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PDP-11

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PDP-11/40 with dual DECtape drives.
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PDP-11/40 with dual DECtape drives.

The PDP-11 was a series of 16-bit minicomputers sold by Digital Equipment Corp. in the 1970s and 1980s. The PDP-11 was a successor to DEC's PDP-8 computer in the PDP series of computers. It had several uniquely innovative features, and was easier to program than its predecessors. While well-liked by programmers, it was eventually superseded by personal computers, including the IBM PC and Apple II.

Contents

Unique Features of the PDP-11 Series

Instruction Set

Programmers liked the PDP-11 design because it had a highly-orthogonal instruction set which allowed a programmer to separately memorize all of the operations and the methods of accessing operands. They could then predict that any access method (or "addressing mode") would work with any operation; they did not have to learn a list of exceptions or special cases in which an operation had a special or restricted set of addressing modes.

The instruction set architecture of the PDP-11 influenced the idiomatic use of the C programming language. The register increment and decrement addressing modes correspond to the --i and i++ constructs in C. If i and j were both register variables, an expression such as *(--i) = *(j++) could be compiled to a single machine instruction. The lack of different opcodes for single and double floating point operations resulted in the omission of the single precision operations in the language,

In some logical sense, the set of addressing modes provided one "basis", and the set of operations provided another. Each two-operand instruction was separated into two six-bit operand identifiers (each consisting of a three-bit register number, and a three-bit addressing mode) and a four-bit op-code; single-operand instructions had one six-bit operand identifier, and a ten-bit op-code. All op-codes operated with any operand identifier address mode (or combination of them, for the two-operand instructions). Of the 8 registers (numbered 0 through 7), 7 were general-purpose and could be used for most purposes, although register 6 was specially recognized by the hardware as the stack pointer for some instructions; register 7 was the program counter. This latter innovation, together with some of the addressing modes, provided constants, absolute addresses, and relative (position independent) addressing.

32-bit words were typically stored in a middle-endian format. Due to the popularity of the PDP-11, this format is still sometimes referred to as pdp-endian.

No Dedicated I/O Bus

In the most radical departure from other, earlier computers, 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 addresses in memory, so no special I/O instructions were needed. The interrupt system was intentionally designed to be as simple as possible, while ensuring that no event in an interrupt sequence could be missed. A device would request an interrupt by asserting a common input into one of four priority lines; the processor would respond over an interrupt daisy chain grant line, one for each priority level. (A daisy chain is a sequence of logic gates arranged in series to order events. Generally the first logic gate has first access to the grant. The daisy chain order established the order of the devices at that priority level.)

In the case of the PDP-11 design, this meant that the interrupt grant order was determined by how close the physical hardware was to the CPU on the bus. When the CPU responded, the device would place its vector address on the bus; this was the address of a 4-byte block of memory. The CPU would then load the status register and program counter from the vector table; the new contents of the status register would generally temporarily disable interrupts. The address in the program counter would be the starting address of the code to run for the interrupt. The interrupt code would then service the device, and in the process, write to the interrupting device to re-enable the interrupt signal. Finally, a special RTI (return from interrupt) instruction would return the CPU to where it was before the interrupt (which might have been in a lower-priority interrupt). Note that this process prevents loss of interrupts; at every stage, if the interrupt is not serviced, it remains in place, to be sensed on the next cycle. If a sequence were erroneously started, the CPU would time out, generating a special spurious interrupt; the spurious interrupt would warn users of bad hardware.

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 noncritical. It used a wire-wrapped backplane. That is, the printed circuit board plugged into a backplane connector. The backplane connector had terminals that could be connected to by wrapping wires around them. The terminal would cut the insulation around the wire and bite into the wire to form a gas-tight (i.e. corrosion-proof, therefore reliable) connection. The connector blocks were very similar to telephone connection blocks.

The LSI-11

The LSI-11 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). 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 (which the Unibus did not support).

The CPU's microcode includes a debugger that directly communicated to a standard RS-232 terminal. This was innovative because the microcode is the part of the irreducible guts of the computer, a critical part of the control unit. If it doesn't work, there is no computer. The debugger provided a way to examine the computer's registers, memory and input and output devices. Thus, if the CPU worked at all, it was possible to examine and correct the computer's internal state. The built-in debugger avoided the expense and inconvenience of a front panel with an array of switches and lights, which was then the typical way to enter digital data into a near-dead computer.

The microcode also included a generic bootstrap, to which all DEC disk drives were compatible.

These two innovations meant that most of the time, the computer just worked. If it did not boot from its big disk, it would boot from its floppy. If the hardware worked at all, it talked to you through a terminal in a familiar way.

The Decline of the PDP-11

The basic design was extremely good, and was continually updated to use newer technologies. Ultimately, however, the 16-bit architecture proved to be a limitation which could not be overcome by tweaks and add-ons. While some models could support larger physical address spaces using memory mapping hardware, all programs were restricted to a 16-bit virtual address space. When inexpensive VLSI memory chips became available, PDP-11 software was not capable of using large amounts of memory easily.

DEC's own successor to the PDP-11, the VAX (for "Virtual Address Extension (to the PDP-11)") addressed all of these issues, but was initially aimed at the high-end market.

As engineers migrated to architectures that supported a larger address space, 32-bit computing began to be supported on microprocessor chips such as the Motorola 68000 and Intel 386 families; eventually the economics of large-scale production of those chips made them so cheap there was no cost advantage for the PDP-11.

DEC discontinued the final PDP-11 models in 1997. The PDP-11 design and operating system licenses were finally sold to Mentec, Inc., an Irish producer of LSI-11 based boards for Q-Bus and ISA architecture personal computers.

Architectural Details

The following information is found in DEC's PDP-11 Processor Handbook (see Gordon Bell's [1969 edition]).

General register (R is a general register, 0 to 7; (R) is the contents of that register.)
0. Register - the value is to or from a register: OPR R ; R contains operand
1. Register deferred - register is used as a memory address to read or write: OPR (R) ; R contains address
2. Autoincrement: OPR (R)+ ; R contains address, then increment (R)
3. Autoincrement deferred: OPR @(R)+ ; R contains address of address, then increment (R) by 2
4. Autodecrement: OPR -(R) ; Decrement (R), then R contains address
5. Autodecrement deferred: OPR @-(R) ; Decrement (R) by 2, then R contains address of address
6. Index: OPR X(R) ; (R)+X is address, second word of instruction
7. Index deferred: OPR @X(R) ; (R)+X is address (second word) of address

Program Counter The program counter (PC) can also be used as a general purpose register, providing the following effectively additional addressing modes, using the mechanisms of the addressing modes above:
2. Immediate: OPR #N ; Operand is contained in the instruction
3. Absolute: OPR @#A ; Absolute address is contained in the instruction
6. Relative: OPR A ; PC+2+X is address. PC+2 is updated PC
7. Relative deferred: OPR @A ; PC+2+X is address of address. PC+2 is updated PC

PDP-11 instructions

15 6 5 3 2 0
OP-Code Mode Register

15 12 11 9 8 6 5 3 2 0
OP-Code Mode Register Mode Register

15 8 7 0
OP-Code Offset

Assembly Language Programming Example

A papertape used for PDP-11
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A papertape used for PDP-11

A complete "Hello, world!" program in PDP-11 macro assembler, to run under RT-11: 

.TITLE  HELLO WORLD
.MCALL  .TTYOUT,.EXIT
HELLO:: MOV     #MSG,R1 ;STARTING ADDRESS OF STRING
1$:     MOVB    (R1)+,R0 ;FETCH NEXT CHARACTER
BEQ     DONE    ;IF ZERO, EXIT LOOP
.TTYOUT         ;OTHERWISE PRINT IT
BR      1$      ;REPEAT LOOP
DONE:   .EXIT


MSG:    .ASCIZ /Hello, world!/
.END    HELLO
If this file is HELLO.MAC, the RT-11 commands to assemble, link and run (with console output shown) are as follows: 
.MACRO HELLO
ERRORS DETECTED:  0


.LINK HELLO


.R HELLO
Hello, world!
.
(The RT-11 command prompt is ".")

For a more complicated example of MACRO-11 code, two examples chosen at random are Kevin Murrell's [KPUN.MAC], or Farba Research's [JULIAN] routine. More extensive libraries of PDP-11 code can be found in the [Metalab] freeware and [Trailing Edge] archives.

You can try out the above for yourself on a PDP-11 emulator. Bob Supnik's outstanding SIMH emulates the PDP-11 and a variety of other architectures, and includes software kits for native operating systems (including RT-11).

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

The following models used the Unibus as their principal bus:

Q-bus models

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

Models without standard bus

The PDT series were desktop systems marketed as "smart terminals". The /110 and /130 were housed in a VT100 terminal enclosure.

These 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 CPUs were from the LSI-11 line running 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.

Models that were planned but never introduced

Special purpose versions

DEC GT40 running Lunar Lander
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DEC GT40 running Lunar Lander

Clandestine clones

The PDP-11 was sufficiently popular that several unauthorized clones were produced behind the Iron curtain. 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:

External links

 

From Wikipedia, the Free Encyclopedia. Original article here. Support Wikipedia by contributing or donating.
All text is available under the terms of the GNU Free Documentation License See Wikipedia Copyrights for details.

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