interrupt

All posts tagged interrupt

A very important component of any demo is having a kick arse SID chip track backing it. That means, we need to be able to play a track while performing some mastery of the raster bar. It was something I was dreading looking at, but turned out to be a very straight forward thing and only ended up taking about 10 minutes to get working.

C64 Music player running
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Simple c64 interrupt

Having done some work with interrupts now, I’ve realised just how powerful they can be for demo coding and how time sensitive some effects / routines are. I’ve looked at a few different ways to set them up and thought I’d recap.

Interrupts occur every 60th of a second. On the C64, memory locations $0314 and $0315 (788 & 789 in decimal) contain a vector to the execution address of the normal interrupt service routine. The default values here are $31 (49) and $ea (234), which form $ea31.
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C64 Raster Colour Cycle

Time to start getting familiar with using the zero page and use an interrupt for some simple effects. I decided to create a basic colour cycle effect that updates the colour map, shifting the character colours in a direction. Back in the day, I couldn’t get my head around how to actually achieve this via assembly language, but I’ve had a lot of experience in programming since then, so have a much better idea of how to achieve the effect.
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