[Coco] 6309's FSWI

[William Astle]

On 13-05-03 11:16 AM, tim lindner wrote:
> I was thinking today about the 6309's V register. I decided you could use
> it to make a "Fast Software Interrupt" instruction.
>
> So the idea is you have a subroutine that will be called often, but you
> don't want to inline. You would put the address of the subroutine in V and
> EXG V,PC to call it.
>
> The routine would stack any registers it uses, perform the function, the
> unstack the registers and then EXG V,PC to return to the main program. You
> would save one additional register stacking because you wouldn't use V for
> any other purpose.
>
> You could also achieve something like self-modifying code by changing the V
> register to point to alternative subroutines.

This seems like one of the more useful things that have been proposed 
for using V. Since V is only accessible through the inter-register 
operations (tfr, exg, subr, etc.), it's not terribly useful. It can't 
even be stacked directly and is not saved by regular interrupts either.