1975 · Bipolar Bit-Slice Processor · Work 894 of 1,871
A microcode laboratory in Sunnyvale in California late at night, on a long workbench a large breadboard forty centimetres wide densely set with small black chips in sockets, in its middle a straight row of four long violet ceramic packages five centimetres long, each with a nearly black glossy square lid and twenty silver legs on each side, all plain smooth and unmarked, a hand in a rolled-up pale blue shirt sleeve fixing a small grey test clip onto the legs of the second violet package, a tall fan-folded stack of pale green-striped continuous paper beside it, a paper coffee cup, a cork wall, cool fluorescent light.
What it was used for
The Am2901 was a part from which engineers assembled a processor: a 4-bit arithmetic unit with scratchpad registers and an accumulator that could be cascaded to any word width, with the 2909 sequencer and the 2910 controller alongside. CPU Shack gives the introduction in August 1975 as the "100 ns Bipolar microprocessor" and about 10 MHz, which was fast for the time. Second sources followed quickly: Motorola under an agreement of November 1975, Raytheon of December 1975, Thomson-CSF through its SESCOSEM division in 1976 for markets outside the US and Japan, later National, Fairchild, NEC, Signetics, OKI, Cypress, IDT and WSI; Soviet copies began around 1985.
What was new
What marked it was what was not on the chip: no instruction set. Word width and instructions were set by the user through microprogram; the part was a slice, not a computer. Against the older 74181 arithmetic unit, which CPU Shack cites as the comparison, the 2901 added scratchpad registers and an accumulator. Technically it was the opposite road from the MOS single-chip processors of the same years: the bipolar 2901 ran at about 10 MHz where the Intel 8080 is given as 2 MHz, but needed several packages per machine. CPU Shack writes that most bit-slice designs still rest on the 2901 and that InnovASIC still makes it as the 59032 - rarely used for new designs, it still runs much equipment in service.
What the sources leave open: Only one source on the 2901, and it names not a single machine that used it - only, in general terms, that it still powers much equipment in use. Buyer groups, unit numbers and prices are therefore omitted. The 2 MHz figure for the Intel 8080 comes from the 8080 source, not from CPU Shack.