;************************************************************************* ;Name: R.P.I.T. Somasiri ;E-mail: ishanthilina@gmail.com ;------------------------------------------------------------------------- ;the ultimate boot-strap loader ;to load a file from a DOS FAT32 Disk as the OS ;************************************************************************* ORG 0x7c00 jmp START nop bs_OEM db "MSDOS5.0" A_BF_BPB_BytesPerSector DW 200h A_BF_BPB_SectorsPerCluster DB 8h A_BF_BPB_ReservedSectors DW 01f0h A_BF_BPB_NumberOfFATs DB 02 A_BF_BPB_RootEntries DW 0 A_BF_BPB_TotalSectors DW 0 A_BF_BPB_MediaDescriptor DB 0f8h A_BF_BPB_SectorsPerFAT DW 0 A_BF_BPB_SectorsPerTrack DW 03fh A_BF_BPB_Heads DW 0ffh A_BF_BPB_HiddenSectors DW 080h A_BF_BPB_HiddenSectorsHigh DW 0h A_BF_BPB_BigTotalSectors DW 3f00h A_BF_BPB_BigTotalSectorsHigh DW 03ch A_BF_BPB_BigSectorsPerFat DW 0f08h A_BF_BPB_BigSectorsPerFatHi DW 0h A_BF_BPB_ExtFlags DW 0 A_BF_BPB_FS_Version DW 0 A_BF_BPB_RootDirStrtClus DW 2 A_BF_BPB_RootDirStrtClusHi DW 0 A_BF_BPB_FSInfoSec DW 1h A_BF_BPB_BkUpBootSec DW 6h A_BF_BPB_Reserved DW 6 DUP (?) BS_physical db 80h db 00 BS_bootrecord db 29h volumeid dd 0AE6DA6B2h volumelabel db "No Name " filesystem 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[A_BF_BPB_SectorsPerCluster] ; compute location of the begining of the Data area and store in ax mov al, BYTE [A_BF_BPB_NumberOfFATs] ; Total number of FATs mul WORD[A_BF_BPB_BigSectorsPerFat] ; Number of sectors for a FAT add ax, WORD [A_BF_BPB_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[A_BF_BPB_RootDirStrtClus] 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 [BS_physical] ; 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 [A_BF_BPB_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) * A_BF_BPB_SectorsPerCluster(0x08)) ;************************************************************************* ClusterLBA: sub ax, 0x0002 ; zero base cluster number xor cx, cx mov cl, BYTE [A_BF_BPB_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 [A_BF_BPB_SectorsPerTrack] ; calculate inc dl ; adjust for sector 0 mov BYTE [absoluteSector], dl xor dx, dx ; prepare dx:ax for operation div WORD [A_BF_BPB_Heads] ; 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 ;*************************************************************************