1. ; Tell NASM that the code's base will be at 7c00h.
2. ; Otherwise it will assume offset 0 when calculating addresses.
3. org 7c00h
4.  
5. ; jump over data
6. jmp short init
7.  
8. message:
9.     db "Error", 0 ; null-terminated message
10.  
11. clearmsg:
12.     db "     ", 0
13.  
14. init:
15.     ; Video initalisation
16.     mov ah, 0x00 ; Set video mode
17.     mov al, 0x12 ; VGA mode
18.     int 0x10     ; Video function call interrupt
19.     ; Register initialisation
20.     xor ax, ax   ; clear ax
21.     mov ds, ax   ; ds needs to be 0 for lodsb
22.     cld          ; clear direction flag for lodsb
23.     mov dh, 0x0a    ; row 10
24.     mov dl, 0x28    ; col 40
25.  
26. main:
27.     ; Set cursor position
28.     mov ah, 0x02    ; cursor position
29.     mov bh, 0x00    ; page 0
30.     int 0x10        ; Video function call interrupt
31.     mov si, clearmsg
32.     call string
33.  
34.     mov ah, 0x02    ; cursor position
35.     mov bh, 0x00    ; page 0
36.     inc dh
37.     inc dl
38.     int 0x10        ; Video function call interrupt
39.     mov si, message ; move the message's address into si for lodsb
40.     call string      ; jump to the string routine
41.     jmp waitblah
42.  
43. ; Displays a character
44. ; al: character
45. char:
46.     mov ah, 0x0e ; Teletype output mode
47.     mov bh, 0x00 ; page number
48.     mov bl, 0x0c ; color (bright red)
49.     int 0x10     ; Video function call interrupt
50.  
51. ; print a string
52. string:
53.     lodsb       ; lodsb loads ds:esi into al
54.     cmp al, 0x0
55.     jnz char    ; display character if not null
56.     mov cx, 0xFFFF
57.     ret
58.  
59. waitblah:
60.     mov bx, 0xFF
61.     dec cx
62.    
63. waitinner:
64.     dec bx
65.     cmp bx, 0x0
66.     jnz waitinner
67.  
68.     cmp cx, 0x0
69.     jnz waitblah
70.     jmp main
71.  
72.  
73. ; infinite loop
74. end:
75.     jmp short end
76.  
77. times 0x0200 - 2 - ($ - $$)  db 0 ; NASM directive: zerofill up to 510 bytes
78.  
79. dw 0x0AA55 ; Magic boot sector signature
80.