In this blog I will show you how to interface an Atari-style joystick to the Altera DE1 FPGA board running a Spectrum implementation, how to change the ROM to enable you to input some game-cheat pokes and a few games I eventually completed using this setup.
...continue reading "ZX Spectrum ROM mods"


In the last article I presented a different way of architecturally modelling a Zilog Z80 processor. It is time to do something really useful with it and what could be better than reliving the past for a moment? Let's recreate an old computer and load in and play some games!
...continue reading "ZX Spectrum on FPGA using A-Z80 CPU"

In the first post I described the sequencer, a circuit that provided discrete timing signals to space operations apart. In the second post I mentioned the Timing matrix that was run by these signals and orchestrated a dance of control signals in time.

This article is about making it all alive and kicking within an FPGA solution.
...continue reading "A Z80 : The Soul"


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.
...continue reading "A Z80 From the Ground Up"


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"