Re: [Qemu-devel] [PATCH] ppc32 guests: fix computation of XER.{CA, OV} in addme, subfme, mullwo

[Tristan Gingold]

On Jun 19, 2008, at 3:36 AM, Julian Seward wrote:

> > > addme   00000000 XER.CA=1  = 00000000 (cr 00000000 xer 20000000)
> >
> > Did I miss the obvious, but why does 0 + 1 - 1 generate a carry ?
>
> Because (as per the docs) it's computing rA + XER.CA + 0xFFFF_FFFF,
> which is 0 + 1 + 0xFFFF_FFFF == 0x1_0000_0000.  Which does indeed
> carry.  I tested now on both a 7447 and a G5 and they both do that.

Ok thanks.  So addme set the carry unless both xer.ca and rA are 0.

Your expression is a little bit complex, especially the second part.
If xer_ca = 1 then res == argL, right ?

For reference:

 void do_addmeo (void)
 {
+    uint32_t argL = T0;
+    uint32_t res  = T0 + xer_ca + (-1);
+    uint32_t carried = res < argL || ((xer_ca & 1) == 1 && res ==  
argL);

Tristan.