In the last article, I described the sequencer, which is the heart of a CPU, and a few other blocks that perform various tasks. But how is it all orchestrated to perform useful work?
Enter the PLA and the Timing matrix – the mind of a CPU.
Category: Retro
Stuff from the past.
A Z80 : The Heart
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This is how it all works.
The sequencer is “the heart” of a CPU. It gets the external clock which in turn toggles two rows of flip-flops that generate machine cycles (M-cycles) and clock periods (T-states).
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A Z80 From the Ground Up
A-Z80 is a conceptual implementation of the venerable Zilog Z80 processor targeted to synthesize and run on a modern FPGA device. It differs from the existing (mostly Verilog) Z80 implementations in that it is designed from the ground up through the schematics and low-level gates.
The Anatomy of a Z80 Gate
The 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.
Data pins (D0-D7) carry arguably the most complex signals on the Z80 since they are both bi-directional and capable of tri-stating. They are located around the +5V pin – four of them on each side. This is a microphotograph of a gate of one of the data pins which we will look at more closely today – a pin for a data line D6.
Z80 Instruction Register deciphered
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 guidelines, which I will describe in this post.
Start with a good and clean die shot. Although the Visual 6502 team had a good one, it was somewhat grainy, and I’ve found a much cleaner version here. There is a slight difference in masks, but the functions are the same. In fact, it may even help to look at several versions when trying to decipher the layout.
ZiLOG Z80 (un)documented behavior
In my previous post, I described an Arduino dongle and the software that can be used to clock a Z80 CPU and dump the 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 a few situations which were not too obvious (to me) after reading various pieces of documentation.
Continue readingArduino and ZiLOG Z80
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.
Sinclair ZX81 lives on!
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 images and how its Z80 CPU boots. I instrumented it and captured scope and logic analyzer images as it was powering on.
What a beauty!
