More Retro Programming

Published on 2005-09-07 by Mark VandeWettering

I’ve been mucking around more with programming the Atari 2600, and have just begun to figure out the vagaries of moving the player missiles around.

It’s complicated.

Basically, as the video hardware scans from left to right, you set the horizontal position of a player by issuing a store to hits horizontal position register. The problem: the 6502 is only fast enough to set the position within 15 pixels of a given location. To refine the horizontal position, you need to issue a shift of -8 to +7. The timing is critical and tricky.

I haven’t really worked it all out yet, but I have players moving behind the playfield here in my example.

I think I need to carefully work out the timing on paper before doing anything more refined.

Addendum: Not really sure where that little black bar is coming from either.