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PC Bootsector Programming Tutorial in ASM

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A word of caution, tampering with the bootsector of a computer can render the machine inoperable. It is advisable to experiment using floppy disks or non-critical hard drives prior to transferring your bootsector onto a live system. The following tutorial will boot any x86 class PC with a floppy disk drive. USB floppies in legacy mode and El Torrito CDROM images will likely boot successfully as well.


There are many reasons for writing a custom bootstrap program. Most prominent is to achieve an increased understanding of just how a computer operates in its rawest form. Programmers that desire to write their own operating systems will generally utilize custom bootstrap code to load and initialize their system. Utilities that allow users to select different operating systems to run at boot time require a fundamental knowledge of the computer boot sequence. Data recovery services performing digital forensics must understand different bootsector formats in order to restore and read lost data. All of these reasons boil back to understanding the first step a computer makes when powering up - the bootstrap.

The bootstrap is a short program loaded by the BIOS (Basic Input Output System) upon system startup. The BIOS has no information about the environment required by the operating system and therefore can do nothing to initialize the system beyond putting the hardware into a known state. This is where the bootstrap program comes into play. The BIOS loads the bootstrap from a known location and transfers control. Operating system specific bootstraps either load the operating system itself or perform a multi-stage boot by loading a more advanced initialization program. It is the bootstrap's responsibility to load code and build an appropriate operating environment.

Bootstrap Basics

A bootstrap is loaded from the first sector on a disk, track zero, head zero, sector one. Which disk the bootstrap is loaded from is dependent upon the BIOS configuration saved in NVRAM (NonVolatile RAM). This single 512 byte sector is loaded into memory at physical address 0000:7C00. The BIOS will then examine the final two bytes of the bootstrap (offset 1FEh) for the value AA55h. This flags the bootsector as a valid, bootable disk instead of just storing disk information. A bootstrap must be exactly 512 bytes long because of the two byte check and the one sector limitation. After this verification, the BIOS will jump to 0000:7C00 and turn control over to the bootstrap.

It is important for the programmer to note the processor is operating in 16bit Real Mode when control is transferred. Programming considerations must be made to ensure that segment registers are initialized and that indexed addressing schemes do not violate 64KB boundaries. Bootstraps generally do not perform processor mode changes, but that does not mean switching to 32bit Protected Mode is impossible. Typically, a second stage loader is employed for more exhaustive configurations to the system because the code will no longer be constrained to the 512 byte limitation.

The simplest bootstrap can be written as follows:

      [BITS 16]
      ORG     0
      INT     0x18
      TIMES   510-($-$$) DB 0
      DW      0xAA55

Henceforth, all code examples will be referred to as bootstrap.asm. This example can be compiled using NASM, the Netwide Assembler, available on-line from SourceForge for free. The code must be compiled into plain binary which can be done using NASM for free.


The quickest way to put the binary file onto the disk is to use DEBUG.EXE. The following example demonstrates using debug to write at memory offset 100h one sector starting at sector zero on disk zero.

      -W 100 0 0 1

The command "W" tells debug to write to the disk. It will begin by copying bytes from memory to the designated disk and sector. The final parameter indicates how many sectors to write. The "L" command uses the same parameters but reads from the disk. Together, these can be used to write new bootsectors or examine existing ones. Type "?" to get a listing of more commands while using the debugger. Look at PC Computer Debug Routines for more information on using this program.

Under the various versions of UNIX, the DD command can be used to copy the image onto the bootsector. Type man dd for detailed usage information.

      dd if=BOOTSTRAP.BIN of=/dev/fd0 

Enhancing the Bootstrap

Bootstrap programs are put to a variety of uses. They are capable of initializing certain pieces of hardware, putting the processor into advanced operating modes, or performing a dedicated processing task. Usually, however, bootstrap programs are used to load a larger file into memory with more functionality than can be placed into a 512 byte block. There are two separate techniques for making this happen. The first assumes that the file to be loaded is located immediately after the bootsector on the disk. The bootstrap only needs to know how many sectors to load and can immediately load the appropriate sectors into memory and transfer control to the loaded file. This, however, typically destroys existing file systems on the disk. Despite the differences between the myriad of file systems available, their physical implementations on hard disks share common trends. The first sector, sharing space with the bootstrap code, contains information on where to find additional file system structures. Oftentimes, the additional structures have static locations on the disk. Thus, while placing the files to be loaded immediately after the first sector will make coding the bootsectoor easier, it becomes increasingly difficult to implement a working file system on the disk afterward.

The more advanced solution requires a bootstrap program of greater complexity. A common solution is to write the bootstrap to be compliant with an existing file system. Doing so allows the bootstrap's target files to be copied or edited directly on the disk. A bootstrap must therefore be able to browse the file systems to both determine the presence of and the location of the secondary file.

The FAT12 file system is commonly used on floppy disks. There are two data blocks inherent to FAT12 formatted disks, the OEM_ID string and the BIOS Parameter Block. The OEM_ID string serves no purpose other than to identify what software performed the disk format. The BIOS Parameter Block, referred to in Microsoft documentation as the BPB, is a record containing fields that describe the physical attributes of the disk. These attributes can be used to determine characteristics such as the disk's total capacity. The BIOS immediately begins executing the code loaded at 0000:7C00, which includes these blocks of data. Therefore, the first step for the bootstrap loader is to perform a JMP operation code located below these required data blocks.

Locating the target file requires understanding how the FAT file system works. FAT12 organizes the disk into a sequential array and calls each data cell a cluster. Thus, depending on how the disk was formatted, the number of sectors per cluster will vary. A structure known as the FAT (File Allocation Table) keeps track of each cluster on the disk. The FAT itself contains an integer value indicating the next cluster in the file. By following the chain of indexes until the EOF (End Of File) value is located, it is possible to locate the contents of any file regardless of fragmentation. FAT12 has one additional structure called the Root Directory, responsible for indexing filenames against their first cluster. Putting everything together, a file is located by searching for its name in the root directory and then following a chain of indexes through the FAT to identify the physical disk locations it is saved on.

Source Code

The source code example below demonstrates how to read the root directory to search for the file, how to traverse the FAT to load the file into memory, and how to begin executing the loaded code.

      [BITS 16]
          ORG 0
          jmp     START
     OEM_ID                db "QUASI-OS"
     BytesPerSector        dw 0x0200
     SectorsPerCluster     db 0x01
     ReservedSectors       dw 0x0001
     TotalFATs             db 0x02
     MaxRootEntries        dw 0x00E0
     TotalSectorsSmall     dw 0x0B40
     MediaDescriptor       db 0xF0
     SectorsPerFAT         dw 0x0009
     SectorsPerTrack       dw 0x0012
     NumHeads              dw 0x0002
     HiddenSectors         dd 0x00000000
     TotalSectorsLarge     dd 0x00000000
     DriveNumber           db 0x00
     Flags                 db 0x00
     Signature             db 0x29
     VolumeID              dd 0xFFFFFFFF
     VolumeLabel           db "QUASI  BOOT"
     SystemID              db "FAT12   "
     ; code located at 0000:7C00, adjust segment registers
          mov     ax, 0x07C0
          mov     ds, ax
          mov     es, ax
          mov     fs, ax
          mov     gs, ax
     ; create stack
          mov     ax, 0x0000
          mov     ss, ax
          mov     sp, 0xFFFF
     ; post message
          mov     si, msgLoading
          call    DisplayMessage
     ; compute size of root directory and store in "cx"
          xor     cx, cx
          xor     dx, dx
          mov     ax, 0x0020                          ; 32 byte directory entry
          mul     WORD [MaxRootEntries]               ; total size of directory
          div     WORD [BytesPerSector]               ; sectors used by directory
          xchg    ax, cx
     ; compute location of root directory and store in "ax"
          mov     al, BYTE [TotalFATs]                ; number of FATs
          mul     WORD [SectorsPerFAT]                ; sectors used by FATs
          add     ax, WORD [ReservedSectors]          ; adjust for bootsector
          mov     WORD [datasector], ax               ; base of root directory
          add     WORD [datasector], cx
     ; read root directory into memory (7C00:0200)
          mov     bx, 0x0200                          ; copy root dir above bootcode
          call    ReadSectors
     ; browse root directory for binary image
          mov     cx, WORD [MaxRootEntries]           ; load loop counter
          mov     di, 0x0200                          ; locate first root entry
          push    cx
          mov     cx, 0x000B                          ; eleven character name
          mov     si, ImageName                       ; image name to find
          push    di
     rep  cmpsb                                       ; test for entry match
          pop     di
          je      LOAD_FAT
          pop     cx
          add     di, 0x0020                          ; queue next directory entry
          loop    .LOOP
          jmp     FAILURE
     ; save starting cluster of boot image
          mov     si, msgCRLF
          call    DisplayMessage
          mov     dx, WORD [di + 0x001A]
          mov     WORD [cluster], dx                  ; file's first cluster
     ; compute size of FAT and store in "cx"
          xor     ax, ax
          mov     al, BYTE [TotalFATs]                ; number of FATs
          mul     WORD [SectorsPerFAT]                ; sectors used by FATs
          mov     cx, ax
     ; compute location of FAT and store in "ax"
          mov     ax, WORD [ReservedSectors]          ; adjust for bootsector
     ; read FAT into memory (7C00:0200)
          mov     bx, 0x0200                          ; copy FAT above bootcode
          call    ReadSectors
     ; read image file into memory (0050:0000)
          mov     si, msgCRLF
          call    DisplayMessage
          mov     ax, 0x0050
          mov     es, ax                              ; destination for image
          mov     bx, 0x0000                          ; destination for image
          push    bx
          mov     ax, WORD [cluster]                  ; cluster to read
          pop     bx                                  ; buffer to read into
          call    ClusterLBA                          ; convert cluster to LBA
          xor     cx, cx
          mov     cl, BYTE [SectorsPerCluster]        ; sectors to read
          call    ReadSectors
          push    bx
     ; compute next cluster
          mov     ax, WORD [cluster]                  ; identify current cluster
          mov     cx, ax                              ; copy current cluster
          mov     dx, ax                              ; copy current cluster
          shr     dx, 0x0001                          ; divide by two
          add     cx, dx                              ; sum for (3/2)
          mov     bx, 0x0200                          ; location of FAT in memory
          add     bx, cx                              ; index into FAT
          mov     dx, WORD [bx]                       ; read two bytes from FAT
          test    ax, 0x0001
          jnz     .ODD_CLUSTER
          and     dx, 0000111111111111b               ; take low twelve bits
         jmp     .DONE
          shr     dx, 0x0004                          ; take high twelve bits
          mov     WORD [cluster], dx                  ; store new cluster
          cmp     dx, 0x0FF0                          ; test for end of file
          jb      LOAD_IMAGE
          mov     si, msgCRLF
          call    DisplayMessage
          push    WORD 0x0050
          push    WORD 0x0000
          mov     si, msgFailure
          call    DisplayMessage
          mov     ah, 0x00
          int     0x16                                ; await keypress
          int     0x19                                ; warm boot computer
     ; PROCEDURE DisplayMessage
     ; display ASCIIZ string at "ds:si" via BIOS
          lodsb                                       ; load next character
          or      al, al                              ; test for NUL character
          jz      .DONE
          mov     ah, 0x0E                            ; BIOS teletype
          mov     bh, 0x00                            ; display page 0
          mov     bl, 0x07                            ; text attribute
          int     0x10                                ; invoke BIOS
          jmp     DisplayMessage
     ; PROCEDURE ReadSectors
     ; reads "cx" sectors from disk starting at "ax" into memory location "es:bx"
          mov     di, 0x0005                          ; five retries for error
          push    ax
          push    bx
          push    cx
          call    LBACHS
          mov     ah, 0x02                            ; BIOS read sector
          mov     al, 0x01                            ; read one sector
          mov     ch, BYTE [absoluteTrack]            ; track
          mov     cl, BYTE [absoluteSector]           ; sector
          mov     dh, BYTE [absoluteHead]             ; head
          mov     dl, BYTE [DriveNumber]              ; drive
          int     0x13                                ; invoke BIOS
          jnc     .SUCCESS                            ; test for read error
          xor     ax, ax                              ; BIOS reset disk
          int     0x13                                ; invoke BIOS
          dec     di                                  ; decrement error counter
          pop     cx
          pop     bx
          pop     ax
          jnz     .SECTORLOOP                         ; attempt to read again
          int     0x18
          mov     si, msgProgress
          call    DisplayMessage
          pop     cx
          pop     bx
          pop     ax
          add     bx, WORD [BytesPerSector]           ; queue next buffer
          inc     ax                                  ; queue next sector
          loop    .MAIN                               ; read next sector
     ; PROCEDURE ClusterLBA
     ; convert FAT cluster into LBA addressing scheme
     ; LBA = (cluster - 2) * sectors per cluster
          sub     ax, 0x0002                          ; zero base cluster number
          xor     cx, cx
          mov     cl, BYTE [SectorsPerCluster]        ; convert byte to word
          mul     cx
          add     ax, WORD [datasector]               ; base data sector
     ; convert "ax" LBA addressing scheme to CHS addressing scheme
     ; absolute sector = (logical sector / sectors per track) + 1
     ; absolute head   = (logical sector / sectors per track) MOD number of heads
     ; absolute track  = logical sector / (sectors per track * number of heads)
          xor     dx, dx                              ; prepare dx:ax for operation
          div     WORD [SectorsPerTrack]              ; calculate
          inc     dl                                  ; adjust for sector 0
          mov     BYTE [absoluteSector], dl
          xor     dx, dx                              ; prepare dx:ax for operation
          div     WORD [NumHeads]                     ; calculate
          mov     BYTE [absoluteHead], dl
          mov     BYTE [absoluteTrack], al

     absoluteSector db 0x00
     absoluteHead   db 0x00
     absoluteTrack  db 0x00
     datasector  dw 0x0000
     cluster     dw 0x0000
     ImageName   db "LOADER  BIN"
     msgLoading  db 0x0D, 0x0A, "Loading Boot Image ", 0x0D, 0x0A, 0x00
     msgCRLF     db 0x0D, 0x0A, 0x00
     msgProgress db ".", 0x00
     msgFailure  db 0x0D, 0x0A, "ERROR : Press Any Key to Reboot", 0x00
          TIMES 510-($-$$) DB 0
          DW 0xAA55

Breaking Down the Code

The source code above is a simple example, following the top-down programming approach for event sequencing. At the bottom are four basic functions to minimize code used for repeated services. DisplayMessage utilizes BIOS routines to output statuses to the screen for keeping the user informed of progress. ReadSectors utilizes BIOS routines to read raw data from the disk into memory. ClusterLBA converts Microsoft's cluster addressing scheme into a Logical Block Address for mapping the file to the disk. LBACHS converts the Logical Block Address into the Cylinder Head Sector format understood by the BIOS for directly accessing the file.

The main program body, identified as START: begins by initializing the processors registers. This step is important to establish a known operating environment. Errant values in the CPU registers may induce unintended side effects when making calls to BIOS functions. With the CPU in a known state, the code calculates the Root Directory's location from the values found in the BPB and loads it into memory. Looping through the Root Directory will identify the first cluster of LOADER.BIN, the target file to load. The next step uses the BPB to locate the FAT and load it into memory for browsing. Using the first cluster for LOADER.BIN, the bootstrap browses the FAT and makes calls to ReadSectors to load the file into memory. At the operation's conclusion, control is passed to LOADER.BIN via a RETF operation.


The bootsector is a simple, yet critical programming component in a computer system. Experimenting with the above source code will reveal techniques for loading different file systems, creating boot-time loaders or debugging a computer with a failed operating system. Security professionals are wise to understand low level coding, a domain frequently exploited by malevolent hackers. Overall, the understanding of a computer's most primitive operations helps programmers develop better software and increases understanding users have of their systems.


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Norton, Peter. "Inside the IBM PC Access to Advanced Features and Programming." Prentice-Hall Publishing : Bowie, MD. 1983.

OS-FAQ WIKI. "FAT12 Document." [on-line] (accessed November 2005); available from

Owen, Gareth. "OS Development." [on-line] (accessed November 2005) available from

Tash, Sean. "NYAOS Boot Sector." [on-line] (accessed November 2005) available from GAZTEK

Weeks, Jeff. "PolyOS Boot Loader Code." [on-line] (accessed November 2005) available from GAZTEK

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