PDP-6
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.
Worldwide, only 23 PDP-6's were sold, the smallest number of any DEC machine, but DEC management still considered the system useful because those sales were to technical leaders such as universities. That gave DEC a number of advantages, including a foothold in that market, access to advice on future technical direction from a group of advanced and technically knowledgeable users, and finally a source of intelligent young employees as the business grew.
It used a 36-bit word length, and had a maximum memory capacity of 262,144 words (256 Kibiwords), although many systems were installed with only 32,768 words of core memory (equivalent to 160 Kbytes on modern machines). The system included discrete-transistor "fast accumulators" which 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 set of "base and bounds" registers which allowed a user's address space to be limited to a set section of main memory.
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