Oldskooler Ramblings

the unlikely child born of the home computer wars

CGADEMO by Codeblasters

Posted by Trixter on November 22, 2014


I don’t normally reblog other people’s blogs, but my own blog has been neglected lately due to work on other vintage computing and programming projects, so I thought I would share a post by my friend Scali that covers a vintage programming problem just as well, if not better, then I would have covered. Enjoy.

Scali's OpenBlog™

Today I want to talk about a rather obscure, yet interesting demo, namely CGADEMO by Codeblasters, from 1992:

As you can read from the scroller, what’s interesting about this demo is that it runs at full framerate (60 Hz) even on the original IBM PC (8088 at 4.77 MHz with CGA). And that there are 16 colours on screen at the same time.

Unstable rasters

To start with the 16 colours… They use a trick similar to my palette switching in the 1991 donut. Namely, they change the background colour of the CGA palette at every scanline, which gives a rasterbar effect. This is very similar to what I have discussed on C64. This demo does not use a stable raster however, since that is very difficult to achieve on a PC anyway. Instead, they use polling of the hblank status bit to determine when a scanline is…

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