The PDP-6, short for Programmed Data Processor model 6, is a computer developed by Digital Equipment Corporation (DEC) during 1963 and first delivered in the summer of 1964.

Using a 36-bit word with 18-bit addresses allowed it to efficiently store the cons structure found in the Lisp language, which made it particularly useful in artificial intelligence labs like Project MAC at MIT.

The lasting influence of the PDP-6 was its re-implementation using modern silicon transistors and the newer Flip-Chip module packaging to produce the PDP-10.

Proposals for a PDP-2 and PDP-3 were turned down, and DEC's next machine, the PDP-4, was essentially a smaller and less-expensive PDP-1 that sold for roughly half the cost.

It used 36-bit words, in common with other large computers at the time from companies like IBM, Honeywell and General Electric.

[8]The sales were so slow that DEC eventually decided to abandon the system and announced that they would not build any more 36-bit machines.

Although it was program-compatible with the PDP-6, it ran roughly twice as fast, had both timesharing and batch processing features, and offered a wide variety of expansions and input/output options.

[11] Addressing remained 18-bit, as in earlier DEC machines, allowing for a 256 kword main memory, about 1 MB in modern terms.

Memory was implemented using magnetic cores; a typical system included 32,768 words (equivalent to 144 kB on modern machines).

[12] This sort of access pattern was common as it allowed tables to be scanned over using a single instruction and then changing the value in memory to point to another location.

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.

The PDP-6 supported time-sharing through the use of a status bit selecting between two operating modes ("Executive" and "User", with access to input/output (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).

In the late 1990s Compaq donated the contents of the DEC internal archives to The Computer Museum History Center.