Everything You Always Wanted to Know About the Z80

The Zilog Z80 8-bit microprocessor holds a special place in the hearts of retrocomputing enthusiasts everywhere in large part because it powered classic computers and consoles in the early days of personal computing, such as the TRS-80, ZX Spectrum, and Sega Master System.

With a relatively low transistor count and simple instruction set architecture, early microprocessors such as the Z80 offer hobbyists a clear window to understand the inner workings of the chip in a way that is obscured in modern chips by their far greater complexity. But of course appropriate tools are still needed to help us gain a deep understanding.

Inspired by the Visual6502, Goran Devic has built the Z80 Explorer — a netlist-level Z80 simulator that can run machine code and also packs in lots of features to help in reverse engineering and better understanding the chip.

As a simulator, Z80 Explorer runs machine code at the netlist-level. As the binary instruction opcodes are sent to the simulated pins, they propagate through the internal nets of transistor gates. The resulting function is identical to what would be observed if running like opcodes on a physical silicon chip. An included waveform view shows signals propagating through the simulator much as a logic analyzer would show for a physical chip.

The simulation speed is not exactly blazing fast — on Devic’s 4GHz i7-4790K CPU, Z80 code runs at about 2.3 kHz. That would be around 2000 times slower than a typical silicon Z80, but that is not really the point of the Z80 Explorer. The most interesting aspects of the tool lie in the exploratory features.

Z80 Explorer loads chip die layer images and netlists from the Z80 dataset. The layers (or combinations of layers) can be viewed and zoomed in on to get a better look at features such as nets, transistors and vias. Adaptive annotations (that show/hide with zoom level) are displayed to label these features. To gain further insight, a schematic can be shown for a net. The lengthy feature list is documented in the user guide.

Check out the following video to see Devic demonstrate some of the many useful features of Z80 Explorer.