“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 game2 to the VAX-11/730 minicomputer, from the Xerox Star workstation to the F-16 fighter’s Magic 372 computer.3 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.
The bit-slice processor
You might wonder how multiple processor chips could work together to support arbitrary word lengths. The key is that a bit-slice processor is a building block, rather than a complete processor,6 and requires separate circuitry to decode instructions and control the system.4 The bit-slice processor chips performed arithmetic or logic operations on the data and contained registers, while a control chip (such as the Am2910) told the bit-slice chips what to do. Each machine instruction was broken down into smaller steps called micro-instructions which were stored in a microcode ROM. Note that the computer’s instruction set was defined by the microcode, not by the Am2901, so almost any instruction set could be supported.5
Bit-slice processors fell in between using a microprocessor chip and building a computer out of simple TTL chips. Building a processor out of TTL chips was much faster than a microprocessor at the time, but required boards full of chips. Using a bit-slice processor kept the speed advantage, but reduced the chip count. The bit-slice processor also provided much more flexibility than a microprocessor, allowing the designer to customize the instruction set and other architectural features.”