Different results with BSR and non-BSR code

[DanielAllsopp]

Evening!

Why would calling code via a BSR act differently to calling the exact same code inline?

For example, in my startup I've got the following to disable the OS:

  lea $dff000,a6
  bsr WaitEOF
  move.l $1c(a6),-(sp)
  move.w 2(a6),-(sp)
  move.l INTVECTORL2(a4),-(sp)
  move.l INTVECTORL3(a4),-(sp)

Now, if I call that code directly within my Start: routine then it works fine, however, if I move it into its own subroutine, i.e.

DisableSystem:
  lea $dff000,a6
  bsr WaitEOF
  move.l $1c(a6),-(sp)
  move.w 2(a6),-(sp)
  move.l INTVECTORL2(a4),-(sp)
  move.l INTVECTORL3(a4),-(sp)
rts

and then call 'bsr DisableSystem' in its place, it hangs on the following line:

  move.l $1c(a6),-(sp)

So I'm thinking it's got something to do with the stack pointer? I've been banging my head against the wall for a good while now over the last two days and I've ran out of ideas.

Anyone else know why this would be the case?

[StingRay]

Just a hint: watch your stack. And ask yourself where the return address comes from. And then check your "DisableSystem" routine.

[paraj]

BSR/RTS also uses the stack, so when you do:

move.l INTVECTORL3(a4),-(sp)
rts

you'll "return" to whatever is stored at INTVECTORL3(a4).

[DanielAllsopp]

Ooooooooh... of course, the return address is stored in the stack isn't it!

[Thomas Richter]

Originally Posted by DanielAllsopp

Anyone else know why this would be the case?

The "why" has been answered, though there is also a solution for you in case you want to use a subroutine for storing data on the stack.

Sub:
  ... do something ...
 move.l (a7)+,d0    ;get return address
 move.l ....,-(a7)    ;store stuff on the stack
 move.l ...,-(a7)    ; more stuff...
 move.l d0,-(a7)   ; restore return address
 rts

All this beside, I believe instead of disabling the system it would be a much better approach to use the system.

[DanielAllsopp]

Originally Posted by Thomas Richter

The "why" has been answered, though there is also a solution for you in case you want to use a subroutine for storing data on the stack.

Sub:
  ... do something ...
 move.l (a7)+,d0    ;get return address
 move.l ....,-(a7)    ;store stuff on the stack
 move.l ...,-(a7)    ; more stuff...
 move.l d0,-(a7)   ; restore return address
 rts

All this beside, I believe instead of disabling the system it would be a much better approach to use the system.

Thanks for that Thomas, much appreciated.

Regarding the system though; the whole point of me jumping into ASM coding on the Amiga in the first place was to learn how to 'bang the metal' as was done in the good old days. I've coded plenty of system friendly Amiga apps in the past so time to go old skool I reckon!