A History of Modern Computing (History of Computing) by Paul Cruzi

Author:Paul Cruzi [Cruzi, Paul]
Language: eng
Format: mobi
Publisher: The MIT Press
Published: 2003-04-07T16:00:00+00:00

Figure 7.5

MITS Altair 8800 Computer. The front panel was copied from the Data General Nova. The machine shown in this photograph was one of the first produced and was owned by Forrest Mims, an electronics hobbyist and frequent contributor to Popular Electronics, who had briefly worked at MITS. (Source: Smithsonian Institution.)

Consider first the technical. None of the other hobbyist projects had the impact of the Altair’s announcement. Why? One reason was that it was designed and promoted as a capable minicomputer, as powerful as those offered by DEC or Data General. The magazine article, written by Ed Roberts and William Yates, makes this point over and over: “a full-blown computer that can hold its own against sophisticated minicomputers”; “not a ‘demonstrator’ or a souped-up calculator”; “performance competes with current commercial minicomputers.”72 The physical appearance of the Altair computer suggested its minicomputer lineage. It looked like the Data General Nova: it had a rectangular metal case, a front panel of switches that controlled the contents of internal registers, and small lights indicating the presence of a binary one or zero. Inside the Altair’s case, there was a machine built mainly of TTL integrated circuits (except for the microprocessor, which was a MOS device), packaged in dual-in-line packages, soldered onto circuit boards. Signals and power traveled from one part of the machine to another on a bus. The Altair used integrated circuits, not magnetic cores, for its primary memory. The Popular Electronics cover called the Altair the “world’s first minicomputer kit”; except for its use of a microprocessor, that accurately described its physical construction and design.73

But the Altair as advertised was ten times cheaper than minicomputers were in 1975. The magazine offered an Altair for under $400 as a kit, and a few hundred more already assembled. The magazine cover said that readers could “save over $1,000.” In fact, the cheapest PDP-8 cost several thousand dollars. Of course, a PDP-8 was a fully assembled, operating computer that was considerably more capable than the basic Altair, but that did not really matter in this case. (Just what one got for $400 will be discussed later.) The low cost resulted mainly from its use of the Intel 8080 microprocessor, just introduced. Intel had quoted a price of $360 for small quantities of 8080s, but Intel’s quote was not based on a careful analysis of how to sell the 8080 to this market. MITS bought them for only $75 each.74

The 8080 had more instructions and was faster and more capable than the 8008 that the Mark-8 and Scelbi-8 used. It also permitted a simpler design since it required only six instead of twenty supporting chips to make a functional system. Other improvements over the 8008 were its ability to address up to 64 thousand bytes of memory (vs. the 8008’s 16 thousand), and its use of main memory for the stack, which permitted essentially unlimited levels of subroutines instead of the 8008’s seven levels.

The 8080 processor was only one architectural advantage the Altair had over its predecessors.

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