> This'd work if TABLE is the first byte of a page, and if $81 is set to the
> number of that page (JMP ($xx) uses a 16-bit pointer.) If you can't set
> up TABLE that way, you'd need to do an add before storing the offset
> (which, of course, screws up the carry you already set...)
JMP ($0080) ; WASTE 5 CYCLES
TABLE NOP ; WASTE 2 CYCLES
NOP ; WASTE 2 CYCLES
NOP ; WASTE 2 CYCLES
NOP ; WASTE 2 CYCLES
NOP ; WASTE 2 CYCLES
BCC JUMP ; WASTE 2 OR 3 CYCLES
END ...
I don't think TABLE has to be the first byte. But the table does have to
fit inside one page. For instance, if I placed the code above at location
$ffe6 (at the end of the page), initialization would place $ff and $e6 in
locations $81 and $80 respectively. Then you just have to remember when
generating your offset that it's from address $ff00, not from $ffe9
(the address of TABLE). It could get confusing in other areas of the code,
but I think it's doable. Your x-coordinate could be referenced from a non
zero location on the previous line, or you just do the add before the LSRA
instruction. That way the carry bit is fine.
Hmmm...actually, I guess the table would need to be contained in the first
half of the page because you lose a bit during the shift.
-Chris
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