On Wed, Jan 21, 2009 at 10:21 AM, Eliot Miranda <eliot.miranda@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 from IA-32 Intel® 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?

TIA

Eliot