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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 floppy.

; 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 you'll see partcopy choke.

Often, you will see a boot signature (0xAA55) at the end. Older versions of 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 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, you might notice two things. One is that the floppy motor will turn off and the other is that you can press Ctrl-Alt-Del to reboot. The point is that 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 floppy motor doesn't turn off and you can't reboot with Ctrl-Alt-Del.

If you try to reduce this even more by removing the loop and merely pad out the sector with zeroes, the BIOS will have something to say about it. On my machine, it was "Operating System Not Found". I have yet to try filling the sector with zeroes except for adding a boot signature.

Not exactly something you would show your friends, but I wanted to show just what the bare minimum is before I elaborate. Unless I'm irritating anyone, in which case I'll desist.

Creating disk image

The code is assembled in NASM and copied to floppy using partcopy, dd, or debug. Then you simply boot from the floppy.


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

Running the binary in QEMU

If you don't have an old machine with floppy drive you can still go through the tutorial by using QEMU.

qemu-system-i386 -fda boot.bin

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

sendkey ctrl-alt-delete

Nowadays even software emulation works fast, so you might want 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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