By: the@womble.co.uk (Questions welcome. Spam not)
More at: http://www.groovyweb.cjb.net, http://www.axion-network.net

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:
push cx pop 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).
; This is a comment
[ORG 0x7C00] ;This just tells the program where it is in the memory. Not important
[BITS 16] ;Not important too.

jmp start ; Jump over BIOS parameter block

start: ;The label for the start of the actual program

push cs ;Put cs onto the stack
pop ds ;Take it out and put it into ds

mov si,Print_loading ;Print loading message
call printstring ;Call is like jump, but it goes back. Like a function

;The complicated bit: Loads the next program
mov ah,02h ;When ah=, int13 reads a disk sector
mov al,4 ;Al is how many sectors to read
mov ch,0 ;The track to read from
mov cl,2 ;Sector Id
mov dh,0 ;Head
mov dl,0 ;Drive (0 is floppy)
mov bx,0x1000 ;Es and Bx put together are where to load the program too (see jmp 0x1000:0x00)
mov es,bx
mov bx,0x00
int 13h ;Int 13 is all functions for disks

mov si,putdot ;Print a ".".
call printstring

jmp 0x1000:0x00 ;Run Bootinit from stack.

printstring: ;Print string routine.
mov ah,0eh ;Mov ah into 0, so int 10 prints
stringloop: ;The following code loads each seperate charcter so it can be printed
lodsb
cmp al,00 ;If al =0, then the string has all been loaded
je endstring
int 10h ;When int 10 is called, and ah=, it prints
jmp stringloop
endstring:
ret ;Ret returns

putdot db '.',0
Print_loading db 13,10,'Loading Easy OS v0.01a...',0
times 425 db 0 ;wastes 425 bytes on purpose, so the sector is full (The program must be 512 bytes long)

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.
; NYAOS Boot Sector (C) Copyright Sean Tash 1998
; assemble with:
; nasm -f bin -o bootsect.bin bootsect.asm
bits 16
org 0x7C00

start: jmp short begin
nop
bsOEM db "NYAOS1.0" ; OEM String
bsSectSize dw 512 ; Bytes per sector
bsClustSize db 1 ; Sectors per cluster
bsRessect dw 1 ; # of reserved sectors
bsFatCnt db 2 ; # of fat copies
bsRootSize dw 224 ; size of root directory
bsTotalSect dw 2880 ; total # of sectors if < 32 meg
bsMedia db 0xF0 ; Media Descriptor
bsFatSize dw 9 ; Size of each FAT
bsTrackSect dw 18 ; Sectors per track
bsHeadCnt dw 2 ; number of read-write heads
bsHidenSect dd 0 ; number of hidden sectors
bsHugeSect dd 0 ; if bsTotalSect is 0 this value is
; the number of sectors
bsBootDrv db 0 ; holds drive that the bs came from
bsReserv db 0 ; not used for anything
bsBootSign db 29h ; boot signature 29h
bsVolID dd 0 ; Disk volume ID also used for temp
; sector # / # sectors to load
bsVoLabel db "NO NAME " ; Volume Label
bsFSType db "FAT12 " ; File System type

begin: cli ; disable interrupts
mov [bsBootDrv],dl ; save drive number
mov ax,0x9000 ; put stack at 0x98000
mov ss,ax
mov sp,0x8000

mov cx,[bsTrackSect] ; update int 1E FDC param table
mov bx,0x0078
lds si,[ds:bx]
mov byte [si+4], cl
mov byte [si+9], 0x0F

sti ; enable interrupts
push ds
mov dl,[bsBootDrv] ; reset controller
xor ax,ax
int 0x13
pop ds
jc bootfail2 ; display error message
jmp _l1
bootfail2: jmp bootfail
_l1:
mov ax,0x0000
mov es,ax
mov ds,ax

mov si,MsgLoad ; display load message
call putstr

; find the root directory

xor ax,ax
mov al,[bsFatCnt]
mov bx,[bsFatSize]
mul bx
add ax,word [bsHidenSect]
adc ax,word [bsHidenSect+2]
add ax,word [bsRessect] ; ax holds root directory location
mov word [BootSig],ax

call checkroot

xor ax,ax
add ax,word [start]
add ax,word [bsVolID] ; sector number
add ax,word [BootSig]
sub ax,2 ; correction for a mis-calc
mov cx,word [bsVolID+2] ; number of sectors

mov bx,0x8000
mov es,bx

nextsector: push ax ; save registers
push cx
push dx
push es

xor bx,bx ; set zero offset
call readsect ; read a sector

mov si,MsgDot ; display a dot
call putstr

pop es ; restore registers
pop dx
pop cx
pop ax
mov bx,es
add bx,20h ; increment address 512 bytes
mov es,bx
inc ax ; read next sector
loopnz nextsector

mov ax,0x8000 ; set segment registers and jump
mov es,ax
mov ds,ax
push ax
mov ax,0
push ax
retf

checkroot:
push ax ; save registers
push bx
push cx
push dx
push si
push di

mov ax,0x8000 ; put root directory at 0x80000
mov es,ax
mov ax,32 ; AX = ((32*RootSize)/512) + 2
mul word [bsRootSize]
div word [bsSectSize]
mov cx,ax ; cx holds # of sectors in root
mov word [start],ax
mov ax,word [BootSig] ; get prev. saved loc. for root dir

r1: xor bx,bx
push cx ; save count
push ax ; save sector number
push es
push dx
call readsect
xor bx,bx
l_1: mov di,bx ; set address to check from
mov cx,11 ; check 11 bytes
mov si,FileName ; address of string to check with
repz cmpsb
je foundit
add bx,32 ; check next entry
cmp bx,[bsSectSize] ; end of sector?
je l_2
jmp l_1
l_2: pop dx ; restore registers
pop es
pop ax
pop cx
inc ax ; read next sector
loopnz r1
jmp bootfail
foundit: pop dx ; get these off the stack
pop es
pop ax
pop cx

mov di,0x1A ; get clustor #
add di,bx
push bx ; save bx for finding # of sectors
mov ax,[es:di]
xor bx,bx ; calculate sector #
mov bl,[bsClustSize]
mul bx ; ax holds sector #
mov word [bsVolID],ax

pop bx ; get location of directory entry
mov di,0x1C
add di,bx
mov ax,[es:di] ; put number of bytes in ax
xor dx,dx
mov bx,[bsClustSize] ; # of bytes / 512
div bx
inc ax
mov word [bsVolID+2],ax ; save number of sectors to load

pop di ; restore registers
pop si
pop dx
pop cx
pop bx
pop ax

ret ; return to caller

putstr: ; SI = address of string to display
lodsb
or al,al
jz short putstrd
mov ah,0x0E
mov bx,0x0007
int 0x10
jmp putstr
putstrd: retn ; return to caller

bootfail: ; display failure message
mov si,MsgBad ; display error message
call putstr
xor ax,ax ; wait for keypress
int 0x16
int 0x19 ; reboot

readsect: ; ES:BX = Location ; AX = Sector
mov si,[bsTrackSect]
div si ; divide logical sect by track size
inc dl ; sector # begins at 1
mov [bsReserv],dl ; sector to read
xor dx,dx ; logical track left in ax
div word [bsHeadCnt] ; leaves head in dl, cyl in ax
mov dh, [bsBootDrv] ;
xchg dl,dh ; head to dh, drive to dl
mov cx,ax ; cyl to cx
xchg cl,ch ; low 8 bits of cyl to ch, hi 2 bits
shl cl,6 ; shifted to bits 6 and 7
or cl, byte [bsReserv] ; or with sector number
mov al,1 ; number of sectors
mov ah,2 ; use read function of int 0x13
int 0x13 ; read sector
jc bootfail ; display error message
ret ; return to caller

padding times 45 db 0
FileName db "OSLOADERCOM"
MsgBad db "Disk Error...",13,10,0
MsgDot db ".",0
MsgLoad db "doors loading",0
BootSig db 0x55, 0xAA

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:
;Funky squares
;
;Assembles with NASM
;Made by Frej somewhere between april and may 2000
;Bits reprogrammed to run within DeviatorOS
;
;This demo is just to show you how small graphical demos can get ;)
;Reprogrammed for DeviatorOS as demo program

org 0x0000
mov ax,cs
mov ds,ax
mov es,ax ; fix segment regs

start: mov bx,cs ;put codesegment to bx
add bh,0x20 ;add 2000 to bx
mov ds,bx ;and put it to ds
mov ax,0x13 ;set ax to videomode 13
int 10h ;and do that
Main: push ds ;put buffer seg to stack
pop es ;and put that into es
in ax,0x40 ;generate "random" number (timer)
shl ax,4 ;multiply random # with 16
mov di,ax ;box offset (random)
mov al,255 ;color of the box
mov bx,50 ;height=50
pl: add di,270 ;di+270 (320-width(50))
mov cx,50 ;# bytes to copy to buffer
rep stosb ;and do it
dec bx ;decrement bx
jnz pl ;jump if bx not zero
mov bh,0xFA ;assume bl = 0 (-> bx = FA00)
Smudge: mov al,[bx+1] ;right color to al
mov cl,[bx-1] ;left color to cl
add ax,cx ;and add it to ax
mov cl,[bx-320] ;upper color to cl
add ax,cx ;and add it to ax
mov cl,[bx+320] ;lower color to cl
add ax,cx ;and add it to ax
shr ax,2 ;divide with 4
mov [bx],al ;and but the avarage color to buffer
dec bx ;decrement bx
jnz Smudge ;jump if bx not zero
mov ax,0xA000 ;vga seg
mov es,ax ;put it to es
mov ch,0xFA ;# bytes to copy to vga
xor di,di ;zero vga offset
xor si,si ;zero buffer offset
rep movsb ;and do that
in al,0x60 ;check for keys
dec al ;was it esc?
jnz Main ;nope, continue
mov ax,3 ;text mode
int 10h ;get back into text mode
xor ah,ah ;yes, return to OS
int 0x18 ;back to good old kernel
;Note:- This was, as you can guess, ripped by from an os. So when it goes back to the 'good ol kernel' it just restarts.

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.

By thewomble
Thanks to www.axion-network.net