stupid questions about PASM2

msrobots

OK the first one

Am I correct that call...ret does NOT return the state of Z or C but preserves the one outside the call unless I do RET WC WZ?

evanh

I wish! It's the other way around.

msrobots

shhhh., I thought I read it somewhere,

so after a RET I get the flags from the sub, right? What you mean by 'other way around'?

Enjoy!

Mike

evanh

I can't use _RET_ and have it preserve the calling flags so that conditional execution works across calls. I have to use RET WCZ instead.

msrobots

so _ret_ or RET will return the changed flags but RET WCZ will preserve them and restore the flags to the point of the call?

curious,

Mike

Cluso99

There are two cases you need to consider.

1. A subroutine wants to return any flags it changes to the calling routine.
eg: on return I set/clear Z &/or C flags to pass results of the subroutine, so Z might be passed/success where NZ is fail/error

2. A subroutine needs to use flags but the calling program does not want them changed.

Both of these methods have their advantages and disadvantages.

The P1 always uses 1. above. If 2. is required, then the subroutine needs to save the flags on entry, and restore them before returning.

On the P2...
1. works with
_RET_ <opcode> <operands>
and
RET

2. works only with
RET {wc/wz/wcz} where the Z &/or C is preserved only if the modifier is used.

msrobots

thank you @Cluso99 that is a useful explanation.

Enjoy!

Mike