Commodore Amiga

The Amiga 1000 launched in July 1985, designed by a team led by Jay Miner, who had driven Atari’s 8-bit custom chips — the ANTIC and its siblings — a few years earlier. Where its 16-bit contemporaries paired a fast CPU with a dumb framebuffer, the Amiga surrounded a Motorola 68000 with co-processors that could run without it: Agnus for DMA, the blitter and the copper; Denise for video output and sprites; Paula for four channels of sampled audio, the floppy, serial and interrupts.

It also shipped with preemptive multitasking in 1985, on a machine with no memory protection and 256K of RAM — something neither the Macintosh nor Windows managed for years afterwards. The Amiga 500 put the same chipset into a keyboard case at a price European households would pay, and that is the machine most people mean when they say Amiga.

Fast facts

• CPU: Motorola 68000 at 7.09379 MHz (PAL) or 7.15909 MHz (NTSC) — a quarter of the ~28.4 MHz master oscillator, and exactly the low-resolution pixel rate. CPU and display are derived from one clock by design.
• Chipset: OCS (1985), ECS (1990), AGA (1992).
• Graphics: planar bitmaps, and how many planes you get depends on the resolution. Lores (320×256 PAL) allows up to six; hires (640×256, or 640×512 interlaced) allows at most four — and four is the ceiling because four hires bitplanes already consume every memory slot a scanline has. Five lores planes give 32 colours from a palette of 4096; the sixth is spent on a mode rather than on more free colours — EHB adds a half-brightness copy of the 32, and HAM6 reaches all 4096 on screen at once. AGA raises the palette to 16.7 million and HAM8 to 262,144.
• Sound: Paula, four channels of 8-bit PCM with independent period and volume, two mixed to each stereo output — sampled audio as standard in 1985.
• RAM: 256K to 512K “chip” RAM on the A1000/A500, expandable; later models far more. Chip RAM is reachable by the custom chips and contended with them; fast RAM is not addressable by the chipset, so access to it never contends.
• OS: AmigaOS — Exec’s preemptive multitasking, AmigaDOS, the Intuition windowing system, the Workbench desktop on top, and a shell.
• Storage: 880K 3.5″ floppy, formatted and read by Paula rather than a dedicated controller.

The machine that programs itself

The Spectrum’s defining constraint is spatial and the C64’s is temporal. The Amiga’s defining idea is delegation: the interesting work happens on chips that are not the CPU.

The copper is a display-list co-processor with three instructions — MOVE a value to a chip register, WAIT for the beam to reach a position, SKIP if it already has. That is the entire language, and it is enough to change the palette on any scanline, split the screen into regions with different resolutions, or reload bitplane pointers mid-frame. The C64 does this by taking a raster interrupt and spending CPU cycles; the Amiga does it with a list the copper executes on its own while the 68000 is busy elsewhere.

The blitter moves rectangular regions of memory through a barrel shifter, combining up to three sources with any one of the 256 possible boolean functions of three inputs, selected by an eight-bit minterm mask — plus line-drawing and area-fill modes. Scrolling, sprite compositing with masks, and polygon fill are all blitter jobs. A demo or game is largely an exercise in keeping it fed.

The catch is the bus, and the hardware manual prices it exactly. Bitplane DMA is served before anything else, and the ceiling is 160 memory time slots a scanline.

In low resolution, four planes or fewer leave the 68000 its alternate memory cycles and cost nothing. Past four, bitplane DMA begins taking cycles the processor would have had, and at six it occupies 160 slots a line — stealing, in the manual’s accounting, half the openings the CPU would have got at four planes. The 32-colour screen is not free; it is paid for at half the processor.

In high resolution each plane fetches 40 words a line rather than 20, and the manual does that sum explicitly: four planes take 160 slots, which is all of them. It is blunt about the consequence — this “effectively locks out the 68000 (as well as the blitter or Copper) from any memory access during the display, except during horizontal and vertical blanking.” A sixteen-colour hires screen suspends the very co-processors that define the machine for the whole visible frame.

That is the real shape of “how many bitplanes can I afford”, and it is why fast RAM exists. The lesson the platform teaches is scheduling work onto parallel hardware, which is a more modern skill than its age suggests — including the part where the parallel hardware contends for one bus.

Why it mattered

• Sampled sound as standard. Four channels of PCM meant real instruments and real voices in 1985, and it made the Amiga the machine the tracker was invented on — Ultimate Soundtracker (1987) and the MOD format that followed shaped how game and demo music was written for a decade.
• The demoscene’s home. C64 groups moved across as budgets allowed and brought their standards with them. Fairlight, Crest and their contemporaries pushed the chipset far past its specification.
• Creative work on a home budget. Deluxe Paint defined bitmap painting; the Video Toaster put broadcast titling and effects on desks that could not have afforded a suite; Babylon 5 rendered its early effects on Amigas.
• Games with a different look. Copper gradients, parallax by bitplane, and 32-colour art gave Amiga games a house style that is recognisable at a glance.

Models

• A1000 (1985) — the original. Signed inside the case by the team that built it.
• A500 (1987) — chipset in a keyboard, and the volume seller in Europe.
• A2000 (1987) — desktop case with Zorro expansion slots; the Video Toaster machine.
• A3000 (1990) — 68030, ECS, and AmigaOS 2.0.
• A600 (1992) — compact, IDE, and no numeric keypad; a commercial misstep.
• A1200 (1992) — AGA in a keyboard case, the A500’s true successor.
• A4000 (1992) — AGA and the high end, sold as an A4000/040 and also in a cheaper 68030 configuration.
• CD32 (1993) — an AGA console with a CD drive; Commodore went bankrupt in 1994 before it could establish itself.

Modern legacy

• Emulation: WinUAE and FS-UAE are the standards; MiSTer provides an FPGA recreation.
• Hardware: Vampire accelerator cards, PiStorm, and a steady supply of recapped and restored machines.
• Community: new games and demos each year, and demoparties that still run Amiga compos.

See also

• Jay Miner — the architect of the chipset.
• The Amiga custom chipset — how Agnus, Denise and Paula fit together.
• Copper and Blitter — the two co-processors that define the platform.
• Agnus, Denise, Paula — the chips themselves.
• 68000 — the CPU.
• Commodore — the company that bought the Amiga and then lost it.
• Commodore 64 — where most Amiga programmers came from.
• Demoscene and the bedroom coder — the culture that moved with them.