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What is a PDP-8?

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  • Written By: Kurt Inman
  • Edited By: Jenn Walker
  • Last Modified Date: 30 October 2016
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A PDP-8 was a 12-bit minicomputer manufactured by Digital Equipment Corporation (DEC) from the mid 1960s until the late 1970s. It included a simple processor, core memory and at least one input/output (I/O) device, such as a teletype. These computers were often used for business applications as well as to control industrial machines and scientific experiments.

Unlike many modern computers, the PDP-8 did not include pre-installed operating system software. A simple program could be keyed in using switches on the front panel of the machine. Typically this was a boot loader, which then transferred other software from a data storage device; paper tape and magnetic tape were the most common PDP storage media at the time. Compilers and interpreters were available for a variety of computer programming languages, including BASIC, DIBOL, FOCAL and FORTRAN. Several operating systems were also developed for this machine; OS/8 was one of the most commonly used.

Many different PDP-8 models were produced. For example, the PDP-8/E introduced an enhanced memory controller to support virtual machines. The /A model increased the core memory capacity beyond the 32K word limit. Other models often added instructions and registers. New types of I/O devices were also introduced over time, including disk drives, cartridge tape drives and analog-to-digital converters.

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The early PDP-8 systems were built with individual transistors, diodes and resistors. By the late 1960s, TTL integrated circuits replaced many of those components, simplifying production. In a final move to save power and streamline the design, custom CMOS integrated circuits were utilized in the late 1970s.

The PDP-8 was very popular in the early 1970s; it was the first powerful computer which could be purchased inexpensively rather than leased. Many companies used them to run their business operations, including accounting and inventory control. Some were even rented to the public by the hour in computer centers; programming, word processing and game playing were common tasks. PDP stands for Programmable Data Processor; DEC typically avoided using the term computer, which was often associated with a large and costly array of machines. By the late 1970s, smaller, lower-priced microcomputers from Apple and Tandy were taking away much of its market.

The PDP-8 processor, memory and control circuitry are not very complex by today's standards; enthusiasts have implemented all of it in a single FPGA chip, making cheap hardware "clones" a reality. Several software emulators exist that can run PDP programs. Much of the original DEC system documentation is also available on the Internet. One website even offers online access to a real, running PDP-8 system. Many of these vintage computers still function; some continue their original duties, while others can be purchased from auction sites and private collectors.

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Soulfox
Post 3

@Vincenzo -- the PDP-8 line probably isn't as lauded as early personal computers because average folks could not afford them. Don't get me wrong. The PDP-8 was affordable for corporations, colleges and other organizations with big budgets, but they were still well out of the price range of the average Joe and Jane.

The computer revolution began in earnest when small, personal computers showed up and were affordable. Also, keep in mind how important games were when it came to early computers. People wanted to play games with graphics and a lot of early personal computers could handle those right out of the gate. The PDP-8s were horrible for that use and that limited their appeal, too.

Vincenzo
Post 2

@Logicfest -- this article does take you back in time, doesn't it? A lot of computer gaming started with these machines. The funny thing is, those were mostly text based games (early adventure games where you would type in "E" to go east or "Look at dog" to interact with things described, for example) as even crude graphics were almost unheard of on these machines.

A lot of people who made a fortune developing games in the 1980s started out programming simple, basic games on PDP-8s.

One thing that I never could understand is why this machine isn't considered as important in the development of personal computers as early systems from the likes of Apple and Radio Shack.

Logicfest
Post 1

Wow. Talking about reaching back in history a few years, huh? The PDP-8 was the first successful, mass produced mini-computer and the series evolved to the point where a lot of people were using them for general purpose computers.

It was a bear to boot up and program, but a lot of the input output devices that we take for granted -- keyboard input, printers, etc. -- were pioneered on the old PDP-8 line. One could argue that the so-called computer revolution that helped define that late 1970s and 1980s really started with these computers.

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