Question

I am reading brokenthorn.com ‘s O/S development tutorials one of the tutorials, the following code is there.

http://www.brokenthorn.com/Resources/OSDev3.html

I don’t understand why this code clear 510 bytes. org, bits, cli, hlt are there in the code too. Shouldn’t it be changed to less than 510 bytes? Could it be typo or something?

Thanks.

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;*********************************************
;   Boot1.asm
;       - A Simple Bootloader
;
;   Operating Systems Development Tutorial
;*********************************************
org 0x7c00  ; We are loaded by BIOS at 0x7C00

bits    16      ; We are still in 16 bit Real Mode

Start:

    cli ; Clear all Interrupts

    hlt ; halt the system

times 510 - ($-$$) db 0 ; We have to be 512 bytes. Clear the rest of the bytes with 0

dw 0xAA55   ; Boot Signiture
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

Answer 1

It’s not clearing 510 bytes, it’s clearing 510 - ($-$$) bytes. Since $ is the current position, and $$ is the start of the section, it’s clearing 510 - (length of the section up to that point) bytes.

This will fill the block correctly up to two bytes from the 512 byte limit, and put the signature on the two last bytes.