Writing OS-Level Code in Assembly Language
for the Intel ix86 / IBM PC computer

Edited by Don Stoner, Revision 2014/03/23 (Always Under Construction)

The author cannot be held responsible for any property damage, loss of data, or loss of health or life that might result from using this information. I'm still trying to sort this stuff out myself.

Since so few people write operating systems, it can be relatively difficult to locate good documentation on how to interface to all of the necessary parts of the computer. These links are intended to be a step toward fixing that problem. Similar attempts to document this information (which you should also check out) can be found: Here, here, here, and here, you can download a simple open-source assembly-language operating system (described as a "learning tool").

What You Will Need:

To write and test OS-level code you will need to have complete control of the computer. This means you can't work under LINUX, Windows, or any other 32-bit, protected- mode operating system. Plain old fashioned 16-bit real-mode DOS turns out to be the right operating system to use. You can use MS DOS if you like it and own a copy, or you can use FreeDOS if you want a newer version of DOS which is both good and free. A FreeDOS boot CD burnable-image can be downloaded off the net here.

You will also need an editor and an assembler which run under DOS. The Tiny Editor (TED) is a free text editor for DOS. You can dowload a copy of it here. The Netwide Assembler (NASM) is a free ix86 assembler for DOS. You can download a copy of it here.

Since you will probably be crashing your system a few times, you will want to use a dedicated computer. You can usually find low-cost and expendable (but still quite good) used computers here (their "more info" link really means "more types of used computers"). You will also want to keep everything you write backed up. Plan on trashing your HDD a few times when you start doing sector writes. (Completely debug all of the reads before you try.) When you start trying to switch to 32-bit protected mode, plan on triple-fault crashing the machine more times than you will want to count.

Setting up a DOS computer:

Warning: Here we are assuming that there is absolutely no information, that you will want to keep, on the targeted hard disk drive; once you have performed the following series of steps, there will certainly be nothing left:

To set up a hard disk to run MS DOS (typically 6.22) starting from a blank disk (or recovering from a complete crash), some or all of the following steps may be necessary:
1) Boot the computer from a bootable DOS floppy (which contains the FDISK and FORMAT programs).
2) Run "FDISK /MBR" (details here). This is only necessary if the Master Boot Record somehow gets messed up; otherwise you can skip this step. The "FDISK /MBR" function is "undocumented" because it isn't needed except by those of us who invalidate warranties as a hobby.
3) Run "FDISK". This step isn't needed as long as you are still able to format your C: partition with the Format command (below). FDISK is a simple menu-driven program which will set up normal DOS partitions for you. (Each partition acts like a different drive, with its own directory etc.) First, remove all existing partitions, one at a time; Then create a single "active" DOS partition (using as much space as is available - up to the 2-gig limit imposed by DOS). You can create more partitions if you plan to make use of more than 2 gigs of space on the disk.
4) Run "FORMAT C:/S" This destroys everything on the active partition (the C: drive). It also formats that partition with blank directories and blank File Allocation Tables (FATS). Then the "/S" option makes it install the DOS operating system (I know: that means "Disk Operating System operating system") so the disk will boot up with DOS. This step isn't needed as long as DOS still boots and runs correctly. You will probably also want to copy your AUTOEXEC.BAT and CONFIG.SYS files from the root directory of the boot floppy to the root directory of your newly created C: drive. (Or you might want to design your own files or edit the old ones to suit the intended use of this new computer; it is a good idea to keep memory-resident applications to a bare minimum, since the point of this exercise is to control the computer's resources yourself.)
5) Finally, make a DOS directory (use "MD DOS"), and copy all of your DOS files over to that directory (using "COPY A:*.* C:\DOS\"). You will probably also want to add your selected text editor and the NASM assembler to this directory. Most entry-level DOS reference books (e.g. DOS for Dummies) will explain these (and other) commands well enough to get you up to speed quickly.

Learning Assembly Language:

And here, I am attempting to put together a sequence of very short assembly-language programs which you can study. Unit1 is the source code for the simplest assembly-language program I could imagine. Subsequent units build in complexity in very small steps. I hope this will help get you started: Unit1, Unit2, Unit3, Unit4, Unit5, Unit6, Unit7, Unit8, Unit9, Unit10, Unit11, Unit12, Unit13, Unit14, Unit15, Unit16, Unit17, Unit18, Unit19, Unit20, Unit21.