cfenollosa_os-tutorial/01-simple-boot-sector

This is very exciting, we're going to create our own boot sector!

Theory

When the computer boots, the BIOS doesn't know how to load the OS, so it delegates that task to the boot sector. Thus, the boot sector must be placed in a known, standard location. That location is the first sector of the disk (cylinder 0, head 0, sector 0) and it takes 512 bytes.

To make sure that the "disk is bootable", the BIOS checks that bytes 511 and 512 of the alleged boot sector are bytes 0xAA55.

This is the simplest boot sector ever:

e9 fd ff 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
[ 29 more lines with sixteen zero-bytes each ]
00 00 00 00 00 00 00 00 00 00 00 00 00 00 55 aa

It is basically all zeros, ending with the 16-bit value 0xAA55 (beware of indianness, x86 is little-endian). The first three bytes perform an infinite jump

Simplest boot sector ever

You can either write the above 512 bytes with a binary editor, or just write a very simple assembler code:

; Infinite loop (e9 fd ff)
loop:
    jmp loop 

; Fill with 510 zeros minus the size of the previous code
times 510-($-$$) db 0
; Magic number
dw 0xaa55 

To compile: nasm -f bin boot_sect_simple.asm -o boot_sect_simple.bin

OSX warning: if this drops an error, read chapter 00 again

I know you're anxious to try it out (I am!), so let's do it:

qemu boot_sect_simple.bin

You will see a window open which says "Booting from Hard Disk..." and nothing else. When was the last time you were so excited to see an infinite loop? ;-)