Why is MOV AH, 1 not supported in 64-bit Intel microprocessor mode?

Question

In the book "INTEL-MICROPROCESSORS" Barry B. Bray recorded that

MOV AH, 1

not allowed in 64-bit mode, but allowed in 32-bit or 16-bit mode. If MOV AL, 1you can resolve in 64 bit mode, what is the problem with MOV AH, 1?

Answer 1

No problem with mov ah,1. It works fine in X64 mode.

The operation code for it b4 01.

The only time when it is mov ahnot allowed is when it movhas a prefix REX.

from: http://www.felixcloutier.com/x86/MOV.html

*** 64- r/m8 , REX: AH, BH, CH, DH.

(AH, BH, CH DH) DIL, SIL, BPL SPL. REX.

REX:

anything with the R8..R15 regs and parts thereof
anything that accesses the new 8 bit regs: DIL, SIL, BPL, SPL
anything that accesses 64 bit registers.

Answer 2

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