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OS_Project/Part1/A4/01_gdb/boot/disk.asm

47 lines
1.5 KiB
NASM
Executable File

; load 'dh' sectors from drive 'dl' into ES:BX
disk_load:
pusha
; reading from disk requires setting specific values in all registers
; so we will overwrite our input parameters from 'dx'. Let's save it
; to the stack for later use.
push dx
mov ah, 0x02 ; ah <- int 0x13 function. 0x02 = 'read'
mov al, dh ; al <- number of sectors to read (0x01 .. 0x80)
mov cl, 0x02 ; cl <- sector (0x01 .. 0x11)
; 0x01 is our boot sector, 0x02 is the first 'available' sector
mov ch, 0x00 ; ch <- cylinder (0x0 .. 0x3FF, upper 2 bits in 'cl')
; dl <- drive number. Our caller sets it as a parameter and gets it from BIOS
; (0 = floppy, 1 = floppy2, 0x80 = hdd, 0x81 = hdd2)
mov dh, 0x00 ; dh <- head number (0x0 .. 0xF)
; [es:bx] <- pointer to buffer where the data will be stored
; caller sets it up for us, and it is actually the standard location for int 13h
int 0x13 ; BIOS interrupt
jc disk_error ; if error (stored in the carry bit)
pop dx
cmp al, dh ; BIOS also sets 'al' to the # of sectors read. Compare it.
jne sectors_error
popa
ret
disk_error:
mov bx, DISK_ERROR
call print
call print_nl
mov dh, ah ; ah = error code, dl = disk drive that dropped the error
call print_hex ; check out the code at http://stanislavs.org/helppc/int_13-1.html
jmp disk_loop
sectors_error:
mov bx, SECTORS_ERROR
call print
disk_loop:
jmp $
DISK_ERROR: db "Disk read error", 0
SECTORS_ERROR: db "Incorrect number of sectors read", 0