Inside the Am2901: AMD’s 1970s bit-slice processor

You’re probably familiar with modern processors made by Advanced Micro Devices. But AMD’s processors go back to 1975, when AMD introduced the Am2901. This chip was a type of processor called a bit-slice processor: each chip processed just 4 bits, but multiple chips were combined to produce a larger word size. This approach was used in the 1970s and 1980s to create a 16-bit, 36-bit, or 64-bit processor (for example), when the whole processor couldn’t fit on a single fast chip.

The Am2901 chip became very popular, used in diverse systems ranging from the Battlezone video game to the VAX-11/730 minicomputer, from the Xerox Star workstation to the F-16 fighter’s Magic 372 computer. The fastest version of this processor, the Am2901C, used a logic family called emitter-coupled logic (ECL) for high performance. In this blog post, I open up an Am2901C chip, examine its die under a microscope, and explain the ECL circuits that made its arithmetic-logic unit work.

A very detailed, technical look at this processor.


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