From Academic Kids


The PDP-6 (Programmed Data Processor-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.

The PDP-6 was DEC's first "big" machine, based on a 36-bit word length common with other business machines from companies like IBM, Honeywell and General Electric. This word size was common at the time due to the use of a small 6-bit character set, allowing the machine to pack six characters into a single word for processing. Addressing remained 18-bit, as in earlier DEC machines, allowing for a 256 kword main memory. Memory was implemented using core, the average machine including 32,768 words equivalent to 160kB on modern machines). Many instructions included an address encoded into the 36-bit instruction, reducing memory accesses and improving performance. 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. The main operating system used on the machine was an early version of what later became TOPS-10, and several sites made custom versions of the system, which was available in source code form.

Worldwide, only 23 PDP-6's were sold, the smallest number of any DEC machine. It was complex and expensive to build, as well as difficult to install and get operational at the customer's site. Additionally the sales force found the PDP-6 to be a "hard sell". After a short period in the market, DEC let it be known that they were exiting the 36-bit market to concentrate on their smaller machines again. Not long after this rumors started to spread that they were, in fact, working on a new 36-bit design, which was eventually released as the PDP-10.

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.

The Boston Computer Museum at one time had a PDP-6 in their collection, however its fate is unknown. Most of the BCM's collection was moved to Mountain View, California to form the Computer Museum History Center, which is now the Computer History Museum (http://www.computerhistory.org), but the PDP-6 was not present. There is no known surviving PDP-6.


  • Bell, C. Gordon, Mudge, J. Craig, McNamara John E. "The PDP-10 Family". (1979). Part V of Computer Engineering: A DEC View of Hardware Systems Design. Digital Equipment Corp.

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