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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 hang: 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 hang: jmp hang times 510-($-$$) db 0 ; 2 bytes less now db 0x55 db 0xAA
Once you've booted, and the cursor is happily blinking on a blank screen, the disk's motor will turn off and you can now press Ctrl-Alt-Del to reboot. This is because interrupts are still being generated.
Try clearing the interrupts flag:
;boot.asm cli hang: jmp hang times 510-($-$$) 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The code is assembled in NASM and copied to floppy (outdated), disk or USB-sticks using partcopy, dd, or debug.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 OR debug boot.bin -W 100 0 0 1 -Q
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:
Because of how fast emulation has become, you might need to slow down emulation speed to 1% to notice the reboots.