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Babystep1: Your first boot sector



Your first boot sector.


The following code is the smallest possible example of booting code from a disk.

; boot.asm
    jmp hang
    times 512-($-$$) db 0

The CPU starts in real mode and the BIOS loads this code at address 0000:7c00. "times 512-($-$$) db 0" is NASM's way of saying fill up 512 bytes with zeroes, and partcopy is going to expect that (200 in Hex = 512 in Decimal). Change it and partcopy will likely fail.

There is often a boot signature (0xAA55) at the end. Older versions of some BIOSes looked for this in order to identify a boot sector on a disk. It is evidently unnecessary nowadays, unless you're running the code on a legacy BIOS, or in QEMU. If it's needed, the last line would be replaced with (or some version of it):

; boot.asm
    jmp hang
    times 510-($-$$) db 0 ; 2 bytes less now
    db 0x55
    db 0xAA

Once you've booted, and the cursor is happily blinking on a blank screen, the disk's motor will turn off and you can now press Ctrl-Alt-Del to reboot. This is because interrupts are still being generated.

Try clearing the interrupts flag:

     jmp hang
     times 510-($-$$) db 0
     db 0x55
     db 0xAA

You may notice that the motor doesn't turn off and you can't reboot with Ctrl-Alt-Del.

Removing the loop and merely padding out the sector with zeroes will usually cause the BIOS to throw an error on boot. On most machines, it will say "Operating System Not Found".

Creating disk image

The code is assembled in NASM and copied to floppy (outdated), disk or USB-sticks using partcopy, dd, or debug. Then you simply boot from that disk.

For a more detailed description, see the Bootable Disk page.


nasmw boot.asm -f bin -o boot.bin
partcopy boot.bin 0 200 -f0 
debug boot.bin
-W 100 0 0 1


nasm boot.asm -f bin -o boot.bin
dd if=boot.bin of=/dev/fd0

To write to a hard drive or USB-stick, use

nasm boot.asm -f bin -o boot.bin
dd if=boot.bin of=/dev/sda

where replace "sda" with sdb, sdc etc. according to your configuration.

Running the binary in QEMU

If you don't have an old machine with floppy drive you can emulate one using QEMU (note "fda").

qemu-system-i386 -fda boot.bin

But it is advisable to forget about floppies altogether, and focus on USB-sticks instead. Also if you're afraid to test your code on your development machine (that would be wise), you can use QEMU (note "hda").

qemu-system-i386 -hda boot.bin

Use the QEMU monitor command to send Ctrl-Alt-Del to the VM:

sendkey ctrl-alt-delete

Because of how fast emulation has become, you might need to slow down emulation speed to 1% to notice the reboots.


  • Instruction Set from the horse's mouth: [1]
  • Easier to read: [2]
  • NASM asembler - docs incl instruction set: [3]
  • Partcopy - download (new link): [4]
  • Interrupts by number: [5]
  • Randall Hyde's look into the bowels of the PC: [6]
  • QEMU [7]
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