What happens when you use a memory override prefix but all the operands are registers?
So, let's say you code mov eax, ebx or add eax, ebxand the default is 32-bit but you use a 67h override.
How does the processor handle that situation?
The Intel Software Developer's Manual*, volume 2, section 2.1, details the behavior of each instruction prefix. It says use of the address-size prefix (67h) with an instruction that doesn't have a memory operand is reserved and may cause unpredictable behavior.
The operand-size prefix (66h) may be used to switch between 16- and 32-bit operand sizes and also as a mandatory prefix with certain SSE2/SSE3/SSSE3/SSE4 instructions. Other use is reserved and may cause unpredictable behavior.
The segment override prefixes are reserved with any branch instruction.
来源：https://stackoverflow.com/questions/46188388/what-happens-when-you-use-a-memory-override-prefix-but-all-the-operands-are-regi