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Code: Select all

[bits 16]						
jmp main

;*********************************************
;	BIOS Parameter Block
;*********************************************
bpbOEM					db "ATLAM OS"
bpbBytesPerSector:  	DW 512
bpbSectorsPerCluster: 	DB 1
bpbReservedSectors: 	DW 2
bpbNumberOfFATs: 		DB 1
bpbRootEntries: 		DW 512
bpbTotalSectors: 		DW 0xFFFF
bpbMedia: 				DB 0xf8  ;; 0xF1
bpbSectorsPerFAT: 		DW 494
bpbSectorsPerTrack: 	DW 18
bpbHeadsPerCylinder: 	DW 2
bpbHiddenSectors: 		DD 0
bpbTotalSectorsBig:     DD 0
DRIVE_NUMBER            equ 7C24h
DRIVE_TYPE              equ 7C3Eh
NUMBER_OF_HEADS         equ 7C1Ah
NUMBER_OF_DRIVES        equ 7C3Fh
SECTORS_PER_TRACK       equ 7C18h
bsExtBootSignature: 	DB 0x29
bsSerialNumber:	        DD 0xa0a1a2a3
bsVolumeLabel: 	        DB "MOS FLOPPY "
bsFileSystem: 	        DB "FAT16   "

;*********************************************
;		End of BIOS PARAMETER BLOCK
;*********************************************
;************************************************;
;	Prints a string
;************************************************;
Print: 
			lodsb				; load next byte from string from SI to AL
			or	al, al			; Does AL=0?
			jz	PrintDone		; Yep, null terminator found-bail out
			mov	ah, 0eh			; Nope-Print the character
			int	10h
			jmp	Print			; Repeat until null terminator found
	PrintDone:
ret								; we are done, so return

;************************************************;
; Convert CHS to LBA
;************************************************;

ClusterLBA:
          sub     ax, 0x0002                          ; zero base cluster number
          xor     cx, cx
          mov     cl, BYTE [bpbSectorsPerCluster]     ; convert byte to word
          mul     cx
          add     ax, WORD [datasector]               ; base data sector
          ret
;************************************************;
; 			Failing to Load Proceedure
;************************************************;		
		
FAILURE:
		
        mov     si, msgFailure
        call    Print
        mov     ah, 0x00
        int     0x16                                ; await keypress
        int     0x19                                ; warm boot computer
;************************************************;
; Convert LBA to CHS
; AX=>LBA Address to convert
;
; 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 [SECTORS_PER_TRACK]           ; calculate
          inc     dl                                  ; adjust for sector 0
          mov     BYTE [absoluteSector], dl
          xor     dx, dx                              ; prepare dx:ax for operation
          div     WORD [NUMBER_OF_HEADS]          ; calculate
          mov     BYTE [absoluteHead], dl
          mov     BYTE [absoluteTrack], al
          ret
;************************************************;
; Reads a series of sectors
; CX=>Number of sectors to read
; AX=>Starting sector
; ES:BX=>Buffer to read to
;************************************************;

ReadSectors:
     .MAIN
          mov     di, 0x0005                          ; five retries for error
     .SECTORLOOP
          push    ax
          push    bx
          push    cx
          call    LBACHS                              ; convert starting sector to CHS
          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 [DRIVE_NUMBER]            ; 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, msgReadSuccess
          call    Print
          pop     cx
          pop     bx
          pop     ax
          add     bx, WORD [bpbBytesPerSector]        ; queue next buffer
		  inc     ax                                  ; queue next sector
          loop    .MAIN                               ; read next sector
          ret

;*********************************************
;	Bootloader Entry Point
;*********************************************


main:
;----------------------------------------------------
     ; code located at 0000:7C00, adjust segment registers
     ;----------------------------------------------------
		 cli						; disable interrupts
		 
         mov     ax, 0x07C0				; setup registers to point to our segment
		 mov     ds, ax
         mov     es, ax
         mov     fs, ax
         mov     gs, ax

     ;----------------------------------------------------
     ; create stack
     ;----------------------------------------------------
     
      mov     ax, 0x0000				; set the stack
      mov     ss, ax
      mov     sp, 0xFFFF
      sti								; restore interrupts
	 ;----------------------------------------------------
	 CLD
	 
DiskParam:
	 
push dx 
push es 
mov ah, 8 
int 13h                       ; get drive parameters 
mov [DRIVE_TYPE], bl 
and cx, 3Fh                   ; maximum sector number 
mov [SECTORS_PER_TRACK], cx 
mov [NUMBER_OF_DRIVES], dl 
movzx dx, dh                  ; maximum head number 
add dx, 1 
mov [NUMBER_OF_HEADS], dx 
pop es 
pop dx 
mov [DRIVE_NUMBER], dl

LOAD:
          xor 	  cx,cx
		  xor 	  ax,ax
		  xor     bx,bx
          mov 	  ax, 0x0210     						; LBA of Sector to read
          mov     bx, 0x0440                            ; Address to Load to
		  mov 	  cx, 0x0002							; Number of sectors to read
		  call    ReadSectors
		  
	mov     cx, 0x000D                 	; Counter String length
	mov		di,0x05A3					; Expected String Location in RAM
	mov     si, String 					; String requiring matching
    repe cmpsb                                         ; test for entry match
	je      KERNL
	jmp     FAILURE
    
KERNL:
mov     si, msgConfirmString
call    Print

jmp 	0x07C0:0x0440
;*********************************************
;		Messages Section
;*********************************************
String 									db "Searching for Operating System"
msgReadSuccess 							db 0x0D, 0x0A, "Krnl Rd Suc", 0x0D, 0x0A, 0x00
msgConfirmString 						db 0x0D, 0x0A, "Kernel Confirmed in Memory", 0x0D, 0x0A, 0x00
msgReadFail 							db 0x0D, 0x0A, "Krnl Rd Fld", 0x0D, 0x0A, 0x00
msgFailure  							db 0x0D, 0x0A, "ERR:Pres Key Rbt", 0x0A, 0x00
;*********************************************
;			Data Section
;*********************************************
absoluteSector 						db 0x00
absoluteHead   						db 0x00
absoluteTrack  						db 0x00     
datasector  						dw 0x0000
cluster    							dw 0x0000
 

;*********************************************
;		Boot Sector Signature
;*********************************************

	 TIMES 510-($-$$) DB 0
     DW 0xAA55

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  org 7C00h

  virtual
  bs BS
  end virtual

  jmp short @f
  nop
  rb BS_SIZE-3
@@:
  ...
  mov [bs.DriveNumber],dl
; or
  mov [$$+BS.DriveNumber],dl
; if virtual block was not declared

Code: Select all

  org 7C00h
  jmp short @f
  nop
  OEMName rb 8
  BytesPerSector dw ?
  ...
@@:
  ...
  mov [DriveNumber],dl ; actually I use a stack to save local variables

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move 3
pass BS_SIZE-3
move 512-BS_SIZE

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struc BS
{
  .Jump rb 3
  .OEMName rb 8
  .BytesPerSector dw ?
  .SectorsPerCluster db ?
  .ReservedSectors dw ?
  .FATs db ?
  .RootEntries dw ?
  .TotalSectors16 dw ?
  .Media db ?
  .FATSize16 dw ?
  .SPT dw ?
  .Heads dw ?
  .HiddenSectors dd ?
  .TotalSectors32 dd ?
  .DriveNumber db ?
  .Reserved1 db ?
  .BootSignature db ?
  .VolumeID dd ?
  .VolumeLabel rb 11
  .FSType rb 8
}