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Data bus on the Z80 processor is 8 bits wide. Data bus wires carrying information within the chip itself do not simply connect to package pins and out to the world - the gate circuitry of each bit is quite complex. This article presents a transistor level schematic of a data bit’s gate which I reverse-engineered from a die photograph.
...continue reading "The Anatomy of a Z80 Gate"


After reading excellent Ken Shirriff's blog on reverse-engineering parts of the Z80 CPU, I decided to learn how to decipher some of the chip die-shots myself. It turns out not to be that difficult if you follow certain guidance which I will describe in this post.
...continue reading "Z80 Instruction Register deciphered"


In my previous post I described an Arduino dongle and the software that can be used to clock a Z80 CPU and dump states of its buses and pins while executing a controlled set of test cases.

Here I show a trace of every single Z80 instruction as run by that setup. I also outlined some of the tests created manually that clarified few situations which were not too obvious (to me) after reading various pieces of documentation.

...continue reading "ZiLOG Z80 (un)documented behavior"


If you want to find out exactly what a venerable Z80 is doing on its bus while executing instructions, in this post I outlined a dongle and the software that will let you see that. Using just a few components and connecting them to an Arduino Mega, you can trace instructions clock by clock and observe what's happening on the bus.
...continue reading "Arduino and ZiLOG Z80"

I got hold of an old Sinclair ZX81 in apparently good and working condition!

ZX81 was the first personal computer I owned. I was 13 years old. At that time, I copied its 8K ROM, byte by byte, into a notebook, and hand-disassembled it (a consequence of which I still suffer from: I still remember some Z80 opcodes), but I never opened it. Finally, now I can do what I missed 🙂

This post shows how it generates TV image and how its Z80 CPU boots. I instrumented it and captured scope and logic analyzer images as it was powering on.
...continue reading "Sinclair ZX81 lives on!"