8086汇编寄存器间接MOV指令

Question

I just wanted to ask: why is it that when I write :

MOV DL, [BX] 

it works, but when I write:

MOV DL, [AX]

it doesn't?

Answer 1

In the current Intel® 64 and IA-32 Architectures Software Developer Manual this is described on page 509 of 4898:

In 16-bit Intel assembly it is impossible to use the AX register for register indirect addressing.

As you can see in the manual, you can use the following registers for indirect addressing ; for all of the following registers there are addressing modes that use them on their own (or in base+index pairs):

BX, BP, SI, DI

These can be combined to (with optional displacements)

 [BX+SI]    [BX+SI]+disp8    [BX+SI]+disp16
 [BX+DI]    [BX+DI]+disp8    [BX+DI]+disp16
 [BP+SI]    [BP+SI]+disp8    [BP+SI]+disp16
 [BP+DI]    [BP+DI]+disp8    [BP+DI]+disp16
 [SI]          [SI]+disp8       [SI]+disp16
 [DI]          [DI]+disp8       [DI]+disp16
 disp16        [BP]+disp8       [BP]+disp16
 [BX]          [BX]+disp8       [BX]+disp16

Hence your first instruction (expanded)

MOV DL, BYTE PTR [BX] 

is valid, but your second one isn't, because in the 8086 (x86_16) ISA there is no ModRM addressing-mode encoding for it in x86 machine code.

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32-bit and 64-bit addressing modes use a different encoding for addressing modes, with an optional SIB (scale+index+base) byte that allows nearly any combination of base and index (except index=RSP), and with a 2-bit shift count (scale factor) for the index. See Referencing the contents of a memory location. (x86 addressing modes) or the other tables in Intel's manual.

mov DL, [EAX] is valid even in 16-bit mode , but only on 386-compatible CPUs.