Hold And Modify - Definition and Overview

This article or section should be merged with HAM.

The Hold and Modify mode (and the HAM8 variation found in later Amiga models) allowed a pallette of 4096 colours to be displayed using only six bits per pixel, with some restrictions.

If bits 4 and 5 for a pixel were zero, then remaining four bits specified a colour from one of 16 user-definable colour look up table values. However, if bit 4 was set to one and bit 5 was set to zero, the remaining four bits specified a new value for the blue component of the pixel (the modify bit of the name); the red and green values would be copied from the pixel immediately to its left (the hold part of the name). Similarly, if bit four was zero and bit five was one then the red component is modified; setting four and five to one changes the green component.

(The AGA chipset added an 8-bit version of this, allowing 64 base colours and a 6-bit modify portion, but in all other respects worked in the same way).

Although not as flexible as a 12-bit (or 24-bit in the case of HAM8) mode and prone to suffering from "fringing" artifacts, judicious selection of the base colours could produce some impressive results. Without close scrutiny a HAM8 picture is indistinguishable from a more traditional 16-bit display, and in many cases looks as good as a 24-bit display.