A simple bootsector/OS.

bootsect.asm

; Simple NASM bootloader, or OS, if you please.
; Boots up and displays a black screen, and prints a message.

[ORG 0x7C00]        ; Specify where to load the code.

msg: db  'Hello!'   ; Define the msg.
len: equ $-msg      ; Define the msg length.

; Create our "PRINT" function, which takes two arguments:
;  1: The message
;  2: The message length
; Note: This is a NASM macro. Any time we reference this, all it does is
;  insert this code at that point, replacing %1, %2 with the arguments.
%macro PRINT 2
    MOV BX, 0       ; Clear register BX.
    MOV CX, 0       ; Clear the count register.
    MOV BX, %1      ; Store the address of the msg.

    pchar:          ; Start point; label.
    MOV AH, 0x0e    ; Prepare to write a char.
    MOV AL, [BX]    ; Copy the char to AL.
    INT 0x10        ; Call the interrupt to print the char.
    INC CX          ; Increment CX.
    INC BX          ; Increment BX.
    CMP CX, %2      ; Compare CX to the msg length.
    JNE pchar       ; If CX is less, jump back to the top.
%endmacro

PRINT msg, len      ; Call the print function on our msg

jmp $               ; Continuously jump to $, which is this line.

times 510 -($ - $$) db 0 ; Pad up to the 510th byte
dw 0xAA55                ; Last two bytes: 0xAA55

; This creates a 512 byte binary file, which is a valid boot sector.
; Copying this to a floppy should yeild a black screen with the output:
;  Hello!