boot.asm

;*************************************************************************
; Name: P.L.B. Sampath
; Index No: 100476M
;*************************************************************************



;*************************************************************************
;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
     TotalSectors dd 0x00FE3B1F
     MediaDescriptor db 0xF8
     SectorsPerFAT dd 0x00000778
     SectorsPerTrack dw 0x003D
     SectorsPerHead dw 0x0002
     HiddenSectors dd 0x00000000
     DriveNumber db 0x00
     Flags dw 0x0000
     Signature db 0x29
     VolumeID dd 0xFFFFFFFF
     VolumeLabel db "QUASI BOOT"
     SystemID db "FAT32 "
     RootDirectoryStart dd 0x00000002
     BackupBootSector dw 0x0006
     
     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
     ; post message
          mov si, msgLoading
          call DisplayMessage



          
;*************************************************************************
;start of function modification
;*************************************************************************
   
       CalcBegOfData:
      ;calculating begining of the data area and assigning to parameter 'datasecter'
          mov al, BYTE [TotalFATs]
          mul WORD[SectorsPerFAT]
          add ax, WORD [ReservedSectors]
          mov WORD [datasector], ax




       ReadFirstCluster:
      ; read 1st data cluster into memory (7C00:0200)
          mov cx, WORD[SectorsPerCluster]
          mov ax, WORD[RootDirectoryStart]
          call ClusterLBA
          mov bx, 0x0200
          call ReadSectors

   


       ReadKernel:
          mov si, msgCRLF
          call DisplayMessage

      .PointIndexReg
          mov di, 0x0200 + 0x20
          
      .PointOffset  
          mov dx, WORD [di + 0x001A]
          mov WORD [cluster], dx

      .SetKernelSeg
          mov ax, 0100h ; set ES:BX = 0100:0000
          mov es, ax
          mov bx, 0

      .ReadClutters
          mov cx, 0x0008
          mov ax, WORD[cluster]
          call ClusterLBA
            call ReadSectors
 
           
;*************************************************************************
;end of function modification
;*************************************************************************
            
     
       

       DONE:
          mov si, msgCRLF
          call DisplayMessage
          push WORD 0x0100
          push WORD 0x0000
          retf




       FAILURE:
          mov     si, msgFailure
          call    DisplayMessage
          mov     ah, 0x00
          int     0x16                                ; await keypress
          int     0x19                                ; warm boot computer




     ;*************************************************************************
     ; 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 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 ClusterLBA
     ; convert FAT cluster into LBA addressing scheme
     ; LBA = (cluster - 2) * sectors per cluster
     ;*************************************************************************
     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
     msgProgress db ".", 0x00
     msgFailure db 0x0D, 0x0A, "Kernel loading failed...", 0x0D, 0x0A, 0x00
     msgCRLF db 0x0D, 0x0A, 0x00
     msgLoading  db 0x0D, 0x0A, "Loading Boot Image ", 0x0D, 0x0A, 0x00
     
          TIMES 510-($-$$) DB 0
          DW 0xAA55
     ;*************************************************************************