1.5 KiB
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