Extended Registers in 32 Bits Addressing Mode

Hello everyone!

I read the AMD64 specs and I'm still not sure about one point,
maybe you can shade some light about it for me.

I was wondering whether AMD64 supports the extended registers (by
using a REX prefix) while addressing memory indirectly with 32 bits
addressing in long mode, of course.

For example (decode in 64 bits):
MOV EBX, [EAX+EBX] results in 67 8B1C18
The address size prefix will make it 32 bits addressing, no problem
yet.

Now, say I have a REX prefix as well, will it affect the registers in
32 bits addressing.
For example (decode in 64 bits):
MOV EBX, [R8D+R11D] results in 67 43 8B1C18

So, I'm not sure, if the address size prefix should cause the REX to be
ignored, or they both get along well.

Thanks,
Arkon

http://ragestorm.net/distorm/

wrote:
Hello everyone!

I read the AMD64 specs and I'm still not sure about one point,
maybe you can shade some light about it for me.

I was wondering whether AMD64 supports the extended registers (by
using a REX prefix) while addressing memory indirectly with 32 bits
addressing in long mode, of course.

Your question is extremely confusingly worded. If you're talking about
"long mode" then you're talking about 64-bit mode. Even if you don't
need the full 64-bits of addressing, and just need 32-bits, you're
still using 64-bit mode. Now, on the other hand, if you're talking
about true 32-bit programs, then you're talking about "compatibility"
mode when used in relationship to 64-bit OS, therefore you have access
to the REX prefixes. Compatibiltiy mode doesn't allow use of the REX
prefixes.

Yousuf Khan