ASM和寄存器

Question

I just have a question because I'm learning ASM languages at my school and I don't understand something:

(BX)=0100H
MOV AX,BX

The address of the source (BX) is empty before the mov statement and I don't understand why. My idea is, that it's because there is nothing in BX. Is that true ?

It's Intel syntax.

Answer 1

It all depends on the context. Does bx contain a pointer or a value? What is the surrounding code?

lea     bx, somestring  ; abc
mov     ax, bx          ; ax contains pointer to abc
mov     ax, [bx]        ; ax will contain ba

Answer 2

It looks like you are moving the value 0100h directly to the bx register.

To take what's in the location 0100h you'll need to specify it as a pointer to a value (of size) at address.

mov bx,WORD PTR ds:0x100 for word size

mov bl,BYTE PTR ds:0x100 for byte size