boot loader's size

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? 开发者_C百科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
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

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.

The boot sector is 512 bytes long, and is identified as such by the final two bytes begin set to 0xAA55. This leaves 510 bytes for the loader's actual code, which is precisely what the provided example fills when assembled. If your resulting binary isn't precisely 512 bytes long then you may need to specify a plain binary output format, though in the case of nasm this is the default setting.

In practice there are other magic bytes which need to be present for partition tables and such, and typically the first stage loader is used for little more than reading in and executing some more code.