chainloader.asm

[BITS 16]

;###############################################################################
;                              -.- Initial Setup -.-
;###############################################################################

;Setup stack
mov  ax, 07C0h
add  ax, 20h
mov  ss, ax
mov  sp, 4096

;Backup drive number
push dx

;Setup data
mov ax,07C0h
mov ds,ax

;Just to known its working
mov si, trCpy
call print

; ## Copy myself ##
;Check if LBA is available
mov dl, [esp]
call check_lba

;Read the first sector
mov ax, 1          ;Read 1 sector
mov bx, chain_addr ;into chain_addr offset
mov cx, 0          ;by starting at sector 0
mov dl, [esp]      ;drive number
call read_lba

;Jumps to my new location
jmp 0:addr_jump

; -- addr_jump is here --
checkpoint:
	mov ax, 0x100
	mov ds, ax
	nop

;Loads the VBR to the right place
mov si, trVbr
call print

;Checks
mov dl, [esp]
call check_lba

;Read the first sector
mov ax, 1        ;Read 1 sector
mov bx, vbr_addr ;into vbr_addr offset
mov cx, 63     ;by starting at the first partition, 2048 win7 or 63 winxp
mov dl, [esp]    ;drive number
call read_lba

;Iterative mode
mov si, pressKey
call print
mov ah, 0
int 0x16

;Jumps to the original VBR
pop  dx
jmp 0:vbr_addr

inf:
	jmp inf


;###############################################################################
;                              -.- Disk Utils -.-
;###############################################################################
;-----------------------------------------------------
;check_lba: Checks if LBA Extensions is Present
;dl: drive number
;-----------------------------------------------------
check_lba:
	mov ah, 0x41
	mov bx, 0x55AA
	int 0x13
	jnc .ret
	mov si, lba_np
	call print
	jmp hang
		
	.ret:
		retn
	
;-----------------------------------------------------
;read_lba: Read sectors from boot media using LBA mode
;ax: number of sectors to read
;bx: offset
;cx: start sector
;dl: drive number 
;-----------------------------------------------------
read_lba:
	mov [blkcnt], ax
	mov [db_add], bx
	mov [d_lba] , cx
	mov si, DAP
	mov ah, 0x42
	int 0x13
	jnc .ret
	.error:
		mov si, lba_fail
		call print
		jmp hang
	
	.ret:
		retn
	
;###############################################################################
;                            -.- Debugging Utils -.-
;###############################################################################
;---------------------------------------
;print: Print a string to screen
;si: pointer to string
;---------------------------------------
print:
	mov ah, 0xE
	mov bx, 0x0007

.printchar:
	lodsb
	cmp al, 0
	je .done
	int 0x10
	jmp .printchar
.done:
	ret
	
;---------------------------------------
;halt: Does nothing
;---------------------------------------
hang:
	jmp hang

;###############################################################################
;                         -.- Some data and constants -.-
;###############################################################################
trCpy    db 'Copying myself...', 13, 10, 0
trVbr    db 'Loading VBR to the right place!', 13, 10, 0
lba_fail db 'Failed while trying to reading using LBA!!!', 13, 10, 0
lba_np   db 'LBA Extensions seems to be not present!!', 13, 10, 0
pressKey db 13, 10, 'Press any key to boot...', 13, 10, 0

vbr_addr   equ 0x7C00
chain_addr equ 0x1000
addr_jump  equ (checkpoint-$$)+chain_addr

;--------------------------------------
; -.- Disk Address Packet structure -.-
;--------------------------------------
DAP:
        db 0x10     ;DAP Size
        db 0        ;Unused
blkcnt:	dw 1        ;Number of sectors to be read
db_add:	dw 0x1000   ;offset
        dw 0        ;segment
d_lba:	dd 1        ;start sector, sector starts in 1
        dd 0        ;start sector, dont need to be used


;Fill up the media
times 510-($-$$) db 0
db 0x55
db 0xAA