Linked by Thom Holwerda on Sat 7th Sep 2013 09:54 UTC
Hardware, Embedded Systems

The 8-bit Z-80 processor is famed for use in many early personal computers such the Osborne 1, TRS-80, and Sinclair ZX Spectrum, and it is still used in embedded systems and TI graphing calculators. I had always assumed that the ALU (arithmetic-logic unit) in the Z-80 was 8 bits wide, like just about every other 8-bit processor. But while reverse-engineering the Z-80, I was shocked to discover the ALU is only 4 bits wide! The founders of Zilog mentioned the 4-bit ALU in a very interesting discussion at the Computer History Museum, so it's not exactly a secret, but it's not well-known either.

I have been reverse-engineering the Z-80 processor using images from the Visual 6502 team. The image below shows the overall structure of the Z-80 chip and the location of the ALU. The remainder of this article dives into the details of the ALU: its architecture, how it works, and exactly how it is implemented.

Ken Shirrif's blog is an absolute must for fans of ultra-low-level hardware stuff. This goes way over my head, but interesting nonetheless.

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I did say that 4bit ALU's were common in those days.
The IC technology available then was by today's standards pretty crude so making 8,12 or even 16 bit ALU's was for a while impossible.
I went on to give some examples of some other uses for them
The 2901 could be used on its own. It didn't have to be used with others.
A lot of engineers quickly realised that 4bits was very limiting especially as many of the other CPU's(non microprocessor) around in those days had far longer word lengths. could this be why we chained 4bit devices together?
Intel realised this as well. How long did the 4004 last before they came out with the 8008?

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