boot_fat12.asm
ostin-project_ostin-kernel/bootloader/boot_fat12.asm
;;======================================================================================================================
;;///// boot_fat32.asm ///////////////////////////////////////////////////////////////////////////////////// GPLv2 /////
;;======================================================================================================================
;; (c) 2007-2011 KolibriOS team <http://kolibrios.org/>
;; (c) 2002 Alex Nogueira Teixeira <alexwal@siteplanet.com.br>
;;======================================================================================================================
;; This program is free software: you can redistribute it and/or modify it under the terms of the GNU General Public
;; License as published by the Free Software Foundation, either version 2 of the License, or (at your option) any later
;; version.
;;
;; This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied
;; warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.
;;
;; You should have received a copy of the GNU General Public License along with this program. If not, see
;; <http://www.gnu.org/licenses/>.
;;======================================================================================================================

lf		equ	0ah
cr		equ	0dh

pos_read_tmp	equ	0700h			;position for temporary read
boot_program	equ	07c00h			;position for boot code
seg_read_kernel	equ	01000h			;segment to kernel read

	jmp	start_program
	nop

; Boot Sector and BPB Structure
include 'floppy1440.inc'
;include 'floppy2880.inc'
;include 'floppy1680.inc'
;include 'floppy1743.inc'

start_program:

	xor	ax,ax
	mov	ss,ax
	mov	sp,boot_program
	push	ss
	pop	ds

	; print loading string
	mov	si,loading+boot_program
loop_loading:
	lodsb
	or	al,al
	jz	read_root_directory
	mov	ah,0eh
	mov	bx,7
	int	10h
	jmp	loop_loading

read_root_directory:
	push	ss
	pop	es

	; calculate some disk parameters
	; - beginning sector of RootDir
	mov	ax,word [BPB_FATSz16+boot_program]
	xor	cx,cx
	mov	cl,byte [BPB_NumFATs+boot_program]
	mul	cx
	add	ax,word [BPB_RsvdSecCnt+boot_program]
	mov	word [FirstRootDirSecNum+boot_program],ax	; 19
	mov	si,ax

	; - count of sectors in RootDir
	mov	bx,word [BPB_BytsPerSec+boot_program]
	mov	cl,5				; divide ax by 32
	shr	bx,cl				; bx = directory entries per sector
	mov	ax,word [BPB_RootEntCnt+boot_program]
	xor	dx,dx
	div	bx
	mov	word [RootDirSecs+boot_program],ax		; 14

	; - data start
	add	si,ax				; add beginning sector of RootDir and count sectors in RootDir
	mov	word [data_start+boot_program],si		; 33
	; reading root directory
	; al=count root dir sectrors !!!! TODO: al, max 255 sectors !!!!
	mov	ah,2				; read
	push	ax

	mov	ax,word [FirstRootDirSecNum+boot_program]
	call	conv_abs_to_THS			; convert abs sector (AX) to BIOS T:H:S (track:head:sector)
	pop	ax
	mov	bx,pos_read_tmp			; es:bx read buffer
	call	read_sector

	mov	si,bx				; read buffer address: es:si
	mov	ax,[RootDirSecs+boot_program]
	mul	word [BPB_BytsPerSec+boot_program]
	add	ax,si				; AX = end of root dir. in buffer pos_read_tmp

	; find kernel file in root directory
loop_find_dir_entry:
	push	si
	mov	cx,11
	mov	di,kernel_name+boot_program
	rep	cmpsb				; compare es:si and es:di, cx bytes long
	pop	si
	je	found_kernel_file
	add	si,32				; next dir. entry
	cmp	si,ax				; end of directory
	jb	loop_find_dir_entry

file_error_message:
	mov	si,error_message+boot_program

loop_error_message:
	lodsb
	or	al,al
	jz	freeze_pc
	mov	ah,0eh
	mov	bx,7
	int	10h
	jmp	loop_error_message

freeze_pc:
	jmp	$				; endless loop

	; === KERNEL FOUND. LOADING... ===

found_kernel_file:
	mov	bp,[si+01ah]			; first cluster of kernel file
	; <diamond>
	mov	[cluster1st+boot_program],bp	; starting cluster of kernel file
	; <\diamond>

	; reading first FAT table
	mov	ax,word [BPB_RsvdSecCnt+boot_program]	; begin first FAT abs sector number
	call	conv_abs_to_THS			; convert abs sector (AX) to BIOS T:H:S (track:head:sector)
	mov	bx,pos_read_tmp			; es:bx read position
	mov	ah,2				; ah=2 (read)
	mov	al, byte [BPB_FATSz16+boot_program]	; FAT size in sectors (TODO: max 255 sectors)
	call	read_sector
	jc	file_error_message		; read error

	mov	ax,seg_read_kernel
	mov	es,ax
	xor	bx,bx				; es:bx = 1000h:0000h


	; reading kernel file
loop_obtains_kernel_data:
	; read one cluster of file
	call	obtain_cluster
	jc	file_error_message		; read error

	; add one cluster length to segment:offset
	push	bx
	mov	bx,es
	mov	ax,word [BPB_BytsPerSec+boot_program]	;\
	movsx	cx,byte [BPB_SecPerClus+boot_program]	; | !!! TODO: !!!
	mul	cx					; | out this from loop !!!
	shr	ax,4					;/
	add	bx,ax
	mov	es,bx
	pop	bx

	mov	di,bp
	shr	di,1
	pushf
	add	di,bp				; di = bp * 1.5
	add	di,pos_read_tmp
	mov	ax,[di]				; read next entry from FAT-chain
	popf
	jc	move_4_right
	and	ax,0fffh
	jmp	verify_end_sector
move_4_right:
	mov	cl,4
	shr	ax,cl
verify_end_sector:
	cmp	ax,0ff8h			; last cluster
	jae	execute_kernel
	mov	bp,ax
	jmp	loop_obtains_kernel_data

execute_kernel:
	; <diamond>
	mov	ax,'KL'
	push	0
	pop	ds
	mov	si,loader_block+boot_program
	; </diamond>
	push	word	seg_read_kernel
	push	word	0
	retf					; jmp far 1000:0000


;------------------------------------------
	; loading cluster from file to es:bx
obtain_cluster:
	; bp - cluster number to read
	; carry = 0 -> read OK
	; carry = 1 -> read ERROR

	; print one dot
	push	bx
	mov	ax,0e2eh			; ah=0eh (teletype), al='.'
	xor	bh,bh
	int	10h
	pop	bx

writesec:
	; convert cluster number to sector number
	mov	ax,bp				; data cluster to read
	sub	ax,2
	xor	dx,dx
	mov	dl,byte [BPB_SecPerClus+boot_program]
	mul	dx
	add	ax,word [data_start+boot_program]

	call	conv_abs_to_THS			; convert abs sector (AX) to BIOS T:H:S (track:head:sector)
patchhere:
	mov	ah,2				; ah=2 (read)
	mov	al,byte [BPB_SecPerClus+boot_program]	; al=(one cluster)
	call	read_sector
	retn
;------------------------------------------

;------------------------------------------
	; read sector from disk
read_sector:
	push 	bp
	mov	bp,20				; try 20 times
newread:
	dec	bp
	jz	file_error_message
	push	ax bx cx dx
	int	13h
	pop	dx cx bx ax
	jc	newread
	pop	bp
	retn
;------------------------------------------
	; convert abs. sector number (AX) to BIOS T:H:S
	; sector number = (abs.sector%BPB_SecPerTrk)+1
	; pre.track number = (abs.sector/BPB_SecPerTrk)
	; head number = pre.track number%BPB_NumHeads
	; track number = pre.track number/BPB_NumHeads
	; Return: cl - sector number
	;	  ch - track number
	;	  dl - drive number (0 = a:)
	;	  dh - head number
conv_abs_to_THS:
	push	bx
	mov	bx,word [BPB_SecPerTrk+boot_program]
	xor	dx,dx
	div	bx
	inc	dx
	mov	cl, dl				; cl = sector number
	mov	bx,word [BPB_NumHeads+boot_program]
	xor	dx,dx
	div	bx
	; !!!!!!! ax = track number, dx = head number
	mov	ch,al				; ch=track number
	xchg	dh,dl				; dh=head number
	mov	dl,0				; dl=0 (drive 0 (a:))
	pop	bx
	retn
;------------------------------------------

loading		db	cr,lf,'Starting system ',00h
error_message	db	13,10
kernel_name	db	'KERNEL  MNT ?',cr,lf,00h
FirstRootDirSecNum	dw	?
RootDirSecs	dw	?
data_start	dw	?

; <diamond>
write1st:
	push	cs
	pop	ds
	mov	byte [patchhere+1+boot_program], 3	; change ah=2 to ah=3
	mov	bp,[cluster1st+boot_program]
	push	1000h
	pop	es
	xor	bx,bx
	call	writesec
	mov	byte [patchhere+1+boot_program], 2	; change back ah=3 to ah=2
	retf
cluster1st	dw	?
loader_block:
		db	1
		dw	0
		dw	write1st+boot_program
		dw	0
; <\diamond>

times	0x1fe-$ db 00h

	db	55h,0aah 			;boot signature
