. All of the dimensions of its pieces were relatively non-critical. It used a wire-wrapped backplane. That is, the printed circuit boards plugged into a backplane connector. The backplane connectors had square pins that could be connected to by wrapping wires around them. The corners of the pins would bite into the wire to form a gas-tight (i.e. corrosion-proof, therefore reliable) connection.

The LSI-11

Q-Bus board with LSI-11/2 CPU
DEC "Jaws-11" (J11) Chipset
DEC "Fonz-11" (F11) Chipset

The LSI-11 (PDP-11/03) was the first PDP-11 model produced using large-scale integration; the entire CPU was contained on 4 LSI chips made by Western Digital (the MCP-1600 chip set; a fifth chip could be added to extend the instruction set, as pictured on the right). It used a bus which was a close variant of the Unibus called the Q-Bus; it differed from the Unibus primarily in that addresses and data were multiplexed onto a shared set of wires, as opposed to having separate sets of wires, as in the Unibus. It also differed slightly in how it addressed I/O devices and it eventually allowed a 22-bit physical address (whereas the Unibus only allowed an 18-bit physical address) and block-mode operations for significantly improved bandwidth (which the Unibus did not support).

The CPU's microcode includes a debugger: firmware with a direct serial interface (RS-232 or current loop) to a terminal. This let the operator do debugging by typing and reading octal numbers, rather than operating switches and reading lights, the typical debugging method at the time. The operator could thus examine and modify the computer's registers, memory, and input/output devices, diagnosing and correcting failures in software and peripherals except a failure that disabled the microcode itself. On a failure that kept the LSI-11 from booting, the operator could command it to boot from a different disk.

Both innovations increased the reliability and decreased the cost of the LSI-11.

Later Q-Bus based systems such as the LSI-11/23, /73, and /83 were based upon chip sets designed in house by Digital Equipment Corporation. Later PDP-11 Unibus systems were designed to use similar Q-Bus processor cards, sometimes with a special memory bus for improved speed, using a Unibus adapter to support existing Unibus peripherals.

There were significant other innovations in the Q-Bus lineup. A system variant of the PDP-11/03 introduced full system Power-On Self-Test (POST) and the 11/83 introduced a primitive (by today's standards) anticipatory CPU cache pre-load as well as a high-speed private memory interconnect (bus). On the other hand, this was among the last computers to allow the use of magnetic core memory configured as up to three quad-wide modules providing 8KB of memory each.[citation needed]

The chip set was not restricted to implementing a PDP-11; a published book[3] gives the complete source listing of microcode that instead implements the APL programming language.

The decline of the PDP-11

The basic design of the PDP-11 was sound and was continually updated to use newer technologies. However, in the 1980s, inexpensive VLSI memory chips made large amounts of memory affordable and the PDP-11's 16-bit limit on logical addresses proved insurmountable.

The article on PDP-11 architecture describes the hardware and software techniques used to work around this limitation.

DEC's successor to the PDP-11, the VAX (for "Virtual Address Extension") overcame the 16-bit limitation, but was initially a superminicomputer aimed at the high-end time-sharing market. The early VAXes provided a PDP-11 compatibility mode under which much existing software could be immediately used.

Microprocessor chips such as the Motorola 68000 and Intel 80386 began to support 32-bit logical addresses as well. The eventual mass-production of those chips eliminated any cost advantage for the 16-bit PDP-11. A line of personal computers based on the PDP-11, the DEC Professional series, failed commercially, along with other non-PDP-11 PC offerings from DEC.

DEC discontinued PDP-11 production in 1997,[citation needed] and sold the PDP-11 design and system software rights to Mentec, Inc., an Irish producer of LSI-11 based boards for Q-Bus and ISA architecture personal computers. For several years, Mentec produced new PDP-11 processors.

By the late 1990s, not only DEC but most of the New England computer industry which had been built around minicomputers similar to the PDP-11 collapsed in the face of microcomputer-based workstations and servers.

PDP-11 models

The PDP-11 processors tended to fall into several natural groups depending on the original design upon which they are based and which I/O bus they used. Within each group, most models were offered in two versions, one intended for OEMs and one intended for end-users.

Unibus models

PDP-11/70 Front Panel
PDP-11/70, running

The following models used the Unibus as their principal bus:

  • PDP-11 (later renamed the PDP-11/20) and PDP-11/15 — The original, non-microprogrammed processor; designed by Jim O'Loughlin.
  • PDP-11/35 and 11/40 — A microprogrammed successor to the /20; the design team was led by Jim O'Loughlin.
  • PDP-11/45, 11/50, and 11/55 — A much faster microprogrammed processor that could use semiconductor memory instead of or in addition to core memory.
  • PDP-11/70 — The 11/45 architecture expanded to allow 4 MB of physical memory segregated onto a private memory bus, 2 KB of cache memory, and much faster I/O devices connected via the Massbus.[4]
  • PDP-11/05 and 11/10 — A cost-reduced successor to the 11/20.
  • PDP-11/34 and 11/04 — Cost-reduced follow-on products to the 11/35 and 11/05. The PDP-11/09 and 11/39 model names were documented internally to DEC but never produced for sale. The PDP-11/34 concept was created by Bob Armstrong.
  • PDP-11/44 — An extension of the 11/34 that included the cache memory as a standard feature and optional floating point unit. This machine also included a sophisticated serial console and support for 4 MB of physical memory. The design team was managed by John Sofio. NOTE: DEC documentation (PDP-11/44 System User's Guide, EK-11044-UG-003) shows the floating point unit as an option (FPP-11F). Moreover, others have documented the 11/44 as a descendent of the 11/45 and 11/70; the 11/34 was of a different 'lineage', following the 11/05.
  • PDP-11/60 — A PDP-11 with user-writable microcontrol store; this was designed by another team led by Jim O'Loughlin.
  • PDP-11/24 — First VLSI PDP-11 for Unibus, using the "Fonz-11" (F11) chip set
  • PDP-11/84 — Using the VLSI "Jaws-11" (J11) chip set
  • PDP-11/94 — J11-based, faster than 11/84

Q-bus models

LSI-11/23, cover removed

The following models used the Q-Bus as their principal bus:

  • PDP-11/03 (also known as the LSI-11/03) — The first LSI PDP-11, this system used a chipset from Western Digital.
  • PDP-11/23 — 2nd generation of LSI (F-11), early units only supported 248 KB memory, but could be modified for 4 MB support
  • PDP-11/23+/MicroPDP-11/23 — Improved 11/23 with more functions on processor card (physically a quad-size card rather than dual)
  • MicroPDP-11/73 — The third generation LSI PDP, this system used the "Jaws-11" (J-11) chip set.
  • MicroPDP-11/53 — slower 11/73 with on-board memory
  • MicroPDP-11/83 — faster 11/73 with PMI (private memory interconnect)
  • MicroPDP-11/93 — faster 11/83; final DEC Q-Bus PDP-11 model.
  • Mentec M100 — Mentec redesign of the 11/93, with J-11 chipset at 19.66 MHz, 4 onboard serial ports, 1-4 MB on-board memory, and optional FPU.
  • Mentec M11 — processor upgrade board; last microcode implementation of PDP-11 instruction set by Mentec, using the TI 8832 ALU and TI 8818 microsequencer from Texas Instruments
  • Quickware QED-993 — high performance PDP-11/93 processor upgrade board

Models without standard bus

  • PDT-11/110
  • PDT-11/130
  • PDT-11/150

The PDT series were desktop systems marketed as "smart terminals". The /110 and /130 were housed in a VT100 terminal enclosure. The /150 was housed in a table-top unit which included two 8" floppy drives, 3 asynchronous serial ports, 1 printer port, 1 modem port and 1 synchronous serial port and required an external terminal. All three employed the same chipset as used on the LSI-11/03 and LSI-11/2 in four "microm"s. There was an option which combined two of the microms into one dual carrier, freeing one socket for an EIS/FIS chip. The /150 in combination with a VT-105 terminal was also sold as MiniMINC, a budget version of the MINC-11.

  • PRO-325
  • PRO-350
  • PRO-380

The DEC Professional series were desktop PCs intended to compete with IBM's earlier 8088 and 80286 based personal computers. The models were equipped with 5 1/4" floppy disk drives and hard disks, except the 325 which had no hard disk. The original operating system was P/OS, which was essentially RSX-11M+ with a menu system on top. As the design was intended to avoid software exchange with existing PDP-11 models, their ill fate in the market was no surprise for anyone except DEC. RT-11 was eventually ported to the PRO series. A port to the PRO for RSTS/E was also done internal to DEC, but was not released. The PRO-325 and -350 units were based on the DCF-11 ("Fonz") chipset, the same as found in the 11/23, 11/23+ and 11/24. The PRO-380 was based on the DCJ-11 ("Jaws") chipset, the same as found in the 11/53,73,83 and others, though running only at 10 MHz because of limitations in the support chipset.

Models that were planned but never introduced

  • PDP-11/27 — A Jaws-11 implementation that would have used the VAXBI Bus as its principal I/O bus.
  • PDP-11/68 — A follow-on to the PDP-11/60 that would have supported 4 MB of physical memory.
  • PDP-11/74 — A PDP-11/70 that was extended to contain multiprocessing features. Up to four processors could be interconnected, although the physical cable management became unwieldy. Another variation on the 11/74 contained both the multiprocessing features and the Commercial Instruction Set. A substantial number of prototype 11/74's (of various types) were built and at least two multiprocessor systems were sent to customers for beta testing, but no systems were ever officially sold. A four processor system was maintained by the RSX-11 operating system development team for testing and a uniprocessor system served PDP-11 engineering for general purpose timesharing. The 11/74 was due to be introduced around the same time as the announcement of the new 32 bit product line and the first model - the VAX 11/780. Rumour/Legend or conspiracy theory held that the reason the 11/74 was cancelled was due to its higher performance compared to the 11/780 (see, for example [1]). Marketing was therefore concerned that the availability of a higher performing PDP-11 would slow migration to the new VAX. This was not the case. Rather, the ability to maintain the product in the field was the issue. However conspiracy or not, DEC was never able to successfully migrate its entire PDP-11 customer base to the VAX. The primary reason was not performance, but the PDP-11's superior real-time responsiveness.

Special purpose versions

DEC GT40 running Lunar Lander
  • GT40 — VT11 vector graphic terminal using a PDP-11/05
  • GT42 — VT11 vector graphic terminal using a PDP-11/10
  • GT44 — VT11 vector graphic terminal using a PDP-11/40
  • GT62 — VS60 vector graphic workstation using a PDP-11/34a
  • H-11Heathkit OEM version of the LSI-11/03
  • VT20 — Terminal with PDP-11/05 with direct mapped character display for text editing and typesetting (predecessor of the VT71)
  • VT71 — Terminal with LSI-11/03 and QBUS backplane with direct mapped character display for text editing and typesetting
  • VT103 — VT100 with backplane to host an LSI-11
  • VT173 — A high-end typseset terminal containing an 11/03
  • MINC-11 — Laboratory system based on 11/03 or 11/23; when based on the 11/23, it was sold as a 'MINC-23', but many MINC-11 machines were field-upgraded with the 11/23 processor. Early versions of the MINC-specific software package would not run on the 11/23 processor because of subtle changes in the instruction set; MINC 1.2 is documented as compatible with the later processor.
  • C.mmp — Multiprocessor system from Carnegie Mellon University
  • SBC 11/21 (boardname KXT11) Falcon and Falcon Plus — single board computer on a Qbus card implementing the basic PDP11 instruction set, based on T11 chipset containing 32KB static RAM, 2 ROM sockets, 3 serial lines, 20 bits parallel I/O, 3 interval timers and a 2-channel DMA controller. Up to 14 Falcons could be placed into one Qbus system.

Unauthorized clones

The PDP-11 was sufficiently popular that many unauthorized PDP-11-compatible minicomputers and microcomputers were produced in Eastern Bloc countries. At least some of these were pin-compatible with DEC's PDP-11s and could share peripherals and system software. These include:

Operating systems

Several operating systems were available for the PDP-11

From Digital

From third parties

See also

Use of PDP-11

The PDP-11 family of computers were used for many purposes. It was used as a 'normal' computer but also often as system to control complex systems like traffic-light systems, medical systems or for network-management. An example of the use of PDP-11's was the management of the packet switched network Datanet 1.


  1. ^ Bell, Gordon & Strecker, Bill. What We Learned From the PDP-11. (1975). p139. Accessed September 10, 2008.
  2. ^ a b 16-bit Timeline. Accessed September 10, 2008.
  3. ^ Rodnay Zaks. A microprogrammed APL implementation ISBN 0895880059
  4. ^ 1 MB = 1024 KB
  5. ^ a b c d e f g h i j k


  • pdp11 processor handbook - pdp11/05/10/35/40, Digital Equipment Corporation, 1973 
  • pdp11 processor handbook - pdp11/04/34a/44/60/70, Digital Equipment Corporation, 1979 

Further reading

  • Eckhouse, jr., Richard H.; Morris, L. Robert (1979). Microcomputer Systems Organization, Programming and Applications (PDP-11). Englewood Cliffs, New Jersey: Prentice-Hall. ISBN 0-13-583914-9. 
  • Michael Singer, PDP-11. Assembler Language Programming and Machine Organization, John Wiley & Sons, NY: 1980.

External links

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