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assemblyx86nasmx86-16real-mode

Write to address without segment register

I know this code will actually write data to ds:[100h]

mov [100h], ax

But how can I write to linear address 100H directly without using any segment register for a segment base?

Solution

  • There is no way to get around segment register; every memory access is relative to some segment register. If you want to write to an absolute address, first load a segment register with an appropriate segment:

            xor cx, cx
        mov es, cx        ; es = 0000
        mov [es:100h], ax ; [0000:0100] = ax

    To load a linear address larger than 16 bit on an 8086 or 80286 system, try something like this:

    addr    dd 0x12345        ; the address we want to load from
        ...
        mov bl, [addr]    ; load low part
        xor bh,bh
        mov cx, [addr+1]  ; load high part
        shl cx, 4         ; adjust high part for segment selector
        mov es, cx        ; load segment register
        mov [es:bx], ax   ; store ax