Qfyilyi

I'm working on my own bootloader and I'm using QEMU as a test lab to check/debug it. Right now I want to practice with reading of sectors using BIOS extensions. According to the docs QEMU uses SeaBIOS which should support int 13h AH=42h.
I have this code

bits    16                      ; we are in 16 bit real mode



org 0                       ; we will set regisers later



start:  jmp main                    ; jump to start of bootloader





Print:

        lodsb               ; load next byte from string from SI to AL

        or  al, al          ; Does AL=0?

        jz  PrintDone       ; Yep, null terminator found-bail out

        mov ah, 0eh         ; Nope-Print the character

        int 10h

        jmp Print           ; Repeat until null terminator found

PrintDone:

        ret             ; we are done, so return





ReadSectors:



         mov ah,0x42

         mov dl,0x80

         mov si,dap

         int 0x13  

         jc .error

         jmp .exit

.error:

         mov si,msgFailure

         call Print

         cli

         hlt

.exit:  

    ret





 main:



 ;----------------------------------------------------

 ; code located at 0000:7C00, adjust segment registers

 ;----------------------------------------------------



    cli                     ; disable interrupts

    mov     ax, 0x07C0              ; setup registers to point to our segment

    mov     ds, ax

    mov     es, ax

    mov     fs, ax

    mov     gs, ax



 ;----------------------------------------------------

 ; create stack

 ;----------------------------------------------------



    mov     ax, 0x0000              ; set the stack

    mov     ss, ax

    mov     sp, 0xFFFF

    sti                     ; restore interrupts

    xor ax,ax

    mov ah,0x41

    xor dx,dx

    mov dl,0x80

    mov bx,0xAA55

    int 0x13

 ;----------------------------------------------------

 ; Display loading message

 ;----------------------------------------------------



    mov     si, msgLoading

    call    Print

    call ReadSectors

    mov si,0x200

    call Print

    cli

    hlt               

dap:

packetSize: db 0x10

reserved:   db 0x0

sectorsNumber:  dw 0x1

buf_seg:    dw 0x0000

buf_off:    dw 0x7E00

lba:        dd 0x0

            dd 0x0



msgLoading  db 0x0D, 0x0A, "Loading Boot Image ", 0x0D, 0x0A, 0x00

msgCRLF     db 0x0D, 0x0A, 0x00

msgProgress db ".", 0x00

msgFailure  db 0x0D, 0x0A, "ERROR : Press Any Key to Reboot", 0x0A, 0x00



TIMES 510-($-$$) DB 0

DW 0xAA55

It checks if the extensions are supported using AH=41h function and then reads 1 sector starting from LBA=0h to memory 0000:7E00.
I'm using gdb to connect to qemu machine to check register and memory. So what I see is that

• int 13h AH=42 returns CF=0, so there is no error


• int 13h AH=41h returns CF=0,CX=7, so it means that BIOS supports extensions.
  But then I check memory at address 7E00 and I see only zeros, but I expect to see the code of bootloader as it is stored in the LBA=0h sector.

This is how I create disk image

nasm bootloader.asm -o ./bin/bootloader.bin

dd if=/dev/zero of=./floppy/floppy.img bs=1024 count=1440

dd if=./bin/bootloader.bin of=./floppy/floppy.img conv=notrunc 

And this is how I run qemu

qemu-system-x86_64 -s -S -hda ./floppy/floppy.img

Can you please help with understanding what I'm doing wrong.
Thanks!

In the Disk Access Packet (DAP) the segment:offset pair is stored with offset first followed by segment. This is because the x86 is a little endian processor and the pair is stored in reverse order. Your DAP should be changed to:

dap:

packetSize: db 0x10

reserved:   db 0x0

sectorsNumber:  dw 0x1

buf_off:    dw 0x7E00              ; Place offset before segment

buf_seg:    dw 0x0000

lba:        dd 0x0

            dd 0x0