Z80: Zilog's Legendary Processor
The chip that powered a British gaming revolution
The Zilog Z80 powered the ZX Spectrum, Amstrad CPC, MSX, and countless arcade machines—becoming the CPU of choice for European home computing.
Overview
Designed by Federico Faggin after he left Intel, the Zilog Z80 (1976) improved upon the Intel 8080 while maintaining compatibility. Its rich instruction set, dual register banks, and block operations made it a favourite for both computers and arcade hardware throughout the 1980s.
Fast facts
- Clock speed: 2.5-8 MHz depending on version (Z80A = 4 MHz, Z80B = 6 MHz).
- Data bus: 8-bit with 16-bit address bus (64KB addressable).
- Registers: A, B, C, D, E, H, L, plus shadow set (A’, B’, C’, etc.).
- Index registers: IX and IY for complex addressing.
- Refresh register: R, auto-incremented for DRAM refresh.
Register architecture
The Z80’s register richness was a major advantage over the 6502:
| Registers | Purpose |
|---|---|
| AF | Accumulator + Flags |
| BC, DE, HL | General purpose / address pairs |
| AF’, BC’, DE’, HL’ | Shadow registers (instant swap with EXX) |
| IX, IY | 16-bit index registers |
| SP | Stack pointer |
| PC | Program counter |
| I | Interrupt vector base |
| R | Memory refresh counter |
Key instruction types
- Block operations: LDIR, LDDR for fast memory copies.
- Bit manipulation: SET, RES, BIT for individual bit control.
- Relative jumps: JR with signed 8-bit offset saves bytes.
- Index modes: (IX+d), (IY+d) for structure access.
- I/O instructions: IN/OUT with port addressing.
Z80 vs 6502
| Feature | Z80 | 6502 |
|---|---|---|
| Registers | Many (with shadows) | Few (A, X, Y) |
| Addressing modes | Fewer but powerful | Many variations |
| Block operations | Built-in | Must be coded |
| Code density | More verbose | More compact |
| Learning curve | Steeper | Gentler |
Systems powered
- Home computers: ZX Spectrum, Amstrad CPC, MSX, TRS-80
- Consoles: Sega Master System, Game Gear, Game Boy (modified)
- Arcade: Pac-Man, Galaga, countless others
- Business: Kaypro, Osborne 1, CP/M machines
Cultural impact
The Z80 defined British gaming. Sinclair’s use of the chip in the ZX Spectrum created a generation of bedroom coders who learnt assembly through necessity—the Spectrum’s BASIC was too slow for serious games. The skills forged here powered the UK games industry for decades.