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Mace
Registered: May 2002
Posts: 1799 |
Detect origin of JSR in subroutine?
Is it possible to detect where a JSR came from when you're in a subroutine?
I mean...
BLAH: jsr SUB // make border white
FOO: jsr SUB // make border black
rts
SUB: {some code}// (detect where this came from)
cmp BLAH // or something of this sort
beq SKIP
lda #$00
sta $d020
rts
lda #$01
sta $d020
rts
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WVL
Registered: Mar 2002
Posts: 902 |
Yes, by reading the stack!
pla -> least significant byte
pla -> most significant byte
(or was it the other way around?)
be careful to put the data back on the stack though..
pla
tax
pla
tay
pha
txa
pha
-> origin in x/y
you can also do something like
tsx
lda $0100,x ;not quite sure, maybe it's $0101? or $00ff?
ldy $0101,x ;not quite sure, maybe it's $0102?
Anyway, read up in the programmer's reference guide.
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doynax
Account closed
Registered: Oct 2004
Posts: 212 |
Yes, JSR pushes the return address onto the stack, and you can extract the stack bytes manually to figure out where the function was called from. Beware that JSR actually pushes the return address minus one though!
Here's a quick example of testing for a particular call site:caller:
jsr callee
.
.
.
callee:
tsx
lda $101,x
cmp #<caller+2
lda $102,x
sbc #>caller+2
beq match
.
.
.
edit: WVL beat me to it. |
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Oswald
Registered: Apr 2002
Posts: 5094 |
whatever you need this for this is the worst solution i bet. rather set a flag or smth before calling. |
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Mace
Registered: May 2002
Posts: 1799 |
Ah, right... the stack. Could've know!
Nah, that's too unreliable for debugging, I indeed better try Oswalds advice ;-) |
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Copyfault
Registered: Dec 2001
Posts: 478 |
Does the subroutine really depend on the position it was called from or rather on how often the routine has been called already? Seeing the comments in your example code sniplet made me wonder...
In the second case it may be helpful to install a counter inside the subrout, i.e. like this
SUB:
ldx COUNTER
inx
lda color,x
sta $D020
stx COUNTER
rts
Most probably you won't need $100 different subrout states, so a little masking of COUNTER would have to be added to this...
Otherwise checking the stack is mandatory; maybe in your case it is enough to only check the low-byte of jsr-adress. |
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Oswald
Registered: Apr 2002
Posts: 5094 |
or just use as many subroutines as many you need :) |
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Mace
Registered: May 2002
Posts: 1799 |
@ Copyfault: Of course, my example was only that, an example.
The program I'm working on has more complex subroutines, but some are very similar, apart from a few lines.
I was thinking of putting something inside the routine to switch between the particular bits, depending on the origin of the JSR.
Or rather... it was a thought experiment. :-) |
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Oswald
Registered: Apr 2002
Posts: 5094 |
then put the different lines into another subroutine, and keep what's the same in one ?
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Copyfault
Registered: Dec 2001
Posts: 478 |
@Mace: if the differing lines are occuring as a connected sequence in the subrout, you could place a jump-command there and tweak the jump-adress before calling the subrout.
This way only the differing parts of the subrout must be placed somewhere in memory.
However, Oswald's suggestion to have as many subrouts as needed will ofcourse be optimal when it comes to timing - unfortunatly not for mem usage :(
Looking forward to the result of your experiments! |
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Mace
Registered: May 2002
Posts: 1799 |
Ah, yeah, self modifying code is a cool idea too.
However, you will not see the result, as I'm working on a program to check lottery tickets at someone's request, not some demo, game or useful tool.
And it's in Dutch... |
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