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The well-known opcode 90 hex (short encoding for xchg eax, eax) is a very special case because operations over 32-bit registers in x86-64 clear the upper half of the matching 64-bit destination register (i.e., xchg eax, eax would zero-extend eax into rax, but in fact it is regarded as a nop). Opcode 87 C0 is a larger encoding for xchg eax, eax which will clear the upper half of rax in 64-bit mode. For example, xchg edi, edi would do the same on rdi. These instructions require a prefix to promote the operand size to 64-bit and thus avoid the zero extension.