Read Usb from Mbr

[paulqddinh]

Hi all,

I'm trying to write an OS booting from USB.
My MBR is loaded fine but I don't know how to
load the kernel from Usb.

I tried to use int 13h but it didn't work.
Any other way to read USB content from MBR?
(x86 assembly code pls)

Many thanks

[XanClic]

I tried to use int 13h but it didn't work.

What do you mean with that? int 13h works well for me. The BIOS emulates the USB device as a floppy disk or a hard disk via int 13h (depending on the size, I think).

And I don't think there's another easy way to boot from USB. Of course you could reserve some sectors after the MBR and write a full USB driver there but I don't suggest this at all. Check your int 13h code first because that works well for me (and for GRUB, too).

[paulqddinh]

tkx xanclic, i agree that usb driver cant be < 512byte
help me check whether my code below is ok?

Code: Select all

                        mov     ah,2        ;to read sector
                        mov     al,1        ;read 1 sector
                        
                        mov     dl,80h      ;first hard disk
                        mov     dh,0        ;head 0
                        mov     ch,0        ;cylinder 0
                        mov     cl,1        ;sector 1 (1 of 1 to 63)
                        
                        mov     ax,1000h ;read to 1000:0000, second 64K segment
                        mov     es,ax
                        xor     bx,bx
                        int     13h

my signature is my PC features.

[Combuster]

Code: Select all

mov     dl,80h      ;first *hard disk*

load the kernel from Usb.

Do you see what I see?

Oh and, you may want to use the value in DL that you get when your bootloader is called. The bios provides the right drive number for you.

[paulqddinh]

tkx for the tip of value of DL

when my Mbr (on Usb) is loaded by Bios, DL = 80h (already checked on real machine)
that means my Usb is considered as first HDD (80h)

now I read by int 13h func 42h, but i dont know if this DAP is correct?

Code: Select all

dap:                    db      10h     ;size of Dap
                        db      0       ;0
                        dw      1       ;1 sector
                        dw      0       ;offset  0000h
                        dw      1000h   ;segment 1000h
                        dq      0       ;start from sector 0 (re-read Mbr, just to verify)

[egos]

help me check whether my code below is ok?

Code: Select all

                        mov     ah,2        ;to read sector
                        mov     al,1        ;read 1 sector
                        
                        ...
                        
                        mov     ax,1000h ;read to 1000:0000, second 64K segment
                        mov     es,ax
                        xor     bx,bx
                        int     13h

Do you really want to call subfunction 10h? Try this: mov bx,1000h / mov es,bx

now I read by int 13h func 42h, but i dont know if this DAP is correct?

See EDD Spec.

[paulqddinh]

oh YESS egos,
my terrible mistake , i changed function number AH in AX
hm
tkx tkx

i can load my kernel now, thx guys