PDP-6
The PDP-6 (Programmed Data Processor model 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.
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 256,000 words, although many systems were installed with only about 32,000 words (that's 160 Kbytes). It included simple mechanisms to support time sharing, including a pair of 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.
