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asm | by: MohamedIBrahi10

last edit: Jan, 26th 2012 | jump to bottom

  1. org 0x7c00	; This is where BIOS loads the bootloader
  2.  
  3.  
  4. ; Execution begins here
  5. entry:
  6. 	jmp short begin ; jump over the DOS boot record data
  7.  
  8.  
  9. ; ----------------------------------------------------------------------
  10. ; data portion of the "DOS BOOT RECORD"
  11. ; ----------------------------------------------------------------------
  12. brINT13Flag     DB      90H             ; 0002h - 0EH for INT13 AH=42 READ
  13. brOEM           DB      'MSDOS5.0'      ; 0003h - OEM name & DOS version (8 chars)
  14. brBPS           DW      512             ; 000Bh - Bytes/sector
  15. brSPC           DB      1               ; 000Dh - Sectors/cluster
  16. brResCount      DW      1               ; 000Eh - Reserved (boot) sectors
  17. brFATs          DB      2               ; 0010h - FAT copies
  18. brRootEntries   DW      0E0H		; 0011h - Root directory entries
  19. brSectorCount   DW      2880		; 0013h - Sectors in volume, < 32MB
  20. brMedia         DB      240		; 0015h - Media descriptor
  21. brSPF           DW      9               ; 0016h - Sectors per FAT
  22. brSPH           DW      18              ; 0018h - Sectors per track
  23. brHPC           DW      2		; 001Ah - Number of Heads
  24. brHidden        DD      0               ; 001Ch - Hidden sectors
  25. brSectors       DD      0	        ; 0020h - Total number of sectors
  26. 		DB      0               ; 0024h - Physical drive no.
  27. 		DB      0               ; 0025h - Reserved (FAT32)
  28. 		DB      29H             ; 0026h - Extended boot record sig 
  29. brSerialNum     DD      404418EAH       ; 0027h - Volume serial number (random)
  30. brLabel         DB      'Joels disk '   ; 002Bh - Volume label  (11 chars)
  31. brFSID          DB      'FAT12   '      ; 0036h - File System ID (8 chars)
  32. ;------------------------------------------------------------------------
  33.  
  34.  
  35. ; --------------------------------------------
  36. ;  Boot program code begins here
  37. ; --------------------------------------------
  38. ; boot code begins at 0x003E
  39. begin:
  40. 	xor	ax, ax		; zero out ax
  41. 	mov	ds, ax		; set data segment to base of RAM
  42. 	mov	si, msg		; load address of our message
  43. 	call	putstr		; print the message
  44.  
  45. hang:
  46. 	jmp	hang		; just loop forever.
  47.  
  48. ; --------------------------------------------
  49. ; data for our program
  50.  
  51. msg	db	'Hello, World!', 0
  52.  
  53. ; ---------------------------------------------
  54. ; Print a null-terminated string on the screen
  55. ; ---------------------------------------------
  56. putstr:
  57. 	lodsb		; AL = [DS:SI]
  58. 	or al, al	; Set zero flag if al=0
  59. 	jz putstrd	; jump to putstrd if zero flag is set
  60. 	mov ah, 0x0e	; video function 0Eh (print char)
  61. 	mov bx, 0x0007	; color
  62. 	int 0x10
  63. 	jmp putstr
  64. putstrd:
  65. 	retn
  66. ;---------------------------------------------
  67.  
  68. size	equ	$ - entry
  69. %if size+2 > 512
  70.   %error "code is too large for boot sector"
  71. %endif
  72. 	times	(512 - size - 2) db 0
  73.  
  74. 	db	0x55, 0xAA		;2  byte boot signature
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