- A key concept for a processor is the management of “state”, information that persists over time. Much of a computer is built from logic gates, such as NAND or NOR gates, but logic gates have no notion of time. Processors also need a way to hold values, along with a mechanism to move from step to step in a controlled fashion. This is the role of “sequential logic”, where the output depends on what happened before. Sequential logic usually operates off a clock signal,1 a sequence of regular pulses that controls the timing of the computer. (If you have a 3.2 GHz processor, for instance, that number is the clock frequency.)
- Analyzing the vintage 8008 processor from die photos: its unusual counters (righto.com)
The revolutionary Intel 8008 microprocessor is 45 years old today (March 13, 2017), so I figured it’s 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.