MOS Technology (Commodore) · 1985 · Amiga Original Chip Set (OCS)
Three custom chips with names like a family, and a coprocessor that executes an instruction stream synchronised to the electron beam — the reason Amiga demos looked impossible.
The Amiga's Original Chip Set was conceived by Jay Miner — the same engineer behind the Atari 2600's TIA — and manufactured by Commodore's subsidiary MOS Technology. Its power comes from a design philosophy that was genuinely unusual in 1985: rather than making the CPU do everything, offload as much as possible onto dedicated silicon, and let the 68000 supervise. Agnus is the centre of the machine. It arbitrates all access to chip RAM between the 68000 and the other custom chips through a priority system, and it contains two sub-processors that do the actual work. The Blitter moves and combines blocks of memory at high speed without troubling the CPU, which is why Amiga games could scroll and animate large areas while the processor was busy elsewhere. The Copper — short for co-processor — is the more remarkable of the two: a programmable state machine that executes its own instruction stream, synchronised to the video beam. It can change hardware registers at precise screen positions, mid-frame, which is how the Amiga produces its signature effects — gradient skies, split screens with different resolutions, colour palettes that change halfway down the display. Denise handles the graphics output proper: bitplane display, palette registers, smooth scrolling, hardware sprites, screen object priority and collision detection. Paula is the audio chip, with four independent hardware-mixed 8-bit PCM channels, each with 65 volume levels and sample rates from about 20 Hz up to nearly 29 kHz — and, in one of the era's stranger design decisions, Paula also handles interrupts, the serial port, analogue joysticks and the floppy drive.
| Agnus | Memory arbitration, Blitter, Copper |
|---|---|
| Denise | Bitplanes, palette, hardware sprites, collision detection |
| Paula | 4-channel 8-bit PCM audio, interrupts, floppy, serial, joystick |
| Copper | Beam-synchronised coprocessor with its own instruction stream |
| Audio | 4 channels, 65 volume levels, up to ~29 kHz |