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. It was an expansion of DEC's existing 18-bit systems to use a 36-bit data word, which was at that time a common word size for large machines like IBM mainframes. The system was constructed using the same germanium transistor-based System Module layout as DEC's earlier machines, like the PDP-1 and PDP-4.
The system was designed with real-time computing use in mind, not just batch processing as was typical for most mainframes. This made it popular in university settings and its support for the Lisp language made it particularly useful in artificial intelligence labs like Project MAC at MIT. It was also complex, expensive, and unreliable as a result of its use of so many early-model transistors. Only 23 were sold, at prices ranging from $120,000 to $300,000.
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. The instruction sets of the two machines are almost identical. The PDP-10 was less expensive and more reliable, and about 1500 were sold during its lifetime.
History
DEC's first products were not computers but a series of plug-in circuits known as Digital Laboratory Modules that performed digital logic. Users could wire the modules together to perform specific tasks. DEC soon introduced the PDP-1 which was built out of large numbers of these modules, now known as System Building Blocks or System Modules.
The PDP-1 used an 18-bit word. Word lengths in the early 1960s were generally some multiple of six bits, as the character codes of the era were 6 bits long and it was also a useful size for storing binary coded decimal digits with an optional sign, as commonly used on IBM machines of the era. Large machines generally used a 36-bit word length, but there were many variations. The PDP-1's 18-bit length allowed the machine to be simpler and less expensive than these, at . 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. The PDP-5 was built from the same components, but used a 12-bit word for even greater savings and sold for about .
The PDP-6 was DEC's first "big" machine. It used 36-bit words, in common with other large computers at the time from companies like IBM, Honeywell and General Electric. Unlike those machines, the PDP-6 was supplied with a timesharing system "out of the box". Timesharing had been available for other machines, most famously the PDP-1 at Project MAC, but the PDP-6 was the first such system to be supported by the manufacturer.
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". Nevertheless, the company later considered the PDP-6 to be a success:
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. Fortunately, this decision was reversed and a new 36-bit system was designed using the new Flip-Chip modules and much smaller system boards to improve density and reliability. The resulting design was released in 1966 as the PDP-10. DEC describes this machine as the successor to the PDP-6. 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. It was far more successful and eventually sold about 1,500 machines.
Description
Architecture
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). The use of a 36-bit word allowed two 18-bit addresses to be stored in a single memory location; this made it suited to the storage of a cons, a widely used structure in the Lisp language, meaning the PDP-6 could store a cons in a single word and read and write one in a single operation.