;
;-------------------------------------------------------------------------
;the ultimate boot-strap loader
;to load a file from a DOS FAT32 Disk as the OS
;*************************************************************************
[BITS 16]
[ORG 0x0000]
jmp START
OEM_ID db "QUASI-OS"
BytesPerSector dw 0x0200
SectorsPerCluster db 0x08
ReservedSectors dw 0x0020
TotalFATs db 0x02
MaxRootEntries dw 0x0000
NumberOfSectors dw 0x0000
MediaDescriptor db 0xF8
SectorsPerFAT dw 0x0000
SectorsPerTrack dw 0x003D
SectorsPerHead dw 0x0002
HiddenSectors dd 0x00000000
TotalSectors dd 0x00FE3B1F	
BigSectorsPerFAT dd 0x00000778
Flags dw 0x0000
FSVersion dw 0x0000
RootDirectoryStart dd 0x00000002
FSInfoSector dw 0x0001
BackupBootSector dw 0x0006
TIMES 13 DB 0 ;jumping to next offset
DriveNumber db 0x00
Signature db 0x29
VolumeID dd 0xFFFFFFFF
VolumeLabel db "QUASI BOOT"
SystemID db "FAT32 "
START:
; code located at 0000:7C00, adjust segment registers
cli
mov ax, 0x07C0
mov ds, ax
mov es, ax
mov fs, ax
mov gs, ax
; create stack
mov ax, 0x0000
mov ss, ax
mov sp, 0xFFFF
sti
; size of a cluster in sectors is stored in cx
mov cx, WORD[SectorsPerCluster]
; compute location of the begining of the Data area and store in ax
mov al, BYTE [TotalFATs] ; Total number of FATs
mul WORD[BigSectorsPerFAT] ; Number of sectors for a FAT
add ax, WORD [ReservedSectors] ; Find the start of the Data area
mov WORD [datasector], ax ; Store the begining of the Data area
; read 1st data cluster into memory (7C00:0200)
mov ax, WORD[RootDirectoryStart]
call ClusterLBA
mov bx, 0x0200 ; copy 1st data cluter above bootcode
call ReadSectors
; Point Index register to 1st File Entry
mov di, 0x0200 + 0x20
mov si, msgCRLF
call DisplayMessage
;Point to the offset where the file location information contains
mov dx, WORD [di + 0x001A]
mov WORD [cluster], dx
;Set up the segments where the kernel needs to be loaded
mov ax, 0100h ; set ES:BX = 0100:0000
mov es, ax
mov bx, 0
;Read the cluster which contains the kernel
mov cx, 0x0008	
mov ax, WORD[cluster]
call ClusterLBA
call ReadSectors
mov si, msgCRLF
call DisplayMessage
;Jump to the location where the kernel was loded
push WORD 0x0100
push WORD 0x0000
retf
;An error has occured if this part is executed
mov si, msgFailure
call DisplayMessage
mov ah, 0x00
int 0x16 ; await keypress
int 0x19 ; warm boot computer
;*************************************************************************
; PROCEDURE ReadSectors
; reads ?cx? sectors from disk starting at ?ax? into
;memory location ?es:bx?
;*************************************************************************
ReadSectors:
.MAIN
mov di, 0x0005 ; five retries for error
.SECTORLOOP
push ax
push bx
push cx
call LBACHS
mov ah, 0x02 ; BIOS read sector
mov al, 0x01 ; read one sector
mov ch, BYTE [absoluteTrack] ; track
mov cl, BYTE [absoluteSector] ; sector
mov dh, BYTE [absoluteHead] ; head
mov dl, BYTE [DriveNumber] ; drive
int 0x13 ; invoke BIOS
jnc .SUCCESS ; test for read error
xor ax, ax ; BIOS reset disk
int 0x13 ; invoke BIOS
dec di ; decrement error counter
pop cx
pop bx
pop ax
jnz .SECTORLOOP ; attempt to read again
int 0x18
.SUCCESS
mov si, msgProgress
call DisplayMessage
pop cx
pop bx
pop ax
add bx, WORD [BytesPerSector] ; queue next buffer
inc ax ; queue next sector
loop .MAIN ; read next sector
ret
;*************************************************************************
; PROCEDURE DisplayMessage
; display ASCIIZ string at ds:si via BIOS
;*************************************************************************
DisplayMessage:
lodsb ; load next character
or al, al ; test for NUL character
jz .DONE
mov ah, 0x0E ; BIOS teletype
mov bh, 0x00 ; display page 0
mov bl, 0x07 ; text attribute
int 0x10 ; invoke BIOS
jmp DisplayMessage
.DONE:
ret
;*************************************************************************
;*************************************************************************
; PROCEDURE ClusterLBA
; convert FAT cluster into LBA addressing scheme
; FileStartSector = ((X ? 2) * SectorsPerCluster(0x08))
;*************************************************************************
ClusterLBA:
sub ax, 0x0002 ; zero base cluster number
xor cx, cx
mov cl, BYTE [SectorsPerCluster] ; convert byte to word
mul cx
add ax, WORD [datasector] ; base data sector
ret
;*************************************************************************
; PROCEDURE LBACHS
; convert ?ax? LBA addressing scheme to CHS addressing scheme
; absolute sector = (logical sector / sectors per track) + 1
; absolute head = (logical sector / sectors per track) MOD number of heads
; absolute track = logical sector / (sectors per track * number of heads)
;*************************************************************************
LBACHS:
xor dx, dx ; prepare dx:ax for operation
div WORD [SectorsPerTrack] ; calculate
inc dl ; adjust for sector 0
mov BYTE [absoluteSector], dl
xor dx, dx ; prepare dx:ax for operation
div WORD [SectorsPerHead] ; calculate
mov BYTE [absoluteHead], dl
mov BYTE [absoluteTrack], al
ret
;************************************************************************
absoluteSector db 0x00
absoluteHead db 0x00
absoluteTrack db 0x00
cluster dw 0x0000
datasector dw 0x0000
;*******************************************************************************
;messages that needs to be shown
msgProgress db ".", 0x00
msgFailure db 0x0D, 0x0A, "Kernel loading failed...", 0x0D, 0x0A, 0x00
msgCRLF db 0x0D, 0x0A, 0x00
TIMES 510-($-$$) DB 0
DW 0xAA55
;*************************************************************************