[stella] Cycle-60 double-HMOVE magic

[stella at casperkitty.com]

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. 

Note that emulation does not support this yet, but it works on my real 7800.

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