[squeak-dev] x86 sarl curiosity...

Eliot Miranda eliot.miranda at gmail.com
Wed Jan 21 18:21:41 UTC 2009

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

(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?


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