cfenollosa_os-tutorial/02-bootsector-print

Concepts you may want to Google beforehand: interrupts, CPU registers

Goal: Make our previously silent boot sector print some text

We will improve a bit on our infinite-loop boot sector and print something on the screen. We will raise an interrupt for this.

On this example we are going to write each character of the "Hello" word into the register al (lower part of ax), the bytes 0x0e into ah (the higher part of ax) and raise interrupt 0x10 which is a general interrupt for video services.

0x0e on ah tells the video interrupt that the actual function we want to run is to 'write the contents of al in tty mode'.

We will set tty mode only once though in the real world we cannot be sure that the contents of ah are constant. Some other process may run on the CPU while we are sleeping, not clean up properly and leave garbage data on ah.

For this example we don't need to take care of that since we are the only thing running on the CPU.

Our new boot sector looks like this:

mov ah, 0x0e ; tty mode
mov al, 'H'
int 0x10
mov al, 'e'
int 0x10
mov al, 'l'
int 0x10
int 0x10 ; 'l' is still on al, remember?
mov al, 'o'
int 0x10

jmp $ ; jump to current address = infinite loop

; padding and magic number
times 510 - ($-$$) db 0
dw 0xaa55 

You can examine the binary data with xxd file.bin

Anyway, you know the drill:

nasm -fbin boot_sect_hello.asm -o boot_sect_hello.bin

qemu boot_sect_hello.bin

Your boot sector will say 'Hello' and hang on an infinite loop