The DEC PDP family
The PDP-1 was the first of a long line. PDP stood for Programmed Data Processor, a name Digital Equipment Corporation chose deliberately to avoid the word computer, which in 1959 meant a million-dollar machine in a glass room and frightened the investors and customers DEC needed. Over the next decade and more, DEC built out a whole family of PDPs, cheaper, smaller, interactive machines that put computing within reach of a single laboratory, a single department, even a single user. The PDP-1 was in that minicomputer family which created the interactive computing culture that blossomed into Spacewar!
The Programmed Data Processors
The main general-purpose PDPs, with the figures that matter for comparison. Memory is given as the basic word count with an approximate modern equivalent in brackets, alongside word lengths and the software each machine is known for.
| Model | Year | Word | Memory (basic, ≈ modern) | Price / class | Known for |
|---|---|---|---|---|---|
| PDP-1 | 1959–60 | 18-bit | 4,096 words (≈ 9 KB) | ~$120,000; ~53 built | Spacewar! (1962), the first time-sharing experiments, the MIT hackers |
| PDP-4 | 1962 | 18-bit | 4,096 words (≈ 9 KB) | ~$65,000 (half a PDP-1) | A cheaper PDP-1; ancestor of the 7 / 9 / 15 line |
| PDP-5 | 1963 | 12-bit | 1,024 words (≈ 1.5 KB) | ~$27,000 | Often called the first commercial minicomputer; ancestor of the PDP-8 |
| PDP-6 | 1964 | 36-bit | 16,384 words (≈ 72 KB) | ~$300,000 system; ~23 sold | DEC's first time-sharing machine; ITS began here; ancestor of the PDP-10 |
| PDP-7 | 1964–65 | 18-bit | 4,096 words (≈ 9 KB) | ~$72,000; ~120 sold | The birthplace of UNIX (Bell Labs, 1969); Cambridge's Duel ran on a PDP-7 with a Type 340 |
| PDP-8 | 1965 | 12-bit | 4,096 words (≈ 6 KB) | $18,500; 50,000+ (family) | The first commercially successful minicomputer; everywhere in labs, industry and schools |
| PDP-9 | 1966 | 18-bit | 8,192 words (≈ 18 KB) | ~$35,000 | A faster PDP-7; DEC's first micro-programmed machine |
| PDP-10 | 1966 | 36-bit | 8,192 words and up | mainframe class (from ~$99,000) | ITS, Project MAC, the MIT AI Lab; MacLisp, Macsyma, the origins of Emacs; Stanford's SAIL |
| PDP-11 | 1970 | 16-bit | Unibus architecture; varied | ~$10,800–20,000; ~600,000 sold | DEC's huge success; early Unix and C; an architecture that influenced later CPUs |
| PDP-12 | 1969 | 12-bit | 4,096 words (≈ 6 KB) | ~$27,900 | A PDP-8 that also ran LINC programs; lab and biomedical work |
| PDP-15 | 1969–70 | 18-bit | 4,096 words (≈ 9 KB) | $15,600 (basic) | DEC's last 18-bit machine; the PDP-9 rebuilt in integrated circuits |
Notes. Modern-equivalent memory figures are rounded conversions from the word count (an 18-bit word is about 2.25 bytes, a 12-bit word about 1.5, a 36-bit word about 4.5), offered only for a sense of scale. A few figures carry genuine source conflicts: the PDP-7 is variously dated 1964 or 1965, the PDP-15 announced in 1969 and first shipped in 1970, and the PDP-11/20's base price is given as anywhere from about $10,800 to $20,000. Several numbers (PDP-4 / 9 / 12 memory, the PDP-12 price) rest on a single secondary source. Figures are drawn from Wikipedia, the Computer History Wiki (gunkies.org), the Computer History Museum and soemtron.org; the PDP-1, 8, 10 and 11 rows should be checked against DEC's own handbooks.
PDP-2, 3, 13, 14, 16
The numbering of the PDP line is not continuous, and the reasons for it are interesting. The PDP-2 (a 24-bit design) and PDP-13 were numbers DEC never used. The PDP-3 was a 36-bit paper design DEC chose not to build; the only one ever made was assembled outside DEC, by the CIA's Scientific Engineering Institute, from DEC modules. The PDP-14 and PDP-16 were not general-purpose computers at all but industrial control systems, the PDP-14 a programmable logic controller meant to replace banks of relays, the PDP-16 a kit of "Register Transfer Modules" for building custom digital systems. The family, in other words, was less a tidy sequence than a spread of experiments in what a smaller, cheaper, more specialised computer could be.
The PDP-1 as the first step
Seen through the DEC product line, the PDP-1 is fascinating as its first foray into minicomputers at a time when computing was still unformalised, open and often chaotic. Its 18-bit word, its interactive console, its optional display and the culture of hands-on programming it invited are the seed of everything the line went on to do: time-sharing on the 6 and 10, Unix on the 7 and 11, the minicomputer revolution on the 8.
References
- Computer History Wiki, List of Programmed Data Processors, and the per-model pages there.
- Computer History Museum, PDP-1 Restoration Project, and the Revolution exhibits on the PDP-8 and PDP-11.
- Wikipedia's Programmed Data Processor and per-model articles, cross-checked against the above; and soemtron.org for the PDP-7.