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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RE: Uh
by ferrels on Sun 8th Sep 2013 17:32 UTC in reply to "Uh"
Member since:

What did you expect? It's a blog for goodness sakes, not a novel or hardback book that you purchased at the bookstore for $50. Technically speaking it's a darn good blog. If you want perfection go buy a James Joyce novel.

Edited 2013-09-08 17:34 UTC

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RE[2]: Uh
by henderson101 on Mon 9th Sep 2013 11:51 in reply to "RE: Uh"
henderson101 Member since:

Ah yes, there's a great section on the 6502 instruction set tucked in to the middle of the Dubliners, and totally I forgot about the Z80 primer in the last chapter of Ulysses! Thanks for your extremely helpful comment!

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