PDP is an abbreviation for Programmed Data Processor, the name of a series of computers, several of them ground-breaking and very influential, made by Digital Equipment Corporation. They were given that name because at the time of their introduction, computers had a reputation of being large and expensive machines, and the PDP machines were aimed at a market which couldn't afford the larger computers.
The various PDP machines can generally be grouped into families based on word length. With the notable exception of the 16-bit PDP-11, the architectures show strong similarities, with the 36-bit PDP-6 and PDP-10 architecture being the most elaborate.
PDP-3: First 36-bit machine DEC designed, though DEC did not offer it as a product. The only PDP-3 was built by a customer in 1960. Architecturally it was essentially a PDP-1 stretched to 36-bit word width.
PDP-4: Supposed to be a slower, cleaper alternative to the PDP-1, but not commercially successful; all later PDP 18-bit machines were based on its instruction set.
PDP-5: DEC's first 12-bit machine. Introduced the instruction set later used in the PDP-8. As a cost-saving measure, the memory location at address zero was used as the program counter, rather than the more typical approach of using a dedicated hardware register.
PDP-6: 36-bit timesharing machine. Very elegant architecture. It was considered a large minicomputer or a mainframe.
PDP-7: Replacement for the PDP-4; DEC's first wire-wrapped machine. The first version of Unix was for this machine.
PDP-8: 12-bit machine with a tiny instruction set; DEC's first wildly successful computer. The first successful "personal computer", many were purchased by schools, university departments, and research laboratories. Later models were also used in the DECmate word processor and the VT-78 workstation.
LINC-8: A hybrid of the LINC and PDP-8 computers; two instruction sets. Progenitor of the PDP-12.
PDP-9: Successor to the PDP-7, DEC's first micro-programmed machine.
PDP-10: 36-bit timesharing machine, and fairly successful over several different models. The instruction set was a slightly elaborated form of that of the PDP-6.
PDP-11: 16-bit machine, widely regarded as the best 16-bit instruction set ever created, and another huge hit for DEC. Also the LSI-11, primarily for embedded systems. The VAX series was descended from it.
PDP-12: Descendant of the LINC-8.
PDP-14: A 12-bit machine intended as an industrial controller.
PDP-15: DEC's final 18-bit machine. Their only 18-bit machine constructed from TTL integrated circuits rather than discrete transistors. Later version sof the system were referred to as the "XVM" family.
PDP-16: A "roll-your-own" sort of computer using Register Transfer Modules, mainly intended for industrial control systems with more capability than the PDP-14. The PDP-16/M was introduced as a standard version of the PDP-16.
Related computers
LINC (Laboratory Instrument Computer), originally designed by MIT's Lincoln Labs, some built by DEC. Not in the PDP family, but important as progenitor of the PDP-12.
The LINC and the PDP-8 can be considered the first minicomputers, and perhaps the first personal computers as well. The PDP-8 and PDP-11 were the most popular of the PDP series of machines.
Digital never made a PDP-20, although the term was sometimes used for a PDP-10 running TOPS-20.
DVK personal computers series is a PDP clones developed in USSR in 70s.
Mark Crispin's 1986 list of PDP's (http://www.village.org/pdp11/faq.pages/Crispin.html)
DEC's PDP-6 was the worlds first commercial time-sharing system (http://americanhistory.si.edu/csr/comphist/bell.htm#first%20commercial%20timesharing) Gordon Bell interview at the Smithsonian
Charles Babbage was the first to conceptualize and design a fully programmable computer as early as 1820, but due to a combination of the limits of the technology of the time, limited finance, and an inability to resist tinkering with his design, the device was never actually constructed in his lifetime.
Its job is to read instructions and data from memory or the I/O devices, decode the instructions, providing the ALU with the correct inputs according to the instructions, "tell" the ALU what operation to perform on those inputs, and send the results back to the memory or to the I/O devices.
Instead, earliest computers stored data in Williams tubes — essentially, projecting some dots on a TV screen and reading them again, or mercury delay lines where the data was stored as sound pulses traveling slowly (compared to the machine itself) along long tubes filled with mercury.
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