I wrote:
> lda variable
> cmp #num_entries
> bcs not_in_table
> asl
> tax
> lda jump_table,x
> sta temp ; temp should be 2 bytes somewhere in RAM
> lda jump_table+1,x
> sta temp+1
> jmp (temp) ; jump indirect through temp
Whoops! Forgot about jmp (abs,x) which I remembered as a 65816 opcode.
Sorry... and thanks to Amos Bannister for the reminder (btw, the
instruction works differently than his example.)
The revised version should read:
lda variable
cmp #num_entries
bcs not_in_table
asl
tax
jmp (jump_table,x)
jmp_table:
dc.w framistat
etc.
The only catch being, due to a bug in the 6502 the jump table itself
cannot cross a page boundary. Align it to an even address if you're
not 100% sure that no entry in the table crosses a 256 byte page
boundary.
Finding bugs in the 6502 and exploiting them was a favorite technique
in disguising copy protection code in Apple II and Atari 400/800 disk
based games.
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