Order in which a functions's arguments are set

cjauvin · Newbie Joined: 23 Jun 2012 Posts: 29 Location: Canada

Hi,

I have a function fetching its boolean arguments in the A and X registers. For some reason, the order in which I set the arguments, prior to the function call, seems to matter, because:

Code:

lda #0
ldx #1
jsr func

behaves differently than:

Code:

ldx #1
lda #0
jsr func

Admittedly my ML skills are rather limited, but I really don't see how this could make a difference. Am I missing something?

groepaz · Posted: Sat Jun 23, 2012 4:32 pm Post subject:

i can assure you that it makes no difference whatsoever =)
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cjauvin · Newbie Joined: 23 Jun 2012 Posts: 29 Location: Canada

I found the answer.. of course I was doing something silly: starting the function right away with a branching instruction, which of course relied on the loading order of the registers.

I guess I'd need some ML "best practices", to guard against such mistakes!

Thraka · Posted: Sat Jun 23, 2012 9:13 pm Post subject:

LOL. I too have been starting up learning ASM, and I've been doing funny things like that too. It gets easier. A few nights ago it really started to click and I'm having an easier time reading it all now.

THEWIZ · Forum Junkie Joined: 17 Jun 2006 Posts: 527

cjauvin wrote:

I found the answer.. of course I was doing something silly: starting the function right away with a branching instruction, which of course relied on the loading order of the registers.

I guess I'd need some ML "best practices", to guard against such mistakes!

Best practices: Unless you have inputs into the routine, dont rely on any values from outside the routine.

So you should never do:

routine: bne somewhere

It should be:
routine: bit a (or cpx #1 or...)
bne somewhere

For one thing the caller may not set the two input registers right away,
Of course, we dont always use "best practices" so you may want to define an input into your routine to be whether the carry or zero flag is set.
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