bootloader.asm

BITS 16

        
	jmp short bootloader_start	
	nop				          




OEMLabel		db "BESTSOFT"	
BytesPerSector		dw 512		
SectorsPerCluster	db 1		
ReservedForBoot		dw 1		
NumberOfFats		db 2		
RootDirEntries		dw 224		
					
LogicalSectors		dw 2880		
MediumByte		db 0F0h		
SectorsPerFat		dw 9		
SectorsPerTrack		dw 18		
Sides			dw 2		
HiddenSectors		dd 0		
LargeSectors		dd 0		
DriveNo			dw 0		
Signature		db 41		
VolumeID		dd 00000000h	
VolumeLabel		db "           "
FileSystem		db "FAT12   "	




bootloader_start:
	mov ax, 07C0h			; Set up 4K of stack space above buffer
	add ax, 544			; 8k buffer = 512 paragraphs + 32 paragraphs (loader)
	cli				; Disable interrupts while changing stack
	mov ss, ax
	mov sp, 4096
	sti				; Restore interrupts

	mov ax, 07C0h			; Set data segment to where we are loaded
	mov ds, ax

	; NOTE: A few early BIOSes are reported to improperly set DL

	cmp dl, 0
	je no_change
	mov [bootdev], dl		; Save boot device number
	mov ah, 8			; Get drive parameters
	int 13h
	jc fatal_disk_error
	and cx, 3Fh			; Maximum sector number
	mov [SectorsPerTrack], cx	; Sector numbers start at 1
	movzx dx, dh			; Maximum head number
	add dx, 1			; Head numbers start at 0 - add 1 for total
	mov [Sides], dx

no_change:
	mov eax, 0			; Needed for some older BIOSes



floppy_ok:				
	mov ax, 19			
	call l2hts

	mov si, buffer			
	mov bx, ds
	mov es, bx
	mov bx, si

	mov ah, 2			
	mov al, 14			

	pusha				


read_root_dir:
	popa				
	pusha

	stc				
	int 13h				

	jnc search_dir			
	call reset_floppy		
	jnc read_root_dir		

	jmp reboot			


search_dir:
	popa

	mov ax, ds			
	mov es, ax			
	mov di, buffer

	mov cx, word [RootDirEntries]	
	mov ax, 0			


next_root_entry:
	xchg cx, dx			

	mov si, kern_filename		
	mov cx, 11
	rep cmpsb
	je found_file_to_load		

	add ax, 32			

	mov di, buffer			
	add di, ax

	xchg dx, cx			
	loop next_root_entry

	mov si, file_not_found		
	call print_string
	jmp reboot


found_file_to_load:			
	mov ax, word [es:di+0Fh]	
	mov word [cluster], ax

	mov ax, 1			
	call l2hts

	mov di, buffer			
	mov bx, di

	mov ah, 2			
	mov al, 9			

	pusha				


read_fat:
	popa				
	pusha

	stc
	int 13h				

	jnc read_fat_ok			
	call reset_floppy		
	jnc read_fat			


fatal_disk_error:

	mov si, disk_error		
	call print_string
	jmp reboot			


read_fat_ok:
	popa

	mov ax, 2000h			
	mov es, ax
	mov bx, 0

	mov ah, 2			
	mov al, 1

	push ax				


load_file_sector:
	mov ax, word [cluster]		
	add ax, 31

	call l2hts			

	mov ax, 2000h			
	mov es, ax
	mov bx, word [pointer]

	pop ax				
	push ax

	stc
	int 13h

	jnc calculate_next_cluster	

	call reset_floppy		
	jmp load_file_sector

calculate_next_cluster:
	mov ax, [cluster]
	mov dx, 0
	mov bx, 3
	mul bx
	mov bx, 2
	div bx				
	mov si, buffer
	add si, ax			
	mov ax, word [ds:si]

	or dx, dx			

	jz even				
					

odd:
	shr ax, 4			
	jmp short next_cluster_cont


even:
	and ax, 0FFFh			


next_cluster_cont:
	mov word [cluster], ax		

	cmp ax, 0FF8h			
	jae end

	add word [pointer], 512		
	jmp load_file_sector


end:					
	pop ax				
	mov dl, byte [bootdev]		

	jmp 2000h:0000h			


reboot:
	mov ax, 0
	int 16h				
	mov ax, 0
	int 19h				


print_string:				
	pusha

	mov ah, 0Eh			

.repeat:
	lodsb				
	cmp al, 0
	je .done			
	int 10h				
	jmp short .repeat

.done:
	popa
	ret


reset_floppy:		
	push ax
	push dx
	mov ax, 0
	mov dl, byte [bootdev]
	stc
	int 13h
	pop dx
	pop ax
	ret


l2hts:			
			
	push bx
	push ax

	mov bx, ax			

	mov dx, 0			
	div word [SectorsPerTrack]
	add dl, 01h			
	mov cl, dl			
	mov ax, bx

	mov dx, 0			
	div word [SectorsPerTrack]
	mov dx, 0
	div word [Sides]
	mov dh, dl			
	mov ch, al			

	pop ax
	pop bx

	mov dl, byte [bootdev]		

	ret



	kern_filename	db "KERNEL  BIN"	

	disk_error	db "Floppy error! Press any key...", 0
	file_not_found	db "KERNEL.BIN not found!", 0

	bootdev		db 0 	
	cluster		dw 0 	
	pointer		dw 0 	





	times 510-($-$$) db 0	
	dw 0AA55h		


buffer: