Amstrad CPC
All-in-one colour computing
Alan Sugar’s Amstrad CPC bundled a computer, cassette deck, and monitor—bringing tidy, reliable 8-bit power to the masses.
The Amstrad CPC (Colour Personal Computer) launched in 1984 as a complete kit: computer, cassette deck, and colour or monochrome monitor in one package. It targeted users who wanted less tinkering and more productivity—or, more often, colourful games.
Fast facts
- CPU: Z80A clocked at 4 MHz, running at an effective 3.3 MHz — see below.
- Models: CPC464 (tape), CPC664 (disk), CPC6128 (128K RAM with disk).
- Graphics: three modes, trading resolution against colour — Mode 0 at 160×200 in 16 colours, Mode 1 at 320×200 in 4, Mode 2 at 640×200 in 2.
- Palette: 27 colours. The hardware takes 32 values, but the firmware manual is blunt that “colours 27–31 are not intended for use; they are merely duplicates of other colours available”.
- Sound: AY-3-8912 PSG with three channels.
The Gate Array makes time regular
The CPC’s Z80 is clocked at 4 MHz and does not run at 4 MHz. The reason is the custom Gate Array, and the firmware manual explains it in two sentences:
“Accesses to memory are synchronised with the video logic — they are constrained to occur on microsecond boundaries. This has the effect of stretching each Z80 M cycle (machine cycle) to be a multiple of 4 T states.”
The consequence is a headline number that looks like a loss — “the effective clock rate is approximately 3.3 MHz” — and a property that is worth more than the sixth or so of the clock it costs. Every machine cycle becomes a whole number of microseconds. Instruction timing stops depending on where in the frame you are, or which memory you touched.
Set beside its contemporaries, that is a different answer to the same question. The Atari 8-bit lets ANTIC halt the 6502, more often at higher resolutions. The C64 takes 43 of a scanline’s 63 cycles on a badline and leaves the programmer to track which lines those are. The Amiga lets bitplane DMA take up to every slot there is. All three make the cost variable, and hand the programmer the job of predicting it.
Amstrad made the cost constant. You pay on every access whether the video logic wanted the bus or not, and in exchange nothing is ever contended, so a routine takes the same time on line 1 and line 200. That is why CPC demo and raster work is written to microsecond counts where Spectrum work is written around contention tables — the machine’s clock is slower and its time is honest.
The Gate Array supports the consequence rather than merely permitting it. Mode is not fixed for a frame: the hardware “synchronises mode changing to the next horizontal fly-back in order to aid software that requires different parts of the screen to be handled in different modes”. Splitting a display into a 16-colour panel and a 2-colour scoreboard is a designed-for operation here, not a raster trick smuggled past the video chip.
Why it mattered
- Ready out of the box: monitor included meant no fuzzy TV setup or shared living-room fights.
- Publishing scene: companies like Ocean, Hewson, and Gremlin supported the CPC heavily, porting hits from C64/Spectrum and crafting exclusives.
- Stability: reliable hardware made it popular in France, Spain, and Germany where clones and upgrades thrived.
Modern legacy
- Emulators: WinAPE, Caprice Forever, and Retro Virtual Machine.
- Hardware add-ons: modern SD storage, RAM expansions, and FPGA remakes keep the platform lively.
- Demoscene: still active with productions showing off overscan graphics and sample playback.