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.
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 dxNow 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.
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 <firstname.lastname@example.org> 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.
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.