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The PDP-6 (Programmed Data Processor-6) was a computer model developed by Digital Equipment Corporation (DEC) in 1963. more...

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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. See: Why 36-bit word length?. 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 160kB 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 but operated four times faster.

The PDP-6 supported time sharing through the use of a status bit selecting between two operating modes ("Executive" and "User", with access to I/O, etc., being restricted in the latter), and a single relocation/protection register which allowed a user's address space to be limited to a set section of main memory (a second relocation/protection register for shareable "high segments" was added on the PDP-10). The main operating system used on the machine was an early version of what later became TOPS-10, and several sites made custom versions of the system, which was available in source code form. MIT's ITS operating system also began on the PDP-6.

Worldwide, only 23 PDP-6's were sold, the smallest number of any DEC machine. It was complex and expensive to build, as well as difficult to install and get operational at the customer's site. Additionally the sales force found the PDP-6 to be a "hard sell". After a short period in the market, DEC let it be known that they were exiting the 36-bit market to concentrate on their smaller machines again. Not long after this rumors started to spread that they were, in fact, working on a new 36-bit design, which was eventually released as the PDP-10.

The PDP-6 was infamous because of the 6205 board, a large (11 x 9 inches) board which contained 1 bit of AR, MB, and MQ (thus there were 36 of these). It had 88 transistors, a 2-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.

Read more at Wikipedia.org