PDP-11

The PDP-11 was a 16-bit minicomputer 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 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

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(or "addressing mode") would work with any operation;

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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. 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.

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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. All input and output devices were mapped to addresses in memory, so in addition, no special I/O instructions were needed. The interrupt system was intentionally designed to be as simple as possible, while assuring 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 is erroneously started, the CPU would time out, generating a special spurious interrupt; the spurious interrupt would warn users of bad hardware.

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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. All parts of the computer chassis were constructed from injection-molded plastic, or bent steel rod (lighter than sheet metal). It used a push-bonded backplane. That is, the printed circuit board plugged into a backplane connector. The backplane connector had terminals that could be connected by pushing wires into 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 case was injection-molded plastic that snapped over the steel-rod chassis.

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16-bit: Prominent 16-bit processors include the Intel 8086, Motorola M68000, Intel 80286 and the WDC 65C816....

Minicomputer: Minicomputer is a largely obsolete term for a class of multi-user computers which make up the middle range of the computing spectrum, in between the largest multi-user systems (mainframe computers) and the smallest single-user systems (microcomputers or personal computers). More modern terms for suc...

Digital Equipment Corp.: REDIRECT Digital Equipment Corporation...