assembly - Why MOV AH,1 is not supported in 64 bit mode of intel microprocessor?

Question

In the book "THE INTEL MICROPROCESSORS" of Barry B. Brey, it is written that

MOV AH, 1

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

Answer 1

There is no problem with mov ah,1. It runs just fine in X64 mode.

The opcode for it is b4 01.

The only time when mov ah is not allowed is when the mov has a REX prefix.

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

***In 64-bit mode, r/m8 can not be encoded to access the following byte registers if a REX prefix is used: AH, BH, CH, DH.

In that case the high byte registers (AH, BH, CH, and DH) are redefined as DIL, SIL, BPL and SPL. But this is only if a REX prefix is present.

Instructions with a REX prefix are:

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.