As promised, here is a programming teaser. Explain how the following code
could ever exit?
loop
jsr Tricky ; do we ever return from this subroutine?
inc colour ; yes! but how?!!
lda colour
sta COLUBK
jmp loop
Tricky
ldx #$FF ; 'character'
line number
Continue
inx
stx SET_BANK_RAM ; bankswitch to appropriate RAM bank
for shape data
ldy #10 ; draw scanlines in groups of 21 (ie:
1 character row)
anotherLine
; draw a scanline here (code omitted)
sta WSYNC
dey
bne anotherLine
bmi Continue
This is a conceptual version of the actual kernel from notBoulderDash. I
can assure you, the background colour will flash quite merrily, proving that
the Tricky subroutine does actually return. There is a subtle trick
operating here which means that this is actually a completely valid kernel
which will exit after drawing an appropriate number of scanlines (in my
case, it's currently 168).
Think a bit laterally... can anyone explain how it might be possible for
code like the above to do its work and actually return to the caller? You
will need to make an assumption that is not entirely clear. Can anyone
figure this out?
Cheers
A
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