All posts by Jesder

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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1x1 character text scroller

The text scroller. The single most used demo effect of all time. An effect that always fascinated me on the C64 back in the day, but (like with the colour cycle effect) something I could never get my head around all those years ago. I have often thought to revisit coding on the C64 JUST to successfully produce this, but never got around to it. Now is the time.

The time spent diving back into assembly language over the last two weeks has been quite fruitful. The lessons learned and knowledge gained will come in handy here. The goal is to create a simple, 1×1 character scroller. Once I can do that, it shouldn’t be too difficult to expand it further in the future to jazz it up.
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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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I am in the process of sorting out a few things to improve the site.

Firstly is the logo, which I hope will be sorted out sometime next week. Next up is a proper facebook page where I’ll be posting updates also, along with becoming active on twitter. I am also getting github sorted out so I can commit my source code and make all the routines I post about here available.

My main focus will remain on the coding, but I’d like to try and clean up a few things around here before progressing too much further. I want to be able to look back in 12 months and be amazed at how far I’ve come and what I achieved.

I should have two new routines to put up in the next day or so as well!