## A Calculator – Completed!

It has been a while since I wrote the last post on this project. In the meantime, I could dedicate all my limited free time to work on the implementation. It took a long time, but I did make significant progress. The calculator is pretty much completed.

## A Calculator (4): The Framework

For as long as I remember, I had played with LEGOs. Those simple blocks would transform into complex objects whose final shapes only existed in the supple thoughts of creative imagination. The kind of LEGO blocks we had while growing up were simple: 2×4, 2×2, 1×8. Anything unusual was rare and precious.

## A Calculator (3): Practical Numerical Methods

In the last post, we verified and quantified the precision of the basic four functions (addition, subtraction, multiplication, and division). Now, we can use them as a stepping stone for more complex functions. We can assume they will be available, so our experimentation could simply use built-in C++ functions.

In this article, the third in a series, we will continue with the proof-of-concept, or pathfinding, research.

## A Calculator

The idea for this project came about during a week of freezing winter arctic events here in Austin, Texas, with a failed power grid (as well as the Texas government), while keeping close to a gas fireplace, the only source of heat and light, for a couple of days. With a weak internet over a phone data line, I could only do some preliminary searches and mainly work out various details on a writing pad, growing feeling that the project may be personally exciting and practically doable.

It took a long time, but this FPGA Calculator is now finished and I am really happy with how well it turned out.

## ZX Spectrum ROM mods

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.

## ZX Spectrum on FPGA using A-Z80 CPU

In the last article, I presented a different way of architecturally modeling 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!

Sinclair ZX Spectrum was my second computer, the first one being ZX81. As I was growing up in Croatia, I had a good friend whose brother, living in Germany, regularly sent him tapes with new games. Through this steady stream of games I’ve probably seen most of them (thank you, Krešo!) However, although I could, I had never played them through. Instead, I would load in DevPac – a debugger – and disassemble them trying to decipher how they were made and occasionally find some pokes for infinite lives.

## A Z80 : References

This design would not be possible without Ken Shirriff who reverse-engineered some major portions of Z80 from the picture of a die. These are the portions of the A-Z80 design that are based on his work:

## A Z80 : The Soul

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 the dance of control signals in time.