RetroShield Z80 CP/M 2.2 — Arduino Giga R1 WiFi

CP/M 2.2 running on a real Zilog Z80 processor via the RetroShield Z80, controlled by an Arduino Giga R1 WiFi. Disk I/O is served over WiFi by a Rust sector server.

The Z80's 64KB address space lives in the Giga's internal SRAM. A custom level converter PCB bridges the Giga's 3.3V logic to the RetroShield's 5V. The system boots CP/M, runs DIR, and plays Zork I interactively.

Hardware

Component	Description
Arduino Giga R1 WiFi	STM32H747, 480MHz Cortex-M7, 1MB SRAM, WiFi
RetroShield Z80	Real Zilog Z80 CPU on shield board, 5V logic
Level converter PCB	Custom shield with TXB0108 bidirectional level shifters (design files)
Sector server	Rust TCP server on local network (source)

The TXB0108 Problem

The TXB0108 level converters use auto-direction sensing, which fails for several Z80 bus signals:

• IORQ_N — stuck HIGH (never toggles)
• RD_N — stuck HIGH (never toggles)
• WR_N — works for memory cycles only, not I/O
• Data bus — Z80-to-Arduino direction invisible during I/O

These failures required a fundamentally different approach: instead of passively reading bus signals, the Arduino actively decodes the Z80's instruction stream and maintains software copies of the CPU's internal registers.

Software Architecture

Guard-Only M1 Detection

Each Z80 instruction takes a known number of T-states. A lookup table (tStates[256]) provides the count for every opcode. After detecting one M1 (opcode fetch), a guard timer prevents false M1 detection until the instruction completes. The next MREQ-active memory read after the guard expires is the next M1.

Software PC (softPC)

A software copy of the Z80's program counter, updated at each M1 by analyzing the opcode and computing the next address. Immune to the TXB0108's address bus propagation delay.

Shadow Registers

Software copies of A, B, C, D, E, H, L, F, and SP, updated at M1 time by decoding each opcode from z80RAM[softPC]. Used for:

• I/O data — OUT instructions need the A register value, which can't be read from the bus
• Branch prediction — conditional branches need flag values to determine the taken/not-taken path
• Pre-writes — memory store instructions use shadow register values to write correct data to z80RAM

Pre-Writes and Deferred Writes

Since bus writes go to wrong addresses (TXB0108 propagation delay), all memory stores are handled in software at M1 detection time. Read-modify-write instructions (INC/DEC (HL), CB-prefix on (HL)) use deferred writes to avoid double-applying operations.

Direct GPIO Register Access

All hot-path I/O uses direct STM32H747 GPIO register access (BSRR, IDR, MODER) instead of Arduino HAL digitalRead()/digitalWrite(). This, combined with removing unnecessary pinMode() calls and I-cache optimization, achieves ~690,000 Z80 cycles/sec (~0.69 MHz effective clock).

Files

File	Description
kz80_cpm_giga.ino	Main Arduino sketch (~2,600 lines)
boot.bin	Z80 boot loader binary (331 bytes) — loads CPM.SYS via sector server
CPM.SYS	CP/M 2.2 system image: CCP + BDOS + BIOS (6,607 bytes)
A.DSK	CP/M disk image for drive A (256 KB) — contains ZORK1, ZORK2

Pin Mapping

The RetroShield uses the same physical pin positions on the Giga as on the Mega 2560. The direct GPIO register access requires knowing the STM32H747 port/pin for each Arduino pin:

Function	Arduino Pin	STM32 Port/Pin
CLK	D52	PK2
MREQ_N	D41	PK7
WR_N	D40	PE6
IORQ_N	D39	PI14
INT_N	D50	PI11
RESET_N	D38	PJ7
Data D0–D7	D49–D42	PE4, PK0, PB2, PH15, PI13, PG10, PI10, PI15
Addr A0–A7	D22–D29	PJ12, PG13, PG12, PJ0, PJ14, PJ1, PJ15, PJ2
Addr A8–A15	D37–D30	PJ6, PK6, PJ5, PK5, PJ4, PK4, PJ3, PK3

CP/M Memory Map

0000-00FF   Page Zero (jump vectors, FCBs, command buffer)
0100-DFFF   TPA (Transient Program Area) — 56KB
E000-E7FF   CCP (Console Command Processor)
E800-F5FF   BDOS (Basic Disk Operating System)
F600-FFFF   BIOS (Basic I/O System)

I/O Port Map

Port	Function
0x80	MC6850 ACIA control/status (console serial)
0x81	MC6850 ACIA data (console TX/RX)
0x10	Disk command register
0x11	Disk status register
0x12	Disk data byte
0x13	Disk filename (write char by char, null-terminated)
0x14–0x15	Disk seek position (low, mid bytes)
0x16–0x17	DMA address (low, high bytes)
0x18	Block command (0=read, 1=write)
0x19	Seek position high byte

Prerequisites

• Arduino IDE or arduino-cli with the arduino:mbed_giga board package installed
• A RetroShield Z80 with a level converter board
• A machine on the same WiFi network running the sector server

Configuration

Edit the WiFi credentials and server IP at the top of kz80_cpm_giga.ino:

const char* WIFI_SSID = "YourNetwork";
const char* WIFI_PASS = "YourPassword";
IPAddress SERVER_IP(192, 168, 0, 248);
const int SERVER_PORT = 9000;

Build & Upload

# Using arduino-cli
arduino-cli compile --fqbn arduino:mbed_giga:giga kz80_cpm_giga.ino
arduino-cli upload -p /dev/cu.usbmodem2101 --fqbn arduino:mbed_giga:giga kz80_cpm_giga.ino

# Serial monitor
screen /dev/cu.usbmodem2101 115200

Or open kz80_cpm_giga.ino in the Arduino IDE, select Arduino Giga R1 WiFi as the board, and click Upload.

Usage

1. Start the sector server on your host machine, pointing it at a directory containing boot.bin, CPM.SYS, and disk images:

   sector_server ./cpm_files 9000

2. Upload the sketch to the Giga and open a serial terminal at 115200 baud.

3. The Giga connects to WiFi, connects to the sector server, loads the boot loader, and starts the Z80. You should see:

   RetroShield Z80 Boot Loader
   Copyright (c) 2025 Alex Jokela, tinycomputers.io
   
   Loading CPM.SYS.....................................................
   Boot complete.
   
   RetroShield CP/M 2.2
   56K TPA
   
   a>
   

4. Type dir to see disk contents, or zork1 to play Zork I.

Blog Posts

This project is documented in a three-part series on tinycomputers.io:

1. My Experience Using Fiverr for Custom PCB Design: A $468 Arduino Giga Shield
2. Porting CP/M to the Arduino Giga R1: When Level Converters Fight Back
3. Playing Zork on a Real Z80: From CP/M Boot to the Great Underground Empire

License

The Arduino sketch (kz80_cpm_giga.ino) is licensed under the BSD 3-Clause License. See LICENSE.

The CP/M 2.2 system files (CPM.SYS, boot.bin) and disk image (A.DSK) contain software originally developed by Digital Research, Inc. CP/M was released under its own license by Lifeboat Associates and later placed into free distribution by Tim Olmstead and the Unofficial CP/M Web Site. These files are included for convenience and are not covered by the BSD license.

Author

Alex Jokela — tinycomputers.io