Writing your own operating system
How to write an operating system
Writing an operating system is something that can not only be interesting (if you're one of those people that get turned on by Int 13....) but it is also a great learning experience. Through creating your own operating system you will learn exactly what goes on behind the scenes, elevating you above the average programmer that just writes in Visual Basic.
In this tutorial you will be tought by examples, and by the end you should have created your own operating system.
Tools:
EasyOs easyos.zip 300kb
EasyOS is a very simple operating system, it contains all the tools needed to build an operating system. (Not written by me, although I did add bits to it and mess it up a bit)
A quick explanation of assembly: (see
here for a good tutorial)
si and ah Think of si as something to put text into, and ah as something to put numbers into.
mov This (mov)es data. mov ah,0 would move 0 into ah. (The data on the right is moved into the left)
Int Think of these as functions. Different int's do different things when ah is different. Ahem. Eg. when ah = 0 and you call int 10 it prints si to the screen.
Stuff To put words and stuff in your program you can't just do mov si,'some words' (well, you can but you wont like the resutls) so instead you have to declare the words first. You do this by putting the name of what you want the words to be called by, then the data type (nearly always db) then the words themselves. Eg:
name db 'some words'
Jump To give sections of code a label, just type the label and add a : at the end, eg code: . You can then use jmp to jump to it, eg jmp code
If To do an if in assembly you use cmp, eg cmp al,0 (if al=0). On the next line you then put je code, and if al=0 then the program jumps to the section of code called code. If you use jne code, then if al is not 0 the program will jump to code.
The stack The stack is where stuff is stored. push pushes stuff into it, pop pulls stuff out. The following example would put cx into dx:
Now you know everything there is to know about assembly, you can now understand most of the program that boot's EasyOs. Drives (hard drives and floppy's) are split into lots of bits, all 512 bytes long (enough to fit 512 letters in). These are calle sectors. The first sector is what the computer looks for when it boots. It is called the bootsector.
Open the folder src and then open boot.asm. Or if you are lazy, just look at the code below (its the same).
You may have noticed that numbers, like int 10h, end in h. They don't have to, it just looks funky (there is a real reason, but it's boring).
Anyway, now copy the program called copyboot in the folder called utils to the folder with test.asm in. Open the dos prompt and type: (Make sure a blank floppy is inserted)
copyboot test.com 0
copyboot is the name of the program, test.com the name of the file to copy, and 0 is the sector.
In the program above all it does is print a string then load whats at sector 1. The program that easyos loads under the src folder called bootinit. If you assemble it with nasm, then copy it to sector 1 and restart, the bootsector will load it.
There isn't much more to be learnt from EasyOs, so run either setup.exe or make.bat to build the whole thing. The difference is setup.exe lets you setup a root password for EasyOs. If you just run make.bat the passwords is the default password: monty (named after my psychotic dog).
Now restart and be amazed. Wow. Pretty crappy, but it isn't that bad.
Fat 12
No, its not a fat people supprt group but a file system. If you try and access the floppy disk Windows will say it need s to be formatted. Formatted? You ask. Formatting is basically organising the sectors so you can give them names. Underneath Windows and Fat, you are still just accessing sectors. I won't go into Fat here, check out
http://www.maverick.subnet.dk/ for some info. Anyway, the bootsector needs to have some information in it that when Windows reads it it tells it it is fat. The following BootSector was disgustingly ripped by me of NYAOS (I have no idea what i stands for). With your A* qualification in assembly language you can no doubt understand it.
Anyways, copy the above program, save it, build it with nasm, copy it with copyboot.
As you can guess above, it loads a program called 'OSLOADER.COM' off of the floppy. So, if you want a particularily funky os, build the following with nasm:
Build it with nasm, but rather than faffing around copyboot, just name is as OSLOADER.COM and copy it to the floppy.
Now restart and enjoy the funkiness. Woah dood.
C
Time to escape assembly language. The boot sector has to be written in assembly, but nothing else does. Unfortunately you can't just go and write a cool shell with Visual C++. First of all, its has to be a .com program, not .exe.
.exe programs are just .com with a bit of extra info. at the start giving some info on what the program is. It's very easy to add .exe capabililty to an os, or you can download a program called exe2com.
The serious problem though is that you can create your own ints to make things easier. EasyOs does this (look in kernel.asm under the src folder) and Dos does this too (Dos makes int 21). By default, compilers build a program with these ints.
For linux lovers
The following code is for Gcc on linux and nasm for dos or linux. Dos/ Windows users can download Djgpp from http://www.delorie.com/djgpp/, which is like gcc. The line
gcc -c -O2 -Wall -g -o hello.o hello.c
Tells gcc to build a plain binary. I don't know who wrote the following, e-mail me if you do.
The smallest pmode + C program I could write is shown below, in three files. I compiled with GCC, not GPP, and got no warnings.
This code doesn't check for a 32-bit CPU or V86 mode. If you try to run it inside a DOS box, Windows will kill it.
load.asm is assembled to aout format instead of COFF, because DJGPP COFF doesn't let you mix 16- and 32-bit code.
More on this topic at: http://www.groovyweb.cjb.net, http://www.axion-network.net