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Posted 6 Oct 2006

Building your own operating system

, 11 Oct 2006 CPOL
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Writing your own operating system.


If you know how an operating system works, it will help you a lot in programming, especially for system programs like device drivers; even for non-system-programming, it can help a lot. And also, I'm sure every one would like to have their own operating system.

You would also learn from this article how to read and write raw sectors from a disk.


In this article, I would explain the first part of building an operating system.

Here is what happens when you start your computer:

  1. The BIOS (Basic Input Output System – this is a program that comes with any mother board and it is placed in a chip on the mother board) checks all the computer components to make sure that they are all working.
  2. If all the components are working, the BIOS starts searching for a drive that might have an operating system. (The BIOS can look in hard drives, floppy drives, CD-ROM drives etc. The order the BIOS checks can be set it in the BIOS setup program. To get to the BIOS setup, right when the computer turns on, press the DELETE key until you see the BIOS setup program; in some computers, it can be a different button than DELETE, so look in your mother board specification to find how to get to the BIOS setup program. And also look up (if you don’t now how to do it) how to change the search search of BIOS while looking for an operating system.)
  3. The BIOS checks the first drive to see if he has a valid BOOT sector. (A disk is divided into little regions that are named sectors. The BOOT sector size is 512 bytes in most drives.) If the drive has a valid BOOT sector, the BIOS loads that sector in to the memory at address 0:7c00 (=31,744) and gives control to that area of the memory.
  4. Now this little program that was loaded from the BOOT sector continues to load the operating system, and after the operating system is loaded, it initializes the operating system and gives the control to the operating system.

Making a bootable disk

The steps would be like this:

  1. Take a floppy diskette that you don't need.
  2. Use the program that comes with this article, BOOTSectorUtility.exe, to copy the file BOOT.bin to the floppy diskette BOOT sector.
  3. Make sure that your BIOS is set to BOOT from the floppy drive first, so that our operating system would be loaded.
  4. Restart your computer (make sure that the floppy diskette is in the drive), and watch what our BOOT sector does.

With BOOTSectorUtility.exe, you can also search your regular operating system, by saving the boot sector of the drive of your operating system to a file.

And to read that file, start the command line and type Debug <file path> (if you have a Microsoft operating system).

The Debug command starts a 16 bit debugger (any boot sector is in 16 bit code, because when the computer starts, it is in 16 bit mode, and only after the boot sector is run, can it change the CPU to 32 bit mode or 64 bit mode). Press u <Enter> (u = unassemble) to unassemble the file. Figure 1 shows an example.

Figure 1 Unassembeling 16 bit code.

Reading raw bytes from a drive

You can read raw sectors from a drive like this:

// Reading/writing raw sectors.
//pBuffer has to be at least 512 bytes wide.
BOOL ReadSector(char chDriveName,char *pBuffer,DWORD nSector)
    char Buffer[256];
    HANDLE hDevice;
    DWORD dwBytesReaden;

    //Init the drive name (as a Driver name). 

    hDevice = 
      CreateFile(Buffer,                  // drive to open.
                FILE_SHARE_READ | // share mode.
                NULL,             // default security attributes.
                OPEN_EXISTING,    // disposition.
                0,                // file attributes.
                NULL);            // 

    if(hDrive==INVALID_HANDLE_VALUE)//if Error Openning a drive.
           return FALSE;
    //Move the read pointer to the right sector.
           return FALSE;

    //Read the Sector.

    //if Error reading the sector.
           return FALSE;

    return TRUE;

Making a BOOT program

Now, I will explain the basics of a boot program (to understand this, you need to be familiar with Assembler and Interrupts and Interrupts vector table).

[Interrupts vector table = From Address 0 -> 1,024 holds 256 structures (4 bytes in size) that holds an address in the form: CS:IP = xxxx:xxxx. So you have addresses from INT 1 -> INT 256. Each interrupt has an index into that table. This table is used like this: if you, for example, use the instruction INT 10h, the CPU checks in index 10h in the interrupt table were the address of the routine is that handles INT 10h, and the CPU jumps to that address to execute it].   

Now, I will explain how to print a string, read sectors, and wait for a key press using only the BIOS.

To print a string, I use the INT 10h function 0Ah (AH = 0Ah). To move the cursor, I use the INT 10h function 2h (AH = 2h). To read sectors, I use the INT 13h function 2h (AH = 2h). To wait for a key stroke, use the INT 16h function 0h (AH = 0h).

I used TASM v3.1 and TLINK to make my boot sector, but you can use any x86 16 bit assembler compiler. 

(If you can't get a copy of TASM and TLINK, you can send me an e-mail and if it is legal, I would send you a copy of them).

(TASM and TLINK v3.1 were released in 1992 by Borland International).

Now I would explain the steps the BOOT program does.

  1. Make a stack frame (if not you don't have any stack).
  2. Set the DS (data segment) so you can access the data.
  3. In my boot sector, I added this: (Display a message to the user, Wait for a key stroke, Continue).
  4. Set up the Disk Parameter Block (the Disk Parameter Block is a structure that holds information about the drive, like how much sectors it has etc., the drive controller uses it for knowing how to read the disk in the drive).
  5. To set the Disk Parameter Block, get its address (its address is pointed by INT 1Eh (in the memory, this is 1E x 4 bytes = 78h = 30 x 4 bytes = 120).

    This is an example of how to initialize the Disk Parameter Block for a <!--?xml:namespace prefix = st1 /--><st1:metricconverter productid="3.5 inch" w:st="on">3.5 inch (1.44 MB) floppy Disk.

    StepRateAndHeadUnloadTime                     db      0DFh
    HeadLoadTimeAndDMAModeFlag                    db      2h
    DelayForMotorTurnOff                          db      25h
    BytesPerSector                                db      2h              
    SectorsPerTrack                               db      12h            
    IntersectorGapLength                          db      1bh
    DataLength                                    db      0FFh
    IntersectorGapLengthDuringFormat              db      54h
    FormatByteValue                               db      0F6h
    HeadSettlingTime                              db      0Fh
    DelayUntilMotorAtNormalSpeed                  db      8h
    DisketteSectorAddress(as LBA)OfTheDataArea    db      0
    CylinderNumberToReadFrom                      db      0
    SectorNumberToReadFrom                        db      0
    DisketteSectorAddress (as LBA) OfTheRootDirectory    db      0
  6. And set the address that INT 1E points at to the address of the Disk Parameter Block you set up.
  7. Reset the drive (by using the INT 13h function 0).
  8. Start reading the diskette by using the INT 13h function 2h.
  9. Give control to the loaded operating system (this is done by inserting in the code the op code of a jump to were you loaded the operating system. Let's say you loaded the operating system 512 bytes from were the BIOS loaded the BOOT sector (0:7c00h).
    db 0E9h  ; FAR JMP op code.
    db 512   ; 512 bytes

    or you can use another way: call some address, and there, change the return address in the stack to were you want the jump to, like this:

    call GiveControlToOS
            Pop ax
            Pop ax
            Mov ax,CodeSegement            ;Push the new CS to return.
            Push ax
            mov ax,InstructionPointer      ;Push the new IP to return.
            Push ax
            ret                            ;Return to the modified address.
  10. In the end of the boot sector the bytes would be 0x55h 0x0AAh. If the boot sector doesn’t have this value, the BIOS would not load that boot sector.

After this, the BOOT sector finishes its job and the operating system starts running.

You can download the files I used for my boot sector here, the files are BOOT.asm, BOOT.bin (this is the sector itself).

The BOOT sector program



ORG 7c00h      ;Because BIOS loades the OS at 
                       ; address 0:7C00h so ORG 7C00h 
                       ; makes that the refrence to date 
                       ; are with the right offset (7c00h).

   ; CS = 0 / IP = 7C00h // SS = ? / SP = ?
   ; You are now at address 7c00.
jmp start   ;Here we start the, BIOS gave us now the control.

;//Here goes all the data of the program.

xCursor db 0
yCursor db 0

nSector db 0
nTrack  db 0
nSide   db 0
nDrive  db 0

nTrays  db 0

'Are You Ready to start Loading the OS...',0
szReady                db
'Error Reading Drive, Press any Key to reboot...',0
szErrorReadingDrive    db
;//Done Reading a track.
szPlaceMarker          db  '~~~~',0
szDone                 db  'Done',0

pOS                    dw   7E00h
;//Points to were to download the Operating System. 

;//Disk Paremeter Table.
StepRateAndHeadUnloadTime                     db      0DFh
HeadLoadTimeAndDMAModeFlag                    db      2h
DelayForMotorTurnOff                          db      25h
;// (1 = 256) //(2 = 512 bytes)
BytesPerSector                                db      2h
;// 18 sectors in a track.
SectorsPerTrack                               db      18
IntersectorGapLength                          db      1Bh
DataLength                                    db      0FFh
IntersectorGapLengthDuringFormat              db      54h
FormatByteValue                               db      0F6h
HeadSettlingTime                              db      0Fh
DelayUntilMotorAtNormalSpeed                  db      8h

DisketteSectorAddress_as_LBA_OfTheDataArea    db      0
CylinderNumberToReadFrom                      db      0
SectorNumberToReadFrom                        db      0
DisketteSectorAddress_as_LBA_OfTheRootDirectory      db      0

;//Here the program starts.


CLI     ;Clear Interupt Flag so while setting 
        ;up the stack any intrupt would not be fired.

        mov AX,7B0h    ;lets have the stack start at 7c00h-256 = 7B00h
        mov SS,ax      ;SS:SP = 7B0h:256 = 7B00h:256
        mov SP,256     ;Lets make the stack 256 bytes.

        Mov ax,CS      ;Set the data segment = CS = 0 
        mov DS,ax
        XOR AX,AX      ;Makes AX=0.
        MOV ES,AX      ;Make ES=0

STI     ;Set Back the Interupt Flag after 
        ;we finished setting a stack fram.
        Call ClearScreen       ;ClearScreen()
        LEA AX,szReady         ;Get Address of szReady.
        CALL PrintMessage      ;Call PrintfMessage()
        CALL GetKey                    ;Call GetKey()
        CALL SetNewDisketteParameterTable
        CALL DownloadOS
        CALL GetKey                    ;Call GetKey()
        CALL FAR PTR  GiveControlToOS  ;Give Control To OS.


;//Prints a message to the screen.
PrintMessage PROC

        mov DI,AX      ;AX holds the address of the string to Display.
        Mov xCursor,1  ;Column.

        cmp byte ptr [DI],0    ;Did we get to the End of String.
        JE EndPrintingMessage  ;if you gat to the end of the string return.
        mov AH,2               ;Move Cursor
        mov DH,yCursor         ;row.
        mov DL,xCursor         ;column.
        mov BH,0               ;page number.
        INT 10h
        INC xCursor
        mov AH,0Ah             ;Display Character Function.
        mov AL,[DI]            ;character to display.
        mov BH,0               ;page number.
        mov CX,1               ;number of times to write character
        INT 10h
        INC DI                 ;Go to next character.
        JMP ContinuPrinting    ;go to Print Next Character.
        Inc yCursor            ;So Next time the message would
                               ;be printed in the second line.
        cmp yCursor,25
        JNE dontMoveCorsurToBegin 
        Mov yCursor,0
PrintMessage EndP 
;//Watis for the user to press a key.

        mov ah,0
        int 16h ;Wait for a key press.
GetKey EndP 
;//Gives Control To Second Part Loader.
GiveControlToOS PROC

        LEA AX,szDone
        Call PrintMessage
        CALL GetKey
        db 0e9h        ;Far JMP op code.
        dw 512         ;JMP 512 bytes ahead.
;       POP AX         ;//Another why to make 
                       ;the CPU jump to a new place.
;       POP AX
;       Push 7E0h      ;Push New CS address.
;       Push 0          ;Push New IP address.
               ;The address that comes out is 7E00:0000. 
               ;(512 bytes Higher from were BIOS Put us.)
;       ret
GiveControlToOS EndP 
;//Clear Screen.
ClearScreen PROC

        mov ax,0600h   ;//Scroll All Screen UP to Clear Screen.
        mov bh,07
        mov cx,0
        mov dx,184fh   
        int 10h
        Mov xCursor,0  ;//Set Corsur Position So next 
                        //write would start in 
                        //the beginning of screen.
        Mov yCursor,0

ClearScreen EndP
PrintPlaceMarker PROC

        LEA AX,szPlaceMarker
        CALL PrintMessage  ;Call PrintfMessage()
        CALL GetKey        ;Call GetKey()
PrintPlaceMarker EndP
;//Set New Disk Parameter Table
SetNewDisketteParameterTable PROC

        LEA DX,StepRateAndHeadUnloadTime
        ;//Get the address of the Disk Parameters Block. 
               ;//Int 1E (that is in address 0:78h) 
               ;//holds the address of the disk parametrs 
               ;//block, so now change it to 
               ;//our parametr black.
        ;//DX holds the address of our Parameters block.
        MOV WORD PTR CS:[0078h],DX
        MOV WORD PTR CS:[007Ah],0000
        ;Reset Drive To Update the DisketteParameterTable.
        MOV AH,0
        INT 13H
SetNewDisketteParameterTable EndP
DownloadOS PROC

        mov nDrive,0
        mov nSide,0
        mov nTrack,0
        mov nSector,1
        INC nSector            ;Read Next Sector.
        cmp nSector,19         ;Did we get to end of track.
        JNE StayInTrack
        CALL PrintPlaceMarker  ;Print now '~~~~' so the user would 
                               ;now that we finished reding a track
        INC nTrack             ;If we gat to end of track Move to next track.
        mov nSector,1          ;And Read Next Sector.
        CMP nTrack,5           ;Read 5 Tracks (Modify this value 
                               ;to how much Tracks you want to read).
        JE      EndDownloadingOS
        Call ReadSector
        JMP     ContinueDownload
        ;If diden't yet finish Loading OS.

DownloadOS EndP 
;//Read Sector.
ReadSector PROC

        mov nTrays,0

        mov AH,2               ;//Read Function.
        mov AL,1               ;//1 Sector.
        mov CH,nTrack
        mov CL,nSector         ;//Remember: Sectors start with 1, not 0.
        mov DH,nSide
        mov DL,nDrive
        Mov BX,pOS             ;//ES:BX points to the address 
                               ;to were to store the sector.
        INT 13h

        CMP AH,0               ;Int 13 return Code is in AH.
        JE EndReadSector       ;if 'Sucsess' (AH = 0) End function.

        mov AH,0               ;Else Reset Drive . And Try Again...
        INT 13h
        cmp nTrays,3           ;Chack if you tryed reading 
                               ;more then 3 times.
        JE DisplayError        ; if tryed 3 Times Display Error.
        INC nTrays
        jmp TryAgain       ;Try Reading again.
        LEA AX,szErrorReadingDrive
        Call PrintMessage
        Call GetKey
        mov AH,0                       ;Reboot Computer.
        INT 19h

        ;ADD WORD PTR pOS,512  ;//Move the pointer 
                               ;(ES:BX = ES:pOS = 0:pOS) 512 bytes.
                               ;//Here you set the varible 
                               ;pOS (pOS points to were BIOS 
                               ;//Would load the Next Sector).
ReadSector EndP 
END ProgramStart

Points of interest

It took some time until I got a running boot sector. I would list a couple of bugs I had in the beginning while writing my boot sector.

  1. I didn’t set up a right stack frame.
  2. I didn’t modify the Disk Parameter Block.
  3. I loaded the operating system to areas that are used by BIOS routines (and even to the Interpret table).
  4. In the end of the boot sector, it must have the bytes 0x55h 0x0AAh (this is a signature that it is a valid bootable sector).

    (For using BOOTSectorUtility.exe, you would need the .NET 2.0 Framework installed on your computer. To download the .NET 2.0 Framework, click here.)

If I see that this article interests people, I would write some more articles on this subject.




About the Author

S Keller
Systems Engineer
United States United States
No Biography provided

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QuestionHelp Pin
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GeneralMy vote of 5 Pin
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QuestionIt will be great if you can provide the source code as a VS project? Pin
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QuestionCan I boot this on a virtual pc? Pin
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AnswerRe: Can I boot this on a virtual pc? Pin
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Questioncan you help me ? Pin
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QuestionBooting USB Pin
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AnswerRe: Booting USB Pin
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Questionhow to convert boot.asm to boot.bin Pin
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AnswerRe: how to convert boot.asm to boot.bin Pin
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GeneralRe: how to convert boot.asm to boot.bin Pin
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Questionunable to run BOOTSectorUtility.exe Pin
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memberMember 938153325-Aug-12 2:04 
AnswerRe: unable to run BOOTSectorUtility.exe Pin
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GeneralMy vote of 4 Pin
VivekTomarDP9-Jul-12 1:22
memberVivekTomarDP9-Jul-12 1:22 
nice but hard to understand what exactly going on.
QuestionBOOT/os/windows/mac/mike os/lwig os/HDD/ Pin
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SuggestionTo make Os Pin
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QuestionBootSectorUtility.exe problem Pin
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QuestionReading and Writing to/from files on disk without BIOS interrupts. Pin
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