Hello World OS

asm
;; boot "Hello world!" program

;; usage:
;; nasm -fbin hello.asm -o hello.bin
;; UltraISO -bootfile hello.bin -outfile hello.iso


bits    16                              ; 16 bit instructions
org     0x7C00                          ; BIOS transfers execution here after loading the boot sector

start:
	;; zero out ds, because memory references use ds as base
	;; ds can only be set via a register so reading from relative addresses would work
    mov ax, 0
    mov ds, ax
    
    mov si, string                      ; si points to the string we want to display

loop:
    mov al, [si]                        ; al contains the character we want to display
    mov ah, 0xE                         ; teletype (TTY) mode
    mov bl, 0x7                         ; text color (doesn't matter in tty mode)
    mov bh, 0                           ; page number

    cmp al, 0
    jz loop_end                         ; end loop if we reached the zero byte
    
    int 0x10                            ; BIOS interrupt
    
    inc si
    jmp loop

loop_end:
    jmp $                               ; hog execution forever :D


string  db "Hello world!", 13, 10, 0
times 510-($-$$) db 0                   ; pad program to 512 bytes with zeroes
dw 0xAA55                               ; boot sector closing bytes