Hi,
now, this is something for the hard core coders! (like me :-)
While disassembling LOTR I found an interesting way of coding.
Whenever the programmer wanted to check, if a game has run through a
special code point, he placed a BRK there followed by an id-byte.
Example: (checks, if the code has to set bit 2 of $e3)
lda $e3
and #$04
bne Lb568
brk
.byte $0e ; id-byte
lda $e3 ; <- here we will continue
ora #$04
sta $e3
Lb568:
Now something very interesting is happening. The break-vector
(indirectly) points to this routine:
plp ; remove flags from stack (not needed)
tsx ; load x with stackpointer
inx ; x++
dec $00,x ; adjust return address
lda ($00,x) ; read break-id...
tay ; ...and store in y
Subroutine:
ldx #$00 ; find variable with lower value
lda $ef
cmp $f0
bcc Lbed3
inx
Lbed3:
tya
cmp $ef,x ; compare break-id with the lower value of $ef/$f0
bcc .exit ; if smaller
beq .exit ; or equal then exit
sty $ef,x ; overwrite old id with larger new id
lda Lb36e,y ; load some tables values into some registers
sta $f1,x
lda Lb37e,y
sta $f3,x
.exit:
rts ; the code returns two bytes behind the breakpoint
First I thought, this was debugging code, but then I found the registers
filled in that routine are used for... SOUND!
(Each sound has it's own priority and only the highest two are played.)
So this is another (rather complicated) way for something like:
ldy #value
jsr Subroutine
But it saves 3 bytes with each call! And the overhead is only 8 bytes,
so after only 3 subroutine calls (LOTR has about 20) you are winning
ROM space! Cool, isn't it?
Have fun!
Thomas
_______________________________________________________
Thomas Jentzsch | *** Every bit is sacred ! ***
tjentzsch at web dot de |
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