Re: [Nasm-users] prob with bios int 10h

[Frank K. <fbk...@zy...>]

zak100 wrote:

This is the problem:

> mov ax,0B800h
 > mov es,ax ;

One example you've got puts characters to screen memory, at B800h:????. 
"stosw" is used to put character and color to es:di.

The other example uses bios int 10h, subfunction 13h, which expects 
es:bp to point to the string (unusual parameters to this interrupt!). 
(the tut you've got explains other parameters to this int, but fails to 
mention the es:bp thing)

When our bootsector receives control from the bios, we don't know what 
ds and es are. If we've assembled at "org 0", we want them to be 7C0h. 
If we've assembled at 7C00h, we want them to be zero... to refer to any 
data in our bootsector (if we want screen memory, 0B800h - an old ttl 
mono card wants 0B000h - we probably won't encounter such a thing, but 
really should check... int 11h, IIRC).

The example that prints registers on the screen, is quite 
"non-standard", and probably isn't a good starting point for other 
bootsectors. This one uses a different int 10h subfunction, 0Eh, which 
prints the character in al (put there by lodsb).

;------------------------------------
; same as boot01, pretty much...
; nasm -f bin -o boot01.bin boot01.asm
; dd count=1 if=boot01.bin of=/dev/fd0
;-------------------------------------

org 7C00h
section .text
     jmp short overdata
     nop
     db 'MyOS    '    ; OEM id
     dw 200h          ; bytes/sector
     db 1             ; sectors per cluster
     dw 1             ; sectors before FAT
     db 2             ; number of FATs
     dw 0E0h          ; max rootdir entries
     dw 0B40h         ; total sectors
     db 0Fh           ; media descriptor
     dw 9             ; sectors per FAT
     dw 12h           ; sectors per track
     dw 2             ; number heads
     times 0Ah db 0
     db 29h
     db 0EFh
     db 7

     db 10h
     db 24h
     db 'LINUX BOOT FAT12   '

;----------------------------
overdata:
     xor ax,ax
     mov ds,ax
     mov es,ax
     ; should probably set up a sane stack here, too.
     mov si,msg    ; "our dear string"

     mov bx,7
     mov ah,0Eh
msgloop:
     lodsb
     or al,al
     jz blackhole
     int 10h       ; since we're still in RM, we can use bios.
     jmp msgloop

blackhole:
     jmp blackhole

msg db "We be bootin'2!",0

     times 510-($-$$) db 90h   ; don't think it matters, but we'll use NOP
     db 55h,0AAh               ; boot sector signature
;-------------------------

I'll see if I can work up an example using int 10h subfunction 13h... 
maybe even set up the stack, instead of just saying we should. :) Won't 
be immediately, but I'll try to get to it "soon"...

Best,
Frank