The PDP-10 was a computer manufactured by Digital Equipment Corporation (DEC) from the early 1970s on; the name stands for "Programmed Data Processor model 10". It was the machine that made time-sharing common; it looms large in hacker folklore because of its adoption in the mid-1970s by many university computing facilities and research labs, including MIT's AI Lab and Project MAC, Stanford's SAIL, and CMU.
The original model processor was the KA, which used discrete transistors packaged in DEC's Flip Chip technology. As supplied by DEC, it did not include paging hardware, only two sets of "base and bounds" registers, which allowed each half of a user's address space to be limited to a set section of main memory, thereby allowing the Unix model of separate read-only code segment, and a read-write data/stack segment. Some KA machines (e.g. at MIT and BBN) were modified to add support for paging.
The KA was replaced by the KI, which used TTL IC's, and did support paging; this in turn was replaced by the KL, which was built from ECL. Also, the KA had a maximum main memory capacity of 256 Kwords (about 1150 Kbytes); the latter two processors removed this limitation. The latest KL versions removed the 256 Kword limitation on a user's address space, as well.
The PDP-10 was eventually eclipsed by the VAX supermini machines (descendants of the PDP-11) when DEC recognized that the PDP-10 and VAX product lines were competing with each other and decided to concentrate its software development effort on the more profitable VAX. The machine was finally dropped from DEC's line in 1983, following the failure of the Jupiter Project at DEC to build a new model acceptable to higher management. (Some attempts by other companies to market clones came to nothing; see Foonly and Mars.)
This event spelled the doom of ITS and the technical cultures that had spawned the original jargon file, but by the 1990s it had become something of a badge of honorable old-timerhood among hackers to have cut one's teeth on a PDP-10.
Will Crowther created Adventure, the prototypical computer role-playing game, for a PDP-10. It is rumoured that the Disney science fiction movie TRON has a (presumably un-authorized) PDP-10 connection; the TRON instruction (Test Right-halfword Ones and skip if Not masked) is in fact instruction number 666...
This article is based in part on one in the jargon file; the jargon file is in the public domain.