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__NOTOC__
=== Your first boot sector ===▼
The following code is the smallest possible example of booting code from a floppy. It is assembled in [[NASM]] and copied to floppy using either partcopy or dd. Then you simply boot from the floppy.▼
 
{{Tone}}
{| {{wikitable}} width="100%"
{{Rating|1}}
|-
 
! Windows/DOS▼
{{Infobox Tutorial
! Unix
| name=Babystep1: Your first boot sector
|-
| next=[[Babystep2]]
|
|}}▼
> nasmw boot.asm -f bin -o boot.bin▼
 
> partcopy boot.bin 0 200 -f0▼
▲=== Your first boot sector. ===
|
 
$ nasm boot.asm -f bin -o boot.bin▼
==Code==
$ dd if=boot.bin of=/dev/fd0▼
The following code is the smallest possible example of booting code from a disk.
▲|}
 
<pre>
<syntaxhighlight lang="asm">
; boot.asm
hang:
Line 20:
 
times 512-($-$$) db 0
</syntaxhighlight>
</pre>
The CPU starts in real mode and the [[BIOS]] loads this code at address 0000:7c00. The "<code>times 512..."-($-$$) stuffdb 0</code> is NASM's way of saying fill up 512 bytes with zeros.zeroes, Andand partcopy is going to expect that (200 in Hex = 512 in Decimal). Change it and you'llpartcopy seewill Partcopylikely chokefail.
 
Often,There youis will seeoften a so-called boot signature (0xAA55) at the end. Older versions of [[BIOS|some BIOSes]] looked for this in order to identify a boot sector on a disk. It is evidentiallyevidently 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):
<syntaxhighlight lang="asm">
<pre>
; boot.asm
hang:
Line 32:
db 0x55
db 0xAA
</syntaxhighlight>
</pre>
 
But the thing I'd really like to point out is how onceOnce you've booted, and the cursor is happily blinking on a blank screen, you might notice two things. One is that the floppydisk's motor will turn off and the other is that you can now press Ctrl-Alt-Del to reboot. The pointThis is thatbecause [[interrupts]] (such as INT 0x09) asare still being generated.
 
For kicks tryTry clearing the interrupts flag:
<syntaxhighlight lang="asm">
<pre>
;boot.asm
cli
Line 46:
db 0x55
db 0xAA
</syntaxhighlight>
</pre>
You may notice that the floppy 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 following code is the smallest possible example of booting code from a floppy. It is assembled in [[NASM]] and copied to floppy using(outdated), disk or eitherUSB-sticks using partcopy, dd, or dddebug. Then you simply boot from thethat floppydisk.
 
For a more detailed description, see the [[Bootable Disk]] page.
 
▲! ===Windows/DOS===
<syntaxhighlight lang="bash">
▲ > nasmw boot.asm -f bin -o boot.bin
▲ > partcopy boot.bin 0 200 -f0
OR
debug boot.bin
-W 100 0 0 1
-Q</syntaxhighlight>
===Unix===
<syntaxhighlight lang="bash">
▲ $ nasm boot.asm -f bin -o boot.bin
▲ $ dd if=boot.bin of=/dev/fd0
</syntaxhighlight>
To write to a hard drive or USB-stick, use
<syntaxhighlight lang="bash">
nasm boot.asm -f bin -o boot.bin
dd if=boot.bin of=/dev/sda
</syntaxhighlight>
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 "'''f'''da").
 
<syntaxhighlight lang="bash">
qemu-system-i386 -fda boot.bin
</syntaxhighlight>
 
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 "'''h'''da").
 
<syntaxhighlight lang="bash">
qemu-system-i386 -hda boot.bin
</syntaxhighlight>
 
Use the QEMU monitor command to send Ctrl-Alt-Del to the VM:
If you try to reduce this even more by removing the loop and merely pad out the sector with zeros, 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 zeros except for adding a boot signature.
<syntaxhighlight lang="bash">
sendkey ctrl-alt-delete
</syntaxhighlight>
 
Because of how fast emulation has become, you might need to slow down emulation speed to 1% to notice the reboots.
Not exactly something you would show your girlfriend, 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.
 
==== References ====
* Instruction Set from the horse's mouth: [http://www.intel.com/design/pentiumII/documentation.htm]
* Easier to read: [http://www.baldwin.cx/386htm/toc.htm]
Line 60 ⟶ 103:
* Interrupts by number: [http://www.osdever.net/downloads.php]
* Randall Hyde's look into the bowels of the PC: [http://webster.cs.ucr.edu/]
* QEMU [https://www.qemu.org]
 
[[Category:Babystep]]
[[Category:Bootloaders]]
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