为什么nasm和yasm为mov r16，单词[r64-3]指令生成67个操作码？

Question

In x86-64 assembly I have following instruction

mov        dx, word [esi-5]

Nasm 2.13.03 generates:

66 67 8B 56 FB 

yasm 1.3.0 generates:

67 66 8B 56 FB 

The 66 67 opcodes are modifiers so 8B 56 FB on its own is:

 mov        edx, dword [rsi-5]

I noticed that:

66 8B 56 FB 

also evaluates to:

mov        dx, word [rsi-5]

I have two questions:
1) Why nasm & yasm emit this 67 opcode byte padding? ( 67 on it's own is not enough to reduce edx to dx , it needs to include 66 )
2) Is there a way to emit a shorter 4 byte instruction without 67 in nasm / yasm?

Answer 1

The question made false assumption 66 8B 56 FB

mov        dx, word [rsi-5] 

is equivalent to

`66 67 8B 56 FB` or `67 66 8B 56 FB` 

mov        dx, word [esi-5] 

66 reduces edx to dx
67 reduces [rsi-5] to [esi-5]