PDP-6

The PDP-6 (Programmed Data Processor-6) was a computer model developed by Digital Equipment Corporation (DEC) in 1963. It was influential primarily as the prototype (effectively) for the later PDP-10; the instruction sets of the two machines are almost identical.

The PDP-6 was DEC's first "big" machine. It used 36-bit words, in common with other large computers at the time from companies like IBM, Honeywell and General Electric. Addressing remained 18-bit, as in earlier DEC machines, allowing for a 256 kword main memory. Memory was implemented using magnetic cores; a typical system included 32,768 words (equivalent to 144kB on modern machines).

The instruction set architecture could be categorized as "one-and-a-half address"; instructions contained one full 18-bit memory address, and a second four-bit address that could specify one of the first sixteen memory locations as an "accumulator" or "AC". Another four-bit field in the instruction allowed for any AC other than AC0 to be used as an index register.

Most, if not all, PDP-6 systems were equipped with the optional Type 162 "Fast Memory", which provided 16 memory locations constructed from discrete-transistor flip-flops. The Fast Memory (also known as "fast accumulators" or "fast ACs") substituted for the first 16 words of core memory and operated four times faster.

The PDP-6 was infamous because of the 6205 board, a large (11 × 9 inches) board which contained 1 bit of arithmetic register (AR), memory buffer (MB), and multiplier-quotient register (MQ) (thus there were 36 of these). It had 88 transistors, a two-sided PC etch, two 18-pin and two 22-pin connectors (two on each side of the module). Because of all these connectors, swapping this module was a major undertaking, and the mechanical coupling made it highly likely that fixing one fault would cause another. There was also a great fear of powering off a PDP-6, since it would generally result in at least one 6205 board failing. The experience with the 6205 led the designers of the first models of PDP-10, the KA10 and KI10, to use only small boards. It was not until the KL10 that large boards were used again.

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