Question

EDIT:
I’m sorry, I’ve made a newbie mistake, and have been changing the value of BX myself! 🙂
Thanks anyway.

I’m having fun with NASM and MBR, and for some reason the BX register gets set to “some” value, even though I’m not writing to it (only reading).

Why is that so? I’m sure I’m missing something obvious, but I can seem to find any tutorials that would explain this behaviour.

Code in question:

partfun:
    mov bx, 01beh ; store a starting address to BX
                  ; I will increment it later
    mov cx, 0

    mov al, bl ; BX changes here already, I think!
    mov al, bh 
   ; do something with AL here

.loop:
    mov al, [bx]    
    ; do something with AL here

    add bx, 16 ;BX is 0110h now, not 01ceh, as I would expect!
               ;I wish to increment the stored value (address) by 16

    inc cx
    cmp cx, 4
    jl .loop

    ret

Answer 1

You can access bl is the lower 8 bits of the 16 bit bx, and bh is the higher 8 bits.

So by changing bl you’re changing bx also.