Amiga OCS Palette Calculator

Amiga OCS color palette

The Amiga OCS (Original Chip Set, 1985) pushes video through the Denise chip, which carries a master palette of 4096 colors stored as 12-bit RGB (4 bits per channel, $RGB nibbles). How many of those you actually see in a single frame comes down to the bitplane count: 1 bitplane gives 2 colors, 2 gives 4, 3 gives 8, 4 gives 16, and 5 bitplanes = 32 colors in lowres (320×256 PAL / 320×200 NTSC). The famous HAM6 mode (Hold-And-Modify, 6 bitplanes) gets all 4096 colors on screen by holding two channels and modifying one per pixel, and the price you pay is color fringing. EHB (Extra HalfBrite) spends its 6 bitplanes on 64 colors instead: 32 base plus 32 darkened versions. The 32 palette registers live at $DFF180–$DFF1BE.

Applications

Retrocomputing and Amiga homebrew (vasm, vbcc, AmigaOS 1.3), Amiga demoscene productions like Eurocharts and Breakpoint, moving IFF ILBM images to and from PNG, pixel-art workflows on vintage hardware, and writing emulators such as FS-UAE and WinUAE.

FAQ

Why 12-bit RGB and not 24-bit? The 1985 Denise chip had a modest DAC. Four bits per channel fit the silicon budget of the day and still gave you 4096 distinct colors, which was plenty back then.

Is HAM6 truecolor? Almost. It does show all 4096 palette entries at once, but each pixel can only change one channel relative to its left neighbor, which is why you get color bleed on sharp transitions.

OCS vs ECS vs AGA? OCS is the original from 1985, with its 4096 colors. ECS arrived in 1990 as the enhanced version: same 4096, but higher resolutions. AGA (1992, A1200/A4000) is the big leap, with a 24-bit / 16.7M palette, 256 on-screen colors and HAM8.