[squeak-dev] Re: x86 sarl curiosity...

Eliot Miranda eliot.miranda at gmail.com
Thu Jan 22 02:18:07 UTC 2009

apologies; my bad.  I'd used the wrong branch.  jump greater (if 0 > v) is
not the same as jump (if v) negative .  I live and learn.  Sorry for the

On Wed, Jan 21, 2009 at 10:21 AM, Eliot Miranda <eliot.miranda at gmail.com>wrote:

> Hi All,
>     anyone know the x86/IA32 really well?  If so, read on.  Otherwise save
> yourself the yawn.
> I just tried to save an instruction in Cog;s generated bitShift: primitive.
>  It seems to me that SARL (shift arithmetic right long) should set the sign
> flag based on the result, in fact it says as much in the manual; I quote fro
> m IA-32 Intel(R) Architecture Software Developer's Manual Volume 2B:
>  Instruction Set Reference, N-Z p 4-192
> Flags Affected
> The CF flag contains the value of the last bit shifted out of the
> destination operand; it is unde-
> fined for SHL and SHR instructions where the count is greater than or equal
> to the size (in bits)
> of the destination operand. The OF flag is affected only for 1-bit shifts
> (see "Description"
> above); otherwise, it is undefined. The SF, ZF, and PF flags are set
> according to the result. If the
> count is 0, the flags are not affected. For a non-zero count, the AF flag
> is undefined.
> (my emphasis added).  But neither the Bochs simulator nor my Intel Core Duo
> set the flags when doing sarl $1, %eax when %eax contains -1.  Have I
> misread, or is the manual wrong?
> Eliot
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