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From: Julian Seward <jseward@acm.org>
Subject: Re: [Qemu-devel] [PATCH] ppc32 guests: fix computation of XER.{CA,
	OV} in addme, subfme, mullwo
Date: Thu, 19 Jun 2008 09:36:58 +0200
References: <200805110204.47184.jseward@acm.org>
	<200806180006.51954.jseward@acm.org>
	<2CD863BB-B874-483E-8FF3-63952C6FAD5D@adacore.com>
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To: Tristan Gingold <gingold@adacore.com>
Cc: qemu-devel@nongnu.org


> > 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.

On a real 7447:

mullwo. 000f423f ffffffff  = fff0bdc1 (cr 80000000 xer 00000000)
addme   00000000 XER.CA=1  = 00000000 (cr 00000000 xer 20000000)
subfme  00000000 XER.CA=1  = ffffffff (cr 00000000 xer 20000000)

On a real G5:

mullwo. 000f423f ffffffff  = fff0bdc1 (cr 80000000 xer 00000000)
addme   00000000 XER.CA=1  = 00000000 (cr 00000000 xer 20000000)
subfme  00000000 XER.CA=1  = ffffffff (cr 00000000 xer 20000000)

J