16-bit Assembly: cannot deref some registers

Question

I'm trying the following Intel 16 bit instruction:

mov si, word [reg]

where reg is some register. It compiles fine if reg is bx , but does not when it is ax , cx or dx . I'm using NASM as my assembler. I'm sure this due to some restrictions in the instruction set. Can some one please explain the restriction and the rationale behind it?

Answer 1

Only the following index registers can be used with 16 bit addressing modes:

bx
si
di
bp
bx + si
bx + di
bp + si
bp + di

Likewise, SIB addressing is not available with 16 bit addressing modes.

If you want to use other index registers, you can always use a 32 bit addressing mode, eg [eax] . This works as long as you run your code on an 80386 or newer processor.

This restriction exists because the modr/m byte only has three bits for the (index) register. As you can see above, exactly 8 possible index register combinations exist. I don't know why they designed the addressing modes to be like that, but it sounds pretty reasonable for a 70's era 16 bit processor as the 8086 is.

In 32 bit mode and long mode, this scheme was changed so 7 of the 8 possible index registers refer to eax, ebx, ecx, edx, esi, edi, and ebp while what would refer to esp instead indicates the presence of a sib byte allowing for the well known [base+index*scale] addressing modes.