The revolutionary Intel 8008 microprocessor is 45 years old today (March 13, 2017), so I figured itās time for a blog post on reverse-engineering its internal circuits. One of the interesting things about old computers is how they implemented things in unexpected ways, and the 8008 is no exception. Compared to modern architectures, one unusual feature of the 8008 is it had an on-chip stack for subroutine calls, rather than storing the stack in RAM. And instead of using normal binary counters for the stack, the 8008 saved a few gates by using shift-register counters that generated pseudo-random values. In this article, I reverse-engineer these circuits from die photos and explain how they work.
The image below shows the 8008ās tiny silicon die, highly magnified. Around the outside of the die, you can see the 18 wires connecting the die to the chipās external pins. The 8008ās circuitry is built from about 3500 tiny transistors (yellow) connected by a metal wiring layer (white). This article will focus on the stack circuits on the right side of the chip and how they interact with the data bus (blue).
For the 8008 processorās birthday, Iām using the date of its first public announcement, an article inĀ ElectronicsĀ on March 13, 1972 entitled ā8-bit parallel processor offered on a single chip.ā This article described the 8008 as a complete central processing unit for use in āintelligent terminalsā and stated that chips were available at $200 each.1
for more detail: Analyzing the vintage 8008 processor from die photos: its unusual counters