OSDev.org

Posted: **Sat May 30, 2009 11:21 pm**

I have written code to print a string using int 0x10. It uses lodsb to get a byte at ds:si. In my initialization code, I set all the segment registers, as follows:

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	cli
	mov     ax,     0x07c0
	mov     ds,     ax      ;; for some reason, this breaks printing the message
        mov     es,     ax
        mov     fs,     ax
	mov     gs,     ax

        mov     ax,     0x0000
	mov     ss,     ax
        mov     sp,     0xFFFF
        sti

(This is in a stage-1 bootloader, hence the 0x07c0.) The printing code:

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;; Method: print
;;--------------------------------------
;; ds:si => string to print
;;--------------------------------------
;; Prints a null-terminated string using BIOS interrupt 0x10.
print:
        lodsb
        or      al,     al
        jz      print_end
        mov     ah,     0x0e
        int     0x10
        jmp     print
print_end:
        ret

It uses lodsb, hence needs the data byte to be at ds:si. However, when I call the print routine, as follows:

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        mov     si,     loadmsg
        call    print

it doesn't work. If ds is zeroed (instead of set to 0x07c0), it works fine. Reading this in the bochs debugger, it appears that loadmsg is being replaced by its full address (0x7c3f) rather than just 0x3f, as should be to print properly with lodsb. Confirming this, if I replace the call code above with

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        lea     si,    [loadmsg - 0x7c00]
        call    print

it works. At the start of my bootsector, I use

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org 0x7c00

This seems as if it should work. Am I doing something wrong? Why is this being parsed this way?

Posted: **Sat May 30, 2009 11:55 pm**

If you are using org 0x7c00, then the segment registers should be 0, else you will have odd offset problems.

Either of the following two should work:

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org 0x7c00
xor ax, ax
mov ds, ax

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org 0x0
xor ax, 0x7c0
mov ds, ax

Posted: **Sun May 31, 2009 1:55 pm**

Thanks, that fixed it.

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