Hold-And-Modify

Hold-And-Modify, usually abbreviated as HAM, is a display mode of the Commodore Amiga computer. It uses a highly unusual technique to express the color of pixels, allowing many more colors to appear on screen than would otherwise be possible. HAM mode was commonly used to display digitized photographs or video frames, bitmap art and occasionally animation. At the time of the Amiga's launch in 1985, this near-photorealistic display was unprecedented for a home computer and it was widely used to demonstrate the Amiga's graphical capability. However, HAM has significant technical limitations which prevent it from being used as a general purpose display mode.

Background

The original Amiga chipset uses a planar display with a 12-bit RGB color space that produces 4096 possible colors.

The bitmap of the playfield was held in a section of main memory known as chip RAM, which was shared between the display system and the main CPU. Due to timing considerations, the chipset only had time to read 6 bits per pixel before it was time to draw the next pixel on screen. To reduce the amount of data needed to describe an image and thus fit within this limitation, the display system used an indexed color system with a color palette.