OSDev.org

Code: Select all

TEST dd 0x00
TEST2 dd 0x00

mov dword[TEST], [TEST2]
 

Code: Select all

org 0 ; we start everything at zero
	jmp start     
MANU_DESC 			db 'mkdosfs '
BLOCK_SIZE	 		dw 512
BLOCKS_PER_ALLOCATION_UNIT 	db 128
RESERVED_BLOCKS			dw 1
TOTAL_FATS			db 1
TOTAL_ROOT_ENTRIES		dw 512
TOTAL_BLOCKS			dw 0xffff
MEDIA_DESCRIPTOR		db 0xf8
BLOCKS_PER_FAT			dw 0x01
BLOCKS_PER_TRACK		dw 18
TOTAL_HEADS			dw 0x02
HIDDEN_BLOCKS			dd 0x00
TOTAL_BLOCKS_2			dd 0x00
DRIVE_NUMBER			dw 0x00
EXTENDED_BOOT_SIGNATURE		db 0x29
VOLUME_SERIAL_NUMBER		dd 0x9d86f18c
VOLUME_LABEL			db 'SEAGULL    '
FILE_SYSTEM_IDENTIFIER		db 'FAT16   '

; Calculated in memory
ROOT_DIRECTORY		dd 0x00

DAPACK:
	db 0x10
	db 0
.len:	dw 1
.loc:	dd 0x7e00
.sec:	dd 2
	dd 0
	
start:

	mov ax, 0x7c0
	mov ds, ax
	mov es, ax
	mov sp, 0x1000
	mov bp, 0
	mov ss, bp


; Root Directory Logical Sector =  (TOTAL_FATS * BLOCKS_PER_FAT) + (RESERVED_BLOCKS)

	mov ax, [TOTAL_FATS]
	mul word [BLOCKS_PER_FAT]
	mov word [ROOT_DIRECTORY+2], dx
	push dx
	add ax, [RESERVED_BLOCKS]
	mov word [ROOT_DIRECTORY], ax


; Load root directory


	mov word [DAPACK.sec], ax
	pop dx
	mov word [DAPACK.sec+2], dx

	mov si, DAPACK		; address of "disk address packet"
	mov ah, 0x42		; AL is unused
	mov dl, 0x80		; drive number 0 (OR the drive # with 0x80)
	int 0x13
	jc err

	mov si, 512
	mov bx, 40
	call print_until

	jmp $

err:
	mov si, ERROR
	call print
	jmp $


print:
	pusha
	mov ah, 0eh
.repeat:
	lodsb
	cmp al, 0
	je .done
	int 10h
	jmp .repeat
.done:
	popa
	ret

print_until:
	mov ah, 0eh
.repeat:
	lodsb
	cmp bx, 0
	je .done
	int 10h
	dec bx
	jmp .repeat
.done:
	popa
	ret

ERROR db "Failed to load sector", 10, 13, 0
times 510-($-$$) db 0 ; fill in the remaining space with zero
dw 0xaa55 ; legacy boot signature

Code: Select all

start:
   mov ax, 0x7c0
   mov ds, ax
   mov es, ax
   mov sp, 0x1000
   mov bp, 0        ; <----- Critical point
   mov ss, bp

Code: Select all

start:
	jmp 0x7c0:update_cs
update_cs:

   mov ax, 0x7c0
   mov ds, ax
   mov es, ax
   cli
   mov sp, 0x1000
   mov bp, 0
   mov ss, bp
   sti

Code: Select all

jmp word start

Code: Select all

jmp short start
nop

Code: Select all

start:                  JMP loader
 
                        TIMES 0Bh-$+start DB 0
bpbBytesPerSector:  	 DW 512

Code: Select all

00000000  EB4F              jmp short 0x51

Code: Select all

print_until:
pusha
mov ah, 0Eh
.repeat:
lodsb
int 10h
loop .repeat
popa
ret