Does the REX.B override work with the MOVSS instruction?

Question

I'm generating the following instruction for 64 bit x86:

41 F3 0F 10 46 10       movss   XMM0,014h[R14]

Unfortunately, it seg faults on that line. gdb disassembles it as:

0x0000000000402054 <+320>:   rex.B
0x0000000000402055 <+321>:   movss  0x14(%rsi),%xmm0

Note that the rex.B override is not recognized, and the index is RSI instead of R14.

Is the instruction invalid? I can't find any indication that this encoding is invalid in the AMD 64 bit instruction reference.

objdump also fails to recognize it as a valid instruction:

41                      rex.B
f3 0f 10 46 10          movss  0x10(%rsi),%xmm0

What's going on here?

Answer 1

Finally, I figured it out. The rex byte goes second for this instruction, as in:

F3 41 0F 10 46 10