RZ80 — Z80 ready for FPGA/ASIC, but also runs in C

Latest build: ASIC layouts, GDS and FPGA bitstreams (sky130, IHP SG13G2, GF180MCU with 7- and 9-track cells, and the IHP and GF180 chips with a Z80 pad ring; IcePi Zero, Tang Primer 25K, iCEBreaker, pico-ice, iCESugar).

A reverse-engineered, schematic-faithful Z80-compatible CPU core, implemented in parallel as a C99 hardware-like model and a Verilog-2001 RTL core.

The design follows the original Zilog Z80's internal organization — PLA-style instruction decode, an explicit M-cycle / T-state timing sequencer, named control signals, an internal register / bus datapath, a Shirriff-informed ALU, an explicit flags subsystem (including undocumented behavior), refresh, and interrupts — rather than being a high-level instruction interpreter.

The C model is the fast executable reference; the Verilog RTL implements the same conceptual architecture and is checked against the C model phase-by-phase. For emulators, cfast/ generates the model into a phase engine with the same pins every half clock; its speed against other C Z80 emulators.

Layout

cmodel/    C99 hardware-like model
cfast/     the model generated into a fast phase engine for emulators
           (its speed against other C Z80 emulators: cfast/BENCHMARK.md)
rtl/       Verilog-2001 synthesizable RTL
tests/     unit + instruction + timing-trace tests, ZEX suites, sim TBs
scripts/   Python and C test generators, oracle runners, trace comparators
docs/      design contract, research notes, verification, known differences

Run BASIC!

The very first thing you can do on this faithful Z80 reconstruction is to run classical BASIC.

make basic

This will take you back in time to the BASIC prompt:

Z80 BASIC Ver 4.7c
Copyright (C) 1978 by Microsoft
31948 Bytes free
Ok
10 PRINT "HELLO Z80"
20 GOTO 10
RUN
HELLO Z80
HELLO Z80
HELLO Z80
HELLO Z80
HELLO Z80
Break in 10
Ok

See tests/basic/README.md for the BASIC ROMs we vendor (NASCOM BASIC 4.7 and 1 KiB Tiny BASIC), the I/O conventions, and the runner's interactive controls (Enter, Backspace, Ctrl-C / Ctrl-Space = BREAK, Ctrl-\ = exit).

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