Here's a thing about 6502 assembly I've learned writing my game... some of
you might find it interesting.
Suppose you've to write a subroutine that executes a piece of code
according to an index value - the kind of thing that you would do with a
CASE SELECT-type of statement in an high level language or maybe with
something else in C (I don't know C, I'm still at 6502 assembly level, ok?
;)
Here's the first, obvious, solution, the same I had first put in my
program.
CHOOSE1:
LDA VALUE
CMP #1
BNE NOT1
JMP DO1
NOT1:
CMP #2
BNE NOT2
JMP DO2
NOT2:
CMP #3
........
DO1:
........
RTS
DO2:
........
RTS
and so on.
Later, when I was running out of ROM memory and I had to optimize the code,
I found a better way of doing that. So I've used JMP (indirect) for the
first time, wee! :)
CHOOSE2:
LDA VALUE
ASL
TAX
LDA JMPTABLE,X
STA TEMP
LDA JMPTABLE+1,X
STA TEMP+1
JMP (TEMP)
........
DO1:
........
RTS
DO2:
........
RTS
JMPTABLE:
WORD DO1
WORD DO2
........
Please note these numbers:
Let's say that n is the number of possible choices, then the first solution
takes 2+8*n bytes of ROM on the 2600, while the second 17+3*n bytes.
Hence the JMP() approach is better that the first in terms of memory when
17+3*n<2+8*n, that is when n>3... My n was 12 so I saved 45 bytes!
Exercise for the reader: which program is faster? ;)
Ciao,
P.
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