move sr and rtr

[leonard]

Hi!

I need to call a subroutine but save/restore X cpu flag. To do that I'm using the nice RTR instruction like:

     bsr   foo
     ...

foo:   move.w sr,-(a7)
         ; code that could scratch X flag
         rtr

It works pretty well. But now I wonder if that kind of code runs on any CPU, including a context without operating system ( so I can't rely on any software emulated instruction on 68060)

Is that kind of code ( move sr,-(a7) and rtr ) fully ocmpatible with any amiga CPUs?

[a/b]

Yeah, it's implemented in hardware in 000-060. Just have to run it in supervisor mode because 010+ requires supervisor for move sr,<ea>, while providing move ccr,<ea> (which 000 doesn't have) for user mode.

[leonard]

Originally Posted by a/b

Yeah, it's implemented in hardware in 000-060. Just have to run it in supervisor mode because 010+ requires supervisor for move sr,<ea>, while providing move ccr,<ea> (which 000 doesn't have) for user mode.

oh you're right the supervisor thing is an issue in my context. I'll go with that kind of simple stuff then:

		subx.b d1,d1		; read X flag in d1
		move.w	d1,-(a7)
                bsr MFMDecodeTrackCallback
		move.w (a7)+,d1
		add.b d1,d1		; restore X flag

[ross]

This?:

    subx.w  d1,d1
    move.w  d1,-(sp) 
    ; code that could scratch X flag
    rtr

[leonard]

Originally Posted by ross

This?:

    subx.w  d1,d1
    move.w  d1,-(sp) 
    ; code that could scratch X flag
    rtr

oh it scratchs other ccr flags but I don't care in my case. Pretty smart!

[admiral]

Originally Posted by leonard

Is that kind of code ( move sr,-(a7) and rtr ) fully ocmpatible with any amiga CPUs?

In 010+, if not in supervisor mode (normally Amiga software runs in user mode), it will break.

Whereas move from CCR is 010+, so it will break on plain 68000.

A reliable (and fast!) alternative is to call exec.library's GetCC().

Of course, that assumes execbase is available and consistent. If it's not, what I would do is:

* Make a function for it, and use get from SR or CCR depending on CPU model. Then call as needed.
* If it's used in a single place, implement the detection right there, or set up the right code via self-modifying code.