003 / boot sector

org 0x7C00

bits 16

%define ENDL 0x0D, 0x0A

start:
    jmp main

NASM is told to assemble 16-bit code with an origin of 0x7C00. The entry jumps to main; org describes the expected address rather than loading the code.

main:
    mov ax, 0
    mov ds, ax
    mov es, ax

    mov ss, ax
    mov sp, 0x7C00

    mov si, msg_hello
    call puts

    hlt

.halt:
    jmp .halt

The source initializes DS, ES, and SS to zero, sets SP to 0x7C00, points SI at the message, and calls puts. After printing, it reaches hlt and a loop before the data.

puts:
    push si
    push ax

.loop:
    lodsb

    or al, al
    jz .done

    mov ah, 0x0E
    mov bh, 0x00
    int 0x10

    jmp .loop

.done:
    pop ax
    pop si
    ret

The routine saves SI and AX, reads characters with lodsb, and stops at the zero terminator. BIOS interrupt 0x10 with AH = 0x0E prints each character. The saved registers are restored before returning.

msg_hello: db 'Hello, World!', ENDL, 0

The actual message is followed by ENDL (carriage return and line feed, defined in entry/) and a zero byte that terminates the string.

times 510 - ($ - $$) db 0

dw 0xAA55

NASM pads the sector with zero bytes up to offset 510, then writes the word 0xAA55. Its little-endian byte order is 55 AA. This explains the source directive; it is not a compiled byte dump.