How about using the BRK vector? I was listening to Joe Decuir's keynote
speech from CGE2K1 this morning and he mentioned using the BRK to avoid
using JSR all over the place. It would shave off 3 bytes of ROM!
You mean something a little like this? :o)
==============================================================
BRK SUBROUTINE TRICK
Mark Lesser, Thomas Jentzch
==============================================================
Thomas found this trick in Mark Lesser's Lord of the Rings prototype: You
can use BRK to call a subroutine that needs to be called often and save ROM
space. If you aren't familiar with BRK, it pushes the flags and PC on the
stack and jumps to wherever the vector $FFFE is pointing.
Thomas found BRK commands like this scattered through Lord of the Rings:
brk
.byte $0e ; id-byte
lda $e3 ; <- here we will continue
ora #$04
And the BRK vector was pointing to this routine:
BrkRoutine:
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:
[subroutine code...]
rts
So it ended up being the equivalent of passing a value to a subroutine
similar to this:
ldy #value
jsr Subroutine
But it saves 3 bytes with each call and the overhead is only 8 bytes.
After only 3 subroutine calls (Lord of the Rings has about 20) you are
saving ROM space.
In the case of Lord of the Rings, the subroutine was sound related code
that selected the sound effect to be played based on a priority system.
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