How It Works:
Master Boot Record (MBR)

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Disassembly of a Master Boot Record (MBR)

This article is a disassembly of a Master Boot Record (MBR). The MBR is the sector at cylinder 0, head 0, sector 1 of a hard disk. An MBR is created by the FDISK program. The FDISK program of all operating systems must create a functionally similar MBR. The MBR is first of what could be many partition sectors, each one containing a four entry partition table.

At the completion of your system’s Power On Self Test (POST), INT 19 is called. Usually INT 19 tries to read a boot sector from the first floppy drive. If a boot sector is found on the floppy disk, the that boot sector is read into memory at location 0000:7C00 and INT 19 jumps to memory location 0000:7C00. However, if no boot sector is found on the first floppy drive, INT 19 tries to read the MBR from the first hard drive. If an MBR is found it is read into memory at location 0000:7c00 and INT 19 jumps to memory location 0000:7c00. The small program in the MBR will attempt to locate an active (bootable) partition in its partition table. If such a partition is found, the boot sector of that partition is read into memory at location 0000:7C00 and the MBR program jumps to memory location 0000:7C00. Each operating system has its own boot sector format. The small program in the boot sector must locate the first part of the operating system’s kernel loader program (or perhaps the kernel itself or perhaps a "boot manager program") and read that into memory.

INT 19 is also called when the CTRL-ALT-DEL keys are used. On most systems, CTRL-ALT-DEL causes an short version of the POST to be executed before INT 19 is called.

Where stuff is

Summary of what this thing does

If an active partition is found, that partition’s boot record is read into 0000:7c00 and the MBR code jumps to 0000:7c00 with SI pointing to the partition table entry that describes the partition being booted. The boot record program uses this data to determine the drive being booted from and the location of the partition on the disk.

If no active partition table enty is found, ROM BASIC is entered via INT 18. All other errors cause a system hang, see label HANG.

Notes (VERY IMPORTANT)

  1. The first byte of an active partition table entry is 80.
    This byte is loaded into the DL register before INT 13 is called to read the boot sector. When INT 13 is called, DL is the BIOS device number. Because of this, the boot sector read by this MBR program can only be read from BIOS device number 80 (the first hard disk). This is one of the reasons why it is usually not possible to boot from any other hard disk.
  2. The MBR program uses the CHS based INT 13H AH=02H call to read the boot sector of the active partition. The location of the active partition’s boot sector is in the partition table entry in CHS format. If the drive is >528MB, this CHS must be a translated CHS (or L-CHS, see my BIOS TYPES document). No addresses in LBA form are used (another reason why LBA doesn’t solve the >528MB problem).

Entire MBR record in hex and ASCII


OFFSET 0 1 2 3  4 5 6 7  8 9 A B  C D E F  *0123456789ABCDEF*
000000 fa33c08e d0bc007c 8bf45007 501ffbfc *.3.....|..P.P...*
000010 bf0006b9 0001f2a5 ea1d0600 00bebe07 *................*
000020 b304803c 80740e80 3c00751c 83c610fe *.....t....u.....*
000030 cb75efcd 188b148b 4c028bee 83c610fe *.u......L.......*
000040 cb741a80 3c0074f4 be8b06ac 3c00740b *.t....t.......t.*
000050 56bb0700 b40ecd10 5eebf0eb febf0500 *V.......^.......*
000060 bb007cb8 010257cd 135f730c 33c0cd13 *..|...W.._s.3...*
000070 4f75edbe a306ebd3 bec206bf fe7d813d *Ou...........}.=*
000080 55aa75c7 8bf5ea00 7c000049 6e76616c *U.u.....|..Inval*
000090 69642070 61727469 74696f6e 20746162 *id partition tab*
0000a0 6c650045 72726f72 206c6f61 64696e67 *le.Error loading*
0000b0 206f7065 72617469 6e672073 79737465 * operating syste*
0000c0 6d004d69 7373696e 67206f70 65726174 *m.Missing operat*
0000d0 696e6720 73797374 656d0000 00000000 *ing system......*
0000e0 00000000 00000000 00000000 00000000 *................*
0000f0 TO 0001af SAME AS ABOVE
0001b0 00000000 00000000 00000000 00008001 *................*
0001c0 0100060d fef83e00 00000678 0d000000 *...........x....*
0001d0 00000000 00000000 00000000 00000000 *................*
0001e0 00000000 00000000 00000000 00000000 *................*
0001f0 00000000 00000000 00000000 000055aa *..............U.*

Disassembly of the MBR

This sector is initially loaded into memory at 0000:7c00 but it immediately relocates itself to 0000:0600.


                 BEGIN:                      NOW AT 0000:7C00, RELOCATE

0000:7C00 FA            CLI                     disable int's
0000:7C01 33C0          XOR     AX,AX           set stack seg to 0000
0000:7C03 8ED0          MOV     SS,AX
0000:7C05 BC007C        MOV     SP,7C00         set stack ptr to 7c00
0000:7C08 8BF4          MOV     SI,SP           SI now 7c00
0000:7C0A 50            PUSH    AX
0000:7C0B 07            POP     ES              ES now 0000:7c00
0000:7C0C 50            PUSH    AX
0000:7C0D 1F            POP     DS              DS now 0000:7c00
0000:7C0E FB            STI                     allow int's
0000:7C0F FC            CLD                     clear direction
0000:7C10 BF0006        MOV     DI,0600         DI now 0600
0000:7C13 B90001        MOV     CX,0100         move 256 words (512 bytes)
0000:7C16 F2            REPNZ                   move MBR from 0000:7c00
0000:7C17 A5            MOVSW                      to 0000:0600
0000:7C18 EA1D060000    JMP     0000:061D       jmp to NEW_LOCATION

        NEW_LOCATION:                        NOW AT 0000:0600

0000:061D BEBE07      MOV     SI,07BE           point to first table entry
0000:0620 B304        MOV     BL,04             there are 4 table entries

        SEARCH_LOOP1:                        SEARCH FOR AN ACTIVE ENTRY

0000:0622 803C80      CMP     BYTE PTR [SI],80  is this the active entry?
0000:0625 740E        JZ      FOUND_ACTIVE      yes
0000:0627 803C00      CMP     BYTE PTR [SI],00  is this an inactive entry?
0000:062A 751C        JNZ     NOT_ACTIVE        no
0000:062C 83C610      ADD     SI,+10            incr table ptr by 16
0000:062F FECB        DEC     BL                decr count
0000:0631 75EF        JNZ     SEARCH_LOOP1      jmp if not end of table
0000:0633 CD18        INT     18                GO TO ROM BASIC

        FOUND_ACTIVE:                        FOUND THE ACTIVE ENTRY

0000:0635 8B14        MOV     DX,[SI]           set DH/DL for INT 13 call
0000:0637 8B4C02      MOV     CX,[SI+02]        set CH/CL for INT 13 call
0000:063A 8BEE        MOV     BP,SI             save table ptr

        SEARCH_LOOP2:                        MAKE SURE ONLY ONE ACTIVE ENTRY

0000:063C 83C610      ADD     SI,+10            incr table ptr by 16
0000:063F FECB        DEC     BL                decr count
0000:0641 741A        JZ      READ_BOOT         jmp if end of table
0000:0643 803C00      CMP     BYTE PTR [SI],00  is this an inactive entry?
0000:0646 74F4        JZ      SEARCH_LOOP2      yes

          NOT_ACTIVE:                        MORE THAN ONE ACTIVE ENTRY FOUND

0000:0648 BE8B06      MOV     SI,068B           display "Invld prttn tbl"

         DISPLAY_MSG:                        DISPLAY MESSAGE LOOP

0000:064B AC          LODSB                     get char of message
0000:064C 3C00        CMP     AL,00             end of message
0000:064E 740B        JZ      HANG              yes
0000:0650 56          PUSH    SI                save SI
0000:0651 BB0700      MOV     BX,0007           screen attributes
0000:0654 B40E        MOV     AH,0E             output 1 char of message
0000:0656 CD10        INT     10                   to the display
0000:0658 5E          POP     SI                restore SI
0000:0659 EBF0        JMP     DISPLAY_MSG       do it again

                HANG:                        HANG THE SYSTEM LOOP

0000:065B EBFE        JMP     HANG              sit and stay!

           READ_BOOT:                        READ ACTIVE PARITION BOOT RECORD

0000:065D BF0500      MOV     DI,0005           INT 13 retry count

           INT13RTRY:                        INT 13 RETRY LOOP

0000:0660 BB007C      MOV     BX,7C00
0000:0663 B80102      MOV     AX,0201           read 1 sector
0000:0666 57          PUSH    DI                save DI
0000:0667 CD13        INT     13                read sector into 0000:7c00
0000:0669 5F          POP     DI                restore DI
0000:066A 730C        JNB     INT13OK           jmp if no INT 13
0000:066C 33C0        XOR     AX,AX             call INT 13 and
0000:066E CD13        INT     13                   do disk reset
0000:0670 4F          DEC     DI                decr DI
0000:0671 75ED        JNZ     INT13RTRY         if not zero, try again
0000:0673 BEA306      MOV     SI,06A3           display "Errr ldng systm"
0000:0676 EBD3        JMP     DISPLAY_MSG       jmp to display loop

             INT13OK:                        INT 13 ERROR

0000:0678 BEC206      MOV     SI,06C2              "missing op sys"
0000:067B BFFE7D      MOV     DI,7DFE              point to signature
0000:067E 813D55AA    CMP     WORD PTR [DI],AA55   is signature correct?
0000:0682 75C7        JNZ     DISPLAY_MSG          no
0000:0684 8BF5        MOV     SI,BP                set SI
0000:0686 EA007C0000  JMP     0000:7C00            JUMP TO THE BOOT SECTOR
                                                      WITH SI POINTING TO
                                                      PART TABLE ENTRY

Messages here.

0000:0680 ........ ........ ......49 6e76616c *           Inval*
0000:0690 69642070 61727469 74696f6e 20746162 *id partition tab*
0000:06a0 6c650045 72726f72 206c6f61 64696e67 *le.Error loading*
0000:06b0 206f7065 72617469 6e672073 79737465 * operating syste*
0000:06c0 6d004d69 7373696e 67206f70 65726174 *m.Missing operat*
0000:06d0 696e6720 73797374 656d00.. ........ *ing system.     *

Data not used.

0000:06d0 ........ ........ ......00 00000000 *           .....*
0000:06e0 00000000 00000000 00000000 00000000 *................*
0000:06f0 00000000 00000000 00000000 00000000 *................*
0000:0700 00000000 00000000 00000000 00000000 *................*
0000:0710 00000000 00000000 00000000 00000000 *................*
0000:0720 00000000 00000000 00000000 00000000 *................*
0000:0730 00000000 00000000 00000000 00000000 *................*
0000:0740 00000000 00000000 00000000 00000000 *................*
0000:0750 00000000 00000000 00000000 00000000 *................*
0000:0760 00000000 00000000 00000000 00000000 *................*
0000:0770 00000000 00000000 00000000 00000000 *................*
0000:0780 00000000 00000000 00000000 00000000 *................*
0000:0790 00000000 00000000 00000000 00000000 *................*
0000:07a0 00000000 00000000 00000000 00000000 *................*
0000:07b0 00000000 00000000 00000000 0000.... *............    *

The partition table starts at 0000:07be.  Each partition table
entry is 16 bytes.  This table defines a single primary partition
which is also an active (bootable) partition.

0000:07b0 ........ ........ ........ ....8001 *            ....*
0000:07c0 0100060d fef83e00 00000678 0d000000 *...........x....*
0000:07d0 00000000 00000000 00000000 00000000 *................*
0000:07e0 00000000 00000000 00000000 00000000 *................*
0000:07f0 00000000 00000000 00000000 0000.... *............    *

The last two bytes contain a 55AAH signature.

0000:07f0 ........ ........ ........ ....55aa *..............U.*

This page was last updated on 05 October 1999.