On Mon, Mar 28, 2005 at 05:47:06PM +0200, Andi Kleen wrote:
> "H. J. Lu" <[email protected]> writes:
> > The new assembler will disallow them since those instructions with
> > memory operand will only use the first 16bits. If the memory operand
> > is 16bit, you won't see any problems. But if the memory destinatin
> > is 32bit, the upper 16bits may have random values. The new assembler
>
> Does it really have random values on existing x86 hardware?
The x86 hardwares will only change the first 16bits. The rest bits
are unchanged. A simple test program can verify that.
>
> If it is a only a "theoretical" problem that does not happen
> in practice I would advise to not do the change.
>
It depends on what the initial value in the upper bits is. The
assembler in CVS generates the same binary code as
movw %ds,(%eax)
for
movl %ds,(%eax)
But the previous assemblers will generate
66 8c 18 movw %ds,(%eax)
for
movw %ds,(%eax)
This bug has been fixed for a while. I guess that may be why Linux
kernel uses
movl %ds,(%eax)
H.J.
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