Sign in with
Sign up | Sign in

DEC's PDP-8

A Complete History Of Mainframe Computing
By
DEC's PDP-8

While IBM was busy releasing its magnificent System/360 line, Digital Equipment Corp. (DEC) was about to release a computer that would have a major impact on the future of computing as well, the PDP-8. Although the different computers in System/360 had an enormous range of performance characteristics and capacities, they were still mainframes, and even the lowest-end models were still too expensive for many businesses. This opportunity was not lost on DEC's founder, Ken Olsen.

Although DEC had released computers as early as 1960, these models were only modestly successful and had little impact on the industry. However, the steady advance of technology, mainly in the form of integrated circuits, allowed DEC to sell a much smaller and much less expensive computer than its predecessors. Integrated circuits also allowed for much lower power use, and consequent to that, much less heat dissipation. This freed computers from purpose-built air-conditioned rooms. When released in 1965, the PDP-8 sold for an astonishingly low price of $18,000, which, with the aforementioned housing requirements, put computers within the reach of many companies that previously found them to be prohibitively expensive.

One unique feature of the PDP-1, DEC's first product, was the use of true direct memory access (DMA), which was much cheaper and less complicated than the channels mainframes used, and without much negative impact on the processor performance. In fact, a single mainframe channel cost more than the entire PDP-1. DMA was used on every successive computer DEC made, including the PDP-8. However, not all the cost-cutting comprises made for the PDP-8 were so benign. The 12-bit word length dramatically limited the amount of directly addressable memory, while only 7-bits of the word comprised the address field, allowing only 128 bytes to be directly addressed. There were ways around this drawback, one of which was to use indirect addressing, where the 7-bits pointed to a memory location that contained the actual address that you wanted to access, which was slower, but allowed a full 12-bit access. The other way was to divide memory into segments of 128 bytes, and change segments when necessary (and people thought the 64 K segments of 16-bit x86 processors were bad). Neither solution was desirable, and they severely limited the usefulness of the PDP-8 with high-level languages. The PDP-8 was also no speed demon, and was capable of only 35,000 additions per second.

Despite these compromises, the PDP-8 was remarkably successful, selling over 50,000 machines before it was discontinued. The low purchase price, low continuing costs, and ease of housing it all were more compelling than its deficits were damning. In fact, this modest machine sparked a whole new type of computer, called the mini-computer, which became very successful for over two decades and made DEC the second-largest computer company in the world. Perhaps sadly, the mini-computer did not survive the march of the micro-computer, and is now an extinct species, and thus is more applicably called a dinosaur than the normal recipient of that unflattering term, the mainframe. The mainframe still sits on top of the food chain, capable of things far beyond desktop computers.

See more See less
React To This Article