The PDP -11 is a widely used in the 1970s, 16 -bit computer that was offered by Digital Equipment Corporation. Although not explicitly designed as a successor, he has the previously dominant 12 -bit computer, the PDP -8 replaced in many applications from the Process Control Programmed Data Processor series. The PDP -11 series was introduced in 1970.

The technical concept of the PDP - 11 series was kept simple. A standardized "Universal Bus System" ( Unibus ), communicating with each other via the central processing unit, memory, and Ein-/Ausgabe-Geräte, enabled the upgrading and conversion for a variety of process applications. Therefore, the PDP -11 was often used in experimental science and research.

In contrast to many previous computers the PDP -11 had no special Ein-/Ausgabe-Befehle. Since the Unibus peripherals such as memory were addressed, the periphery could be controlled with "normal" computer commands. Also in the control of power levels, traffic routes and telephone networks, the PDP -11 was used. The open bus system also allowed the third-party cost-effective and high-performance peripherals to be connected to the PDP -11 to bring to the market.

  • 3.1 1 address commands
  • 3.2 2- address commands
  • 3.3 jumps
  • 3.4 addressing Modes 3.4.1 general-purpose registers (R0 to R5)
  • 3.4.2 Stack Pointer
  • 3.4.3 Program Counter (R7 or PC)

PDP-11 models

PDP -11 machine may be classified according to the peripheral bus used.

Unibus models

The following models used the original 18-bit wide Unibus:

  • PDP -11 ( later PDP-11/20 ) and PDP-11/15, the original computer by Jim O'Loughlin with 4K 12 -bit memory
  • PDP-11/35 and 11/40
  • PDP-11/45, 11/ 50 and 11/55 with a significantly faster processor
  • PDP-11/70: 11/45 architecture with up to 4 MiB memory and I / O interfaces on the Massbus
  • PDP-11/05 and 11 /10 cost-reduced 11/20
  • PDP-11/34 and 11/ 04, cost-reduced version on the concept of Bob Armstrong
  • PDP-11/44, improved 11/34 with cache memory and floating point unit, developed by John Sofio
  • PDP-11/60
  • PDP-11/24, first VLSI PDP -11 for Unibus with " Fonz -11" (F11) chipset
  • PDP-11/84, VLSI " Jaws -11" ( J11 ) chipset
  • PDP-11/94, faster variant of the 11/84

Q bus models

The following models used the later introduced cheaper Q- bus, were merged with the address and data lines:

  • PDP-11/03 (also known as LSI-11/03 ) A PDP-11/23, housing cover removed
  • LSI 11/2 ( half - elf ) more compact version of the original LSI -11
  • PDP-11/23 with 248 KB memory
  • MicroPDP-11/23
  • MicroPDP-11/73 with " Jaws -11" ( J -11) chipset
  • MicroPDP-11/53, 11/53 with 1.5 MB memory on board
  • MicroPDP-11/83
  • MicroPDP-11/93, last DEC Q -Bus PDP-11 Model
  • Mentec M100, redesign of the 11/93 of Mentec
  • Mentec M11
  • Quick goods QED -993: PDP-11/93 Processor Upgrade Board

Models without a bus system

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

These models had only the 16 -bit-wide processor and served as intelligent terminals. The series PDT-11/110 and PDT-11/130 used a VT100 terminal housing.

More workstations

  • Per 325 workstation with " Fonz -11" (F11) chipset and floppy
  • Per 350 workstation with " Fonz -11" (F11) chipset, floppy and hard disk
  • Per 380 workstation with " Jaws -11" ( J -11) chipset, floppy and hard disk

Replicas from the Eastern Bloc

The PDP -11 was rebuilt because of their technical importance in the Soviet Union and its allies without a license. Examples are:

  • SM -4, SM -1420, IZOT -1016 (Bulgaria).
  • K 1600 (DDR )
  • Mera (Poland )
  • I- 102 (Romania )
  • SM -4, SM -1420, SM -1600, Elektronika BK -0010, DVK, UKNC (Soviet Union)
  • TPA -51 ( Hungary) " TPA " (Hungarian abbreviation ) " programmable logic analyzer ". Exact replica of the PDP 11/40 from the Institute of Nuclear Physics ( KFKI ) of the Hungarian Academy of Sciences ( MTA). " TPA-11/40 " later in " TPA -51" (11 40) has been renamed.

Operating Systems

Different operating systems were available for the PDP -11:

From DEC:

  • RSX -11, IAS, P / OS
  • CAPS -11
  • RT -11
  • RSTS / E
  • Ultrix -11


  • PEARL Operating System
  • Unix (for example, Version 7 Unix and BSD) DEMOS (Soviet Union)


The PDP 11 has a word length of 16 bits. It can be distinguished Einadressbefehle, Two address commands and jumps. The address is six bits, respectively, where the first three bits are used for addressing the eight and the last three for the selection of one of the eight registers (R0 to R7 ). Many commands are available as word commands and as a byte instructions, that is, they operate with 16- bit or 8 -bit units. The Byteversionen the "double" available commands are given in the following tables in parentheses.

1 address commands

The most important one - address commands are:

  • INC ( INCB) X: incrementing the value by 1
  • DEC ( DECB ) X: decrement the value by 1
  • COM (COMB ) X: Invert X
  • NEG ( NEGB ) X: two's complement of X ( negation )
  • ASR ( ASRB ) X: Arithmetic shift right
  • ASL ( ASLB ) X: Arithmetic Shift to the left
  • ROR ( RORB ) X: Rotate to the right
  • ROL ( rolB ) X: Rotate to the left

2 address commands

2- address commands always follow the pattern " command - source-destination ". When ADD R1, R2 command is thus calculated R2 = R1 R2.

The main 2- address commands are:

  • MOV ( MOVB ) A, B: Copying (B = A)
  • ADD A, B: addition ( B = B A)
  • SUB A, B: subtraction ( B = B - A)
  • MUL A, B: Multiplication (B = B * A)
  • DIV A, B: Division (B = B / A)


When jumping an 8 -bit offset is always specified. This indicates the number of words in order to jump to.

The PDP -11 brings a lot of jump instructions, a total of 18 different.


The addressing modes differ depending on the PDP -11 on whether it is used as registers R0 to R5 ( general purpose registers ), R6 ( stack pointer or stack pointer, SP) or R7 ( program counter, PC).

General-purpose registers (R0 to R5)

An addressing unit is "1" for byte commands, and "2" for word instructions.

Stack pointer

R6 is a pointer to the stack memory which is used by the processor to interrupts for the intermediate storage of the current machine condition. The stack pointer is used to manage the stack, it must basically a word address, ie contain an even address. Therefore, in contrast to the general-purpose registers at the address autoincrement mode or Autodecrement the register R6 is always increased by 2 or decreased, regardless of whether it is a byte or word instruction.

Program counter (R7 or PC)

Use in the 21st century

In facility HASYLAB at the Deutsches Elektronen -Synchrotron in Hamburg served from 1981 to 20 October 2012, a PDP-11/23 at the beamline E1 to control FLIPPER II, a plant for the measurement of photoelectron spectroscopy with synchrotron radiation.

The PDP -11 is still used in nuclear power plants of General Electric. This should remain so until 2050.


In the movie 23 - Nothing is as it seems the PDP -11 is mentioned several times. In the film falsely claimed, a PDP-11 need necessarily a three-phase AC power supply with 380 V. Although there are some " big" PDP-11 models that actually need three-phase alternating current, but there is the vast majority of the PDP-11 computer with a single-phase AC voltage of 110 V or 220 V from. However, since the film is a Einphasenstecker is seen in 32- amp version, could also be meant that the machine can not supply from a normal outlet.

Competing products


There are currently four emulators for PDP-11 server.