HMOVE magic
Entry posted by supercat
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The Atari 2600's HMOVE circuitry is, to put it mildly, interesting. I was just examining the schematics for it earlier today and figured out a trick that might be useful for some applications, most notably kernels in the style of Homestar Runner or Ruby Runner which move some sprites left and right by eight pixels on alternate scan lines.
The "classic" approach, invented by David Crane almost 30 years ago, was to only move sprites by seven pixels instead of eight and then shift the data bytes for alternate scan lines by a bit. This approach works, but it requires having blank columns since the sprites can't reach everywhere. A more modern approach is to use cycle 74 HMOVEs to push sprites eight pixels left and normal HMOVEs to push them right. This can be a little annoying, however, because it requires rewriting on every scan line the HMxx registers for any sprite whose motion is of interest.
There's another way, at least for "normal" Ataris (as distinct from "funny" Juniors).
Perform a "STA HMOVE" that starts at cycle 60. Perform another that starts on or before cycle 72.
Sprites with HMxx values of $00-$70 will behave normally. Sprites with an HMxx value of $80 will move left 8 pixels. Depending upon the amount of time between the HMOVEs, other values may cause sprites to become "confused". Note that $90 will always cause a sprite to become confused, and values $A0-$F0 that do not cause sprites to become confused will simply result in them not moving.
If it's desired to have some sprites move left and right by 8 pixels on alternate scan lines while other sprites stay put, the HMxx values for all the sprites may be set up beforehand: $80 for sprites that move and $00 for those that don't. Before a scan line where the $80 sprites are supposed to move right, do a "normal" HMOVE. On the line before those sprites are supposed to move left, do the funny double HMOVE described above. In this way, all the sprite motions can be handled without having to write HMxx or HMCLR during the kernel--just three HMOVEs every two lines.
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