Another Real Machine: The DEC PDP-8

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Another Real Machine: The DEC PDP-8

As the Digital Equipment Corporation PDP-8 computer, having originated with a very simple instruction set, was also inexpensive enough that it was the first computer within the reach of many organizations, it holds a place in the affections of many.

Of course, a number of other computers might also have been noted as making computing more affordable, such as the IBM 650, the Bendix G-15, the Packard Bell 250, and the Royal McBee LGP 30, for example. There was more to the enduring affection in which the PDP-8 is held than its affordability, but that is a phenomenon I will not attempt to analyze at length.

The diagram below shows an overview of the instruction formats available with the PDP-8 and related computers throughout their history:

The first column shows the basic instructions included with the PDP-5 and all PDP-8 models. The opcodes for memory-reference instructions were:

000 AND And
001 TAD Two’s Complement Add
010 ISZ Increment and Skip if Zero
011 DCA Deposit and Clear Accumulator
100 JMS Jump to Subroutine
101 JMP Jump

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