Writing a Simple Operating System — from Scratch

CHAPTER 3. BOOT SECTOR PROGRAMMING (IN 16-BIT REAL

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; actually read in AL is not equal to the number we expected.

cmp al, <no. sectors expected >

jne disk_error

disk_error :

mov bx, DISK_ERROR_MSG

call print_string

jmp $

; Global variables

DISK_ERROR_MSG: db "Disk read error!", 0

3.6.4 Putting it all Together

As explained earlier, being able to read more data from the disk will be essential for boot-
strapping our operating system, so here we will put all of the ideas from this section into
a helpful routine that will simply read the firstnsectors following the boot sector from
a specified disk device.

; load DH sectors to ES:BX from drive DL

disk_load:

push dx ; Store DX on stack so later we can recall

; how many sectors were request to be read ,

; even if it is altered in the meantime

mov ah, 0x02 ; BIOS read sector function

mov al, dh ; Read DH sectors

mov ch, 0x00 ; Select cylinder 0

mov dh, 0x00 ; Select head 0

mov cl, 0x02 ; Start reading from second sector (i.e.

; after the boot sector)

int 0x13 ; BIOS interrupt

jc disk_error ; Jump if error (i.e. carry flag set)

pop dx ; Restore DX from the stack

cmp dh, al ; if AL (sectors read) != DH (sectors expected)

jne disk_error ; display error message

ret

disk_error :

mov bx, DISK_ERROR_MSG

call print_string

jmp $

; Variables

DISK_ERROR_MSG db "Disk read error!", 0

And to test this routine, we can write a boot sector program as follows: