Hold And Modify (HAM) is a graphics mode on the Amiga computer which allows you to get more colors from the hardware by using two bits from each pixel to control a "hold" and "modify" feature of the base colors. 6-bit HAM (HAM6) used 4 bits for base colors, while 8-bit HAM (HAM8) used 6.

The HAM feature was hacked by Jay Miner, who almost removed from the early model Amiga architecture due to it's slowness. It is fortunate to have survived since it was one of the first graphic modes on a personal computer to show photorealistic images.

It would be awesome if someone could write a really detailed description of how HAM works.