On Wed, Apr 1, 2015 at 6:14 PM, Linus Torvalds <torva...@linux-foundation.org> wrote: > On Wed, Apr 1, 2015 at 1:52 PM, Denys Vlasenko <dvlas...@redhat.com> wrote: >> >> BTW, AMD64 docs do explicitly say that MOVs from segment registers >> to gpregs are zero-extending. > > Yeah, I think anything even *remotely* recent enough to do 64-bit does > zero-extending. > > Even on the 32-bit side, anything that does register renaming is much > better off with zero-extension than with partial register writes. > > And I found the "push" thing. It's actually documented: > > "When pushing a segment selector onto the stack, the Pentium 4, > Intel Xeon, P6 family, and Intel486 processors > decrement the ESP register by the operand size and then write 2 bytes. > If the operand size is 32-bits, the upper > two bytes of the write are not modified" > > but I can't find any similar documentation for the "mov > Sreg->register" thing. So now I'm starting to doubt my own memory. > > Linus
It's in the description of MOV: "When the processor executes the instruction with a 32-bit general-purpose register, it assumes that the 16 least-significant bits of the general-purpose register are the destination or source operand. If the register is a destination operand, the resulting value in the two high-order bytes of the register is implementation dependent. For the Pentium 4, Intel Xeon, and P6 family processors, the two high-order bytes are filled with zeros; for earlier 32-bit IA-32 processors, the two high order bytes are undefined." AMD will always zero-extend, although this applies specifically to 64-bit processors: "When reading segment-registers with a 32-bit operand size, the processor zero-extends the 16-bit selector results to 32 bits. When reading segment-registers with a 64-bit operand size, the processor zero-extends the 16-bit selector to 64 bits." So I think it's safe to assume zero-extension on 64-bit, but not 32-bit. -- Brian Gerst -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/