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Re: i386/x86_64 segment register issuses
On Mon, Mar 28, 2005 at 02:34:16PM -0500, Ross Ridge wrote:
> > The assembler in CVS generates the same binary code as
> >
> > movw %ds,(%eax)
> >
> > for
> >
> > movl %ds,(%eax)
>
> Which is the way I think the GNU assembler should work. In case like
I'd like to disallow
movl %ds,(%eax)
since there is no such an instruction. People can use
mov %ds,(%eax)
with old and new assemblers or
movw %ds,(%eax)
with the newer assembler.
> Linux kernel code you quoted, you really do want a 16-bit move when a
> memory destination is used and a 32-bit move when a register destination
> used. The only problem here is that the Linux kernel doesn't ingore
> the higher 16-bits of the resulting variable, which aren't guaranteed
> to be zero in either case.
>
The higher bits are guaranteed to be zero for x86_64 and Pentium Pro
or above. I am testing Linux kernel patches to make sure that both
old and new assembler will generate optimized binary.
H.J.