Boot sector: hello in 512 bytes
| The setup | |
| Printing in real mode | |
| The signature |
The setup
A boot sector is 512 raw bytes the BIOS loads at physical address 0x7C00 and jumps to. Assemble with -f bin, set the origin, and switch to 16-bit mode, because the CPU starts in real mode:
bits 16
org 0x7C00
start:
; real-mode code here
Printing in real mode
In real mode there are no syscalls: the BIOS provides services through INT 10h. Function 0Eh of INT 10h prints one character in AL to the screen, teletype style:
mov si, msg
print_loop:
lodsb ; AL = [SI], SI++
or al, al
jz done
mov ah, 0x0E
int 0x10
jmp print_loop
The signature
The last two bytes must be 0x55 then 0xAA, or the BIOS rejects the disk. Pad to 510 bytes and append the signature with TIMES:
times 510-($-$$) db 0
dw 0xAA55
Next: Capstone: command-line calculator
Article author: Arthur Isaev
