52 minutes ago, desertfish said:
Yeah I don't know exactly, I just assume undefined behavior when an underflow or overflow occurs. And perhaps it's cheap to clip just the X-position, and is that implemented? Who knows -- use the source, Luke ?
Alas my 6502 assembly was limited to a couple routines I coded trial and error (mostly error) in the Plus4 MONITOR to speed up some game stuff, and as that was 1986 or so I'm not really qualified to read or get what may be the issue:
Anyone with the chops wanna give it a look?: https://github.com/commanderx16/x16-rom/blob/a200d6266038fc5ff506280e70383e5774bd0ac9/basic/graph.s