crackers@xxxxxxxx wrote:
>
> I'm sorry if this is the third time everyone has seen this post, but
> on my end, for some reason, everytime I post this message everything
> goes nuts. I'm hoping the message goes through this time and if it has
> been all along, well consider it a mid-season repeat. ;)
>
> ~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~
>
> I was curious if there was a more efficient way to do something like this.
>
> ----------------------------------------------------------------------------
> lda #$00
> sta CHECK
>
> checkit ldx CHECK
> lda VARIABLE
> cmp CHECK
> beq pointers,x
You can't do the above. What you need is a JMP (ZERO_PAGE_INDIRECT).
There are lots of ways to build a table of offsets, but the bottom line
is putting the low byte, then the hi byte of where you want to jump to
in 2 bytes of zero page memory. In order to save space in both the
table sizes and the amount of code needed, I *usually* try and build a
table only with the low bytes, and put all the target addresses into the
same page of memory. So, I'll end up with something like:
ldx variable
lda jmpTable,x
sta jmpWord
lda #>routinePage ;this byte might need to be loaded only once
sta jmpWord + 1 ;when program inits, unless you are recycling
;this memory with other routines
jmp (jmpWord)
badChoice
...
jmpTable #<firstRoutine,#<secondRoutine,#<thirdRoutine ;etc.
As a rule, I don't like to see code where there might be an illegal
value sent to a routine like this, so I rarely check it. The main
reason for doing this sort of thing is to save space over a long list of
lda/cmp/bne, so cutting out checks for invalid values furthers that
cause... One exception would be something that gets re-used, like a
menuing system, in which case you'd need the checks, as well as indirect
pointers to the list of jmp-to addresses (or pre-load them into zero
page).
--
mor@xxxxxxx
http://www.crl.com/~mor/
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