HAM256 viewer by Andrew Owen and Simon Owen
This software is Public Domain

HAM256 is a software display mode that increased the number of available colours on screen with ULAplus from 64 to 256. It is not really like the Amiga's HAM mode. It is more like the Atari ST's Spectrum 512 mode (but the Spectrum 256 name was already taken).

Images must be in the standard .SCR format with alternate rows using alternate pairs of CLUTs as follows:

row 0: CLUT 0 and 1
row 1: CLUT 2 and 3

and so on.

This gives a maximum of 32 colours per line, and gives the CPU enough time to change the colours for the next pair of CLUTs before they are displayed.

Palette changes are stored in a a 768 byte .PAL file in row order.

If you want to avoid the border stripes you must ensure that every 8th palette entry out of 64 has the same value and set BORDER 0.

The timing has been tested against the real hardware's contention model, although not on the real hardware. The mode works correctly on SpecEmu. Your milage may vary with other emulators.

LOAD your image at 57844 and your palette file at 64756. Call USR 57700 to display the image and the PAUSE 0 (if calling from BASIC). Call USR 57702 to deactivate the IM2 display code.