Direct video memory

Video memory starts at 0x20000

In Mode 4 (512×256 4 colour) the memory is laid out in byte pairs (or words)

Red byte        | Green byte
0 0 0 0 0 0 0 0 | 0 0 0 0 0 0 0 0

Those 16 bits set a series of 8 contiguous horizontal pixels. The values of the Red and Green byte are AND-ed to get the final colour:

Red | Green | Result
1   | 0     | Red
0   | 1     | Green
1   | 1     | White
0   | 0     | Black

Example 1:

0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1

…would draw an 8 pixel horizontal line in Green.

Example 2:

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

…would draw an 8 pixel horizontal line in White.

Example 3:

1 0 1 0 1 0 1 0 0 1 0 1 0 1 0 1

…would draw 8 alternating pixels of Red and Green.

There are a range of development tools and languages for the QL, including the built-in SuperBASIC. But I am more interested in using the same type of tools that I use for the NEC PC-98, MS-DOS, X68000 and more; a reasonably standards compliant C compiler, assembler and linker.

There are a few C compiler options for the Sinclair QL - which are summarised nicely here: http://www.dilwyn.me.uk/c/index.html

I touch on a few of them, including the one I am using (C68, in it's modern cross-compiler incarnation) below.

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