Hold-And-Modify - Overview of Amiga Chip Set

Overview of Amiga Chip Set

The original Amiga chipset uses a planar display with a 12-bit RGB color space (4096 possible colors). In order to describe any arbitrary color in this color space 12 bits are needed (4 bits per color component). However, the chipset can only access up to 6 bits per pixel in a part of memory known as 'chip RAM'. All display modes use an indexed color system with a color palette, where each palette entry can be set to one of the available 4096 colors. As the original chipset only has 32 color registers other techniques must be used to express more than 32 on-screen colors.

A display mode known as Extra Half-Brite uses 5 bit planes to index entries from the 32 color palette, and an extra sixth bit plane to control whether the color should be displayed at full or half luminosity. This allows the sixth bit plane to indicate shadows. With judicious selection of the palette, 64 distinct colors may exist on screen, though this is still far fewer than the full 4096.