Ribos

This little demo is intended to be a well-commented example of how to program a raster interrupt in 6502 assembly language on a Commodore 64.

This (r)aster (i)nterrupt changes the colour of a region of the (bo)rder of the C64 (s)creen; thus, RIBOS. Also, it's the name of a planet from Dr. Who, if that means anything.

How to Run the Demo (using the VICE C64 emulator, x64)

1. Obtain VICE from http://www.viceteam.org/, install it, and run x64

2. Mount this project's directory as drive 8:

Make sure Peripheral settings > Device #8 > Enable IEC Device is checked, then select Peripheral settings > Device #8 > File system directory... and enter the path to the project directory.

1. LOAD "RIBOS.PRG",8,1

2. SYS 49152

3. You should see the colour of the middle of the border change while you get a READY. prompt and can continue working.

How to Assemble the Program (using the p65 assembler)

1. Obtain p65 from http://hkn.berkeley.edu/~mcmartin/P65/ (I used p65-Perl version 1.1) and install it somewhere on your path. If your Perl interpreter isn't located at /usr/bin/perl, change the first line of p65 appropriately.

2. p65 -v -t -b ribos.p65 ribos.prg

The switches aren't necessary, but they make it feel like p65 is doing something difficult and important. It also isn't necessary to add the '.prg' extension on the end of the binary object's filename, since it will appear as a PRG file to VICE anyway, but it's nice as a reminder when you're working in a modern operating system.

1. Follow the steps under 'How to Run this Demo' to see that it worked.

How it Works

Read the source! I've tried to make it very well-commented, including what happens when you leave out some steps.

I wrote this demo because it was a long time since I had done any C64 programming, and, having just obtained a copy of the 'Commodore 64 Programmer's Reference Guide,' I wanted to code something challenging, yet not too involved. I remembered raster interrupts as one of those quintessential C64 low-level graphics tricks, so I decided to try my hand at that. Looking around on the Internet, I found this page:

http://everything2.com/index.pl?node_id=79254

Although it's a fairly detailed description, it took me a couple of frustrating hours to implement it successfully - both the everything2 article and the Reference Guide were pretty muddy on a couple of points. What I learned in the process is written into the comments.

Happy raster-interrupting!

-Chris Pressey
April 10, 2007
Vancouver, BC