An Examination of the
MBR ( Master Boot Record )
[ Embedded in Microsoft's® FDISK Programs
for: MS-Windows™
95B, 98, 98SE and ME ]

Web Presentation and Text are Copyright©2003 by Daniel B. Sedory
NOT to be reproduced in any form without Permission of the Author !

This page examines the Win 95B MBR which was introduced in 1996 when Microsoft® released its FAT32 File System. FAT32 required a number of changes in their utility programs. The Standard MBR embedded in FDISK was replaced by the expanded code which you see here; it added routines to check for things that only applied to Win 95B at that time, but the same MBR code was later used in the Windows 98, 98SE and Windows ME FDISK programs.

Basically, this MBR was created so that Microsoft's FAT32 Boot Sectors could be located in a partition that's beyond the reach of the Standard MBR (which is limited to cylinder 1024 or less, since it does not use the Extended INT 13 Function 42h which is found in this MBR).

Other Microsoft MBRs:
  An Examination of the Standard MBR created by MS-DOS FDISK
  The MBR created by MS-Windows 2000/XP


And OS Volume Boot Record pages:
  An Examination of the MS-Windows 95B/98 OS Boot Record (MSWIN4.1)
  An Examination of the NTFS Boot Record created by Windows 2000/XP



Confused? Send me an email if you have a specific question about the MBR or any other Boot Records...

IMPORTANT: One of the first things that any PC user should do after setting up a new hard disk (or creating a new partition with a utility such as Partition Magic) is to make a copy of its MBR; especially if you have more than one partition on the disk! Why? If you accidentally overwrite this sector, or are infected by a Boot sector virus, you may never be able to access some or even all of your disk again!   Even the most expensive HD utility might not correctly restore the Partition Table of a multi-partitioned hard disk!
Some advice: Save the Partition Table data on floppy disks or even on paper(!); it does no good to have the data you need to access your HD on the un-accessible HD itself! There are many ways you can do this... See the MBR Tools Page. Any good Disk Editor will allow you to manually enter data you've written down under an easy to use Partition Table View, or you can use Power Quest's excellent little utility program "MBRutil" (under any version of Windows!) and "MBRUTILD" (under DOS) to save the binary data to a file on a floppy diskette and later restore the MBR from the saved file.




Introduction

 Absolute Sector 0 (Cylinder 0, Head 0, Sector 1)

        0  1  2  3  4  5  6  7  8  9  A  B  C  D  E  F
 0000  33 C0 8E D0 BC 00 7C FB 50 07 50 1F FC BE 1B 7C  3.....|.P.P....| 
 0010  BF 1B 06 50 57 B9 E5 01 F3 A4 CB BE BE 07 B1 04  ...PW........... 
 0020  38 2C 7C 09 75 15 83 C6 10 E2 F5 CD 18 8B 14 8B  8,|.u........... 
 0030  EE 83 C6 10 49 74 16 38 2C 74 F6 BE 10 07 4E AC  ....It.8,t....N. 
 0040  3C 00 74 FA BB 07 00 B4 0E CD 10 EB F2 89 46 25  <.t...........F%
 0050  96 8A 46 04 B4 06 3C 0E 74 11 B4 0B 3C 0C 74 05  ..F...<.t...<.t.
 0060  3A C4 75 2B 40 C6 46 25 06 75 24 BB AA 55 50 B4   .u+@.F%.u$..UP.
 0070  41 CD 13 58 72 16 81 FB 55 AA 75 10 F6 C1 01 74  A..Xr...U.u....t
 0080  0B 8A E0 88 56 24 C7 06 A1 06 EB 1E 88 66 04 BF  ....V$.......f..
 0090  0A 00 B8 01 02 8B DC 33 C9 83 FF 05 7F 03 8B 4E  .......3.......N
 00A0  25 03 4E 02 CD 13 72 29 BE 46 07 81 3E FE 7D 55  %.N...r).F..>.}U
 00B0  AA 74 5A 83 EF 05 7F DA 85 F6 75 83 BE 27 07 EB  .tZ.......u..'..
 00C0  8A 98 91 52 99 03 46 08 13 56 0A E8 12 00 5A EB  ...R..F..V....Z.
 00D0  D5 4F 74 E4 33 C0 CD 13 EB B8 00 00 00 00 00 00  .Ot.3...........
 00E0  56 33 F6 56 56 52 50 06 53 51 BE 10 00 56 8B F4  V3.VVRP.SQ...V..
 00F0  50 52 B8 00 42 8A 56 24 CD 13 5A 58 8D 64 10 72  PR..B.V$..ZX.d.r
 0100  0A 40 75 01 42 80 C7 02 E2 F7 F8 5E C3 EB 74 49  .@u.B......^..tI
 0110  6E 76 61 6C 69 64 20 70 61 72 74 69 74 69 6F 6E  nvalid partition
 0120  20 74 61 62 6C 65 00 45 72 72 6F 72 20 6C 6F 61   table.Error loa
 0130  64 69 6E 67 20 6F 70 65 72 61 74 69 6E 67 20 73  ding operating s
 0140  79 73 74 65 6D 00 4D 69 73 73 69 6E 67 20 6F 70  ystem.Missing op
 0150  65 72 61 74 69 6E 67 20 73 79 73 74 65 6D 00 00  erating system..
 0160  00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
 0170  00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
 0180  00 00 00 8B FC 1E 57 8B F5 CB 00 00 00 00 00 00  ......W.........
 0190  00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
 01A0  00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
 01B0  00 00 00 00 00 00 00 00 00 00 00 00 00 00 80 01  ................
 01C0  01 00 0B 7F BF FD 3F 00 00 00 C1 40 5E 00 00 00  ......?....@^...
 01D0  00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................ 
 01E0  00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................ 
 01F0  00 00 00 00 00 00 00 00 00 00 00 00 00 00 55 AA  ..............U. 
        0  1  2  3  4  5  6  7  8  9  A  B  C  D  E  F



An Examination of the Assembly Code


7C00 33C0          XOR   AX,AX         ; Zero out both the Accumulator and
7C02 8ED0          MOV   SS,AX         ;    the Stack Segment register.
7C04 BC007C        MOV   SP,7C00       ; Set Stack Pointer to 0000:7C00
7C07 FB            STI                 ; Enable Interrupts
7C08 50            PUSH  AX            ; (AX=0)
7C09 07            POP   ES            ; Zero-out Extra Segment
7C0A 50            PUSH  AX            ; (AX=0)
7C0B 1F            POP   DS            ; Zero-out Data Segment
7C0C FC            CLD                 ; Clear Direction Flag
7C0D BE1B7C        MOV   SI,7C1B       ; Source Index: Copy from here...
7C10 BF1B06        MOV   DI,061B       ; Destination Index: Copy to here:
                                       ;  Code will begin at: 0000:061B
7C13 50            PUSH  AX            ; Set up Segment(AX) and Offset(DI)
7C14 57            PUSH  DI            ;  for jump to 0000:061B.
7C15 B9E501        MOV   CX,01E5       ; Set up Counter (CX) to copy
                                       ; (1E5h) 485 bytes of code.

; After executing the instructions above, if you were able to run MS-DEBUG
; to debug this code, its CPU Registers display would appear as follows:
;
; AX=0000  BX=0000  CX=01E5  DX=0080  SP=7BFC  BP=0000  SI=7C1B  DI=061B
; DS=0000  ES=0000  SS=0000  CS=0000  IP=7C18   NV UP EI PL ZR NA PE NC
; 0000:7C18 F3A4   REP MOVSB BYTE PTR ES:[DI], BYTE PTR DS:[SI]

7C18 F3            REP                 ; REPeat the following MOVSB
                                       ; instruction for 'CX' times.
7C19 A4            MOVSB               ;  Copy one byte at a time.

7C1A CB            RETF                ; Uses RETF to do Jump to where we
                                       ;  copied the code: 0000:061B.

; Since the preceding routine copies the remainder of the code to 0000:061B
; through 0000:07FF and continues its execution there, the following
; addresses have been changed to reflect the code's actual location in
; memory at the time of execution.


; This next section of code tries to find an ACTIVE (i.e., bootable) entry in
; the Partition Table.  The first byte of an entry indicates if it's bootable
; (normally an 80h, but in this case, anything from 81h through FFh as well!)
; or not (a 00h); any other values (from 01h through 7Fh) in these locations
; means the Table is invalid!  If none of the four entries in the Table is
; active, then the INT 18h instruction will be executed; for which most BIOS
; today (see note at 062B below) will display "PRESS A KEY TO REBOOT".

061B BEBE07        MOV   SI,07BE         ; Location of first entry
                                         ; in the partition table
                                         ; (see Sample Table here).
061E B104          MOV   CL,04           ; Maximum of 4 entries.
0620 382C          CMP   [SI],CH         ; We know (from Counter decrement
                                         ; above) that CH=0, so CoMPare 1st
                                         ; byte of entry [SI] to zero. Only
                                         ; if the high bit is set (80h thru
                                         ; FFh), will it be 'less than':
0622 7C09          JL    062D            ; Found a possible boot entry, but
                                         ; let's check it out more at 062D,
                                         ; or... continue searching table.
0624 7515          JNZ   063B            ; If not a zero byte (01h to 7Fh),
                                         ; then print out error message...
                                         ; -> "Invalid partition table"
0626 83C610        ADD   SI,+10          ; Checking the next entry...
                                         ; (10h = 16 bytes per entry)
0629 E2F5          LOOP  0620            ; Go back & check next Entry...
                                         ;   unless CL=0 (tried all four).
062B CD18          INT   18              ; Checked all 4; NONE of them
                                         ;   were bootable, so start
                                         ; ROM-BASIC (only available on
                                         ; some IBM machines!) Many BIOS
                                         ; simply display "PRESS A KEY TO
                                         ; REBOOT" when Int 18h is run.


; An Active partition was found, but we must check all other entries
; for being non-bootable (first byte = 00h), or there's an error!
; (Obviously, only one entry at a time can be marked as 'Active'!)

; Before doing so, we load the Head and Drive data into DX for use by
; the DOS Interrupt 13 commands later....

062D 8B14          MOV     DX,[SI]       ; This causes the Drive # (Yes,
                                         ; the Active BootFlag bytes in
; Partition Tables are Drive Numbers) to be placed into -> DL  and its
; Head # -> DH. For the Standard MBR, DL will always be 80h (see offset
; 0622 there with an 80h 'hardcoded' into the instruction) making only
; the first drive bootable! [So, that part of the Standard MBR code was
; often changed by MBR replacements! If you have an OS that can boot up
; from another drive, this new Win95B-ME MBR makes it easier to do so.]

062F 8BEE          MOV     BP,SI         ; Save offset of this entry...
0631 83C610        ADD     SI,+10        ;   see if any more are left.
0634 49            DEC     CX            ; If CX=0, then we're done here,
0635 7416          JZ      064D          ;   so Jump to ID-type checking!
0637 382C          CMP     [SI],CH       ; CoMPare entry's first byte [SI]
0639 74F6          JZ      0631          ;   to Zero (CH=0). If it's not
                                         ; zero, we fall into the error
                                         ; routine immediately following...

; If there was an error, then this next routine displays the message that
; SI points to.  After printing the ASCII-Z string (null terminated), the
; program 'hangs' by going into an infinite loop of executions between 
; 063E and 0642 as soon as it hits the Zero-byte at the end of the error
; message.
;   This is an interesting way to form an endless loop:  Since the LODSB
; instruction increments SI whenever it's executed, the decrement of SI
; at 063E forces the CPU to load the same byte at 0726 (a 00h) into the
; AL register over and over again!

 063B BE1007        MOV     SI,0710       ; To make SI point to: (070F;
                                          ; or "Invalid partition table")
                                          ;   after the decrement of SI.
|063E 4E            DEC     SI            ; Used only to 'hang' machine!
|063F AC            LODSB                 ;   Load byte at SI into AL
|                                         ;   and increment SI register.
|0640 3C00          CMP     AL,00         ; Is it the string terminator?
|0642 74FA          JZ      063E          ;   If so, 'lock-up' machine!
 0644 BB0700        MOV     BX,0007       ; Use normal White on Black text
 0647 B40E          MOV     AH,0E         ;  and Function 0Eh of INT 10 to
 0649 CD10          INT     10            ;  display the character in AL
                                          ;  on the video screen.
 064B EBF2          JMP     063F          ; Get next string character...


; At this point, the BP Register contains the offset to the Active entry
; (see 062F) and AX should still be zero. The first instruction in the code
; below appears rather strange, since this overwrites data in the 3rd entry!
; These are Memory locations; not bytes on the hard disk, and the only data
; which still matters is in the Boot entry. But why use an offset of +25 Hex
; (i.e., 37 bytes) when each entry is only 16 bytes long?  If we boot from
; the first entry (making BP=07BE), this first instruction will affect the
; two bytes at offsets 07E3h and 07E4h.  See below for why this was done:

064D 894625        MOV     [BP+25],AX   ; Zero-out the two bytes at offsets
                                        ; BP plus 37 (25 hex) and 38.

0650 96            XCHG    SI,AX        ; The contents of SI should be 07FE
                                        ; at this point (from checking the
                                        ; last entry in the Table), so this
                                        ; makes AX -> 07FE and SI -> zero.
                                        ; But after the next 2 instructions
                                        ; the 07FE is gone! So this is just
                                        ; a quick way to zero-out SI!

0651 8A4604        MOV     AL,[BP+04]   ; This is the Partition Type ID
                                        ;     for the bootable entry!
0654 B406          MOV     AH,06        ; 06h = FAT16 (>32MB) Partition.
0656 3C0E          CMP     AL,0E        ; 0Eh = FAT16 LBA-mapped Partition.
0658 7411          JZ      066B         ; If Partition Type is 0E hex,
                                        ;   Jump to 066B with AX=060E.

065A B40B          MOV     AH,0B        ; 0Bh = FAT32 Partition.
065C 3C0C          CMP     AL,0C        ; 0Ch = FAT32 LBA-mapped Partition;
                                        ;     uses Extended INT 13 code to
                                        ;     begin past cylinder 1024.
065E 7405          JZ      0665         ; If Partition Type is 0C hex,
                                        ;   Jump to 0665 with AX=0B0C.

0660 3AC4          CMP     AL,AH        ; AH should still be 0Bh.
0662 752B          JNZ     068F         ; For ID Types other than 06,0B,
                                        ;   0C or 0E, skip tests for BIOS
                                        ;   Extensions, jump to 068F with
                                        ;   AX=0Bxx where xx = ID Type.

0664 40            INC     AX           ; Partition Type is 0B hex, but we
                                        ;   make it a 0C hex in AL (this
                                        ;   changes the Zero Flag to a
                                        ;   Non-zero which forces a jump
                                        ;   to 068F at 0669 below!)

0665 C6462506      MOV     BYTE PTR [BP+25],06  ; Make the byte at BP
                                                ;  plus 37 a 06 and..
0669 7524          JNZ     068F         ; Jump to 068F with AX=0B0C only
                                        ;   if we have an ID Type = 0Bh.
					;
                                        ;  Otherwise (e.g., type=0Ch)...
066B BBAA55        MOV     BX,55AA      ;|
066E 50            PUSH    AX           ;|      Check for INT 13
066F B441          MOV     AH,41        ;|     Extended Functions
0671 CD13          INT     13           ;| Any of the 3 tests below may
                                        ;|   indicate the check failed.
0673 58            POP     AX
0674 7216          JB      068C         ;/ If Carry Flag (CF=1) is set,
                                        ;\  Extensions are not present!
0676 81FB55AA      CMP     BX,AA55      ;/ BX must equal AA55h or:
067A 7510          JNZ     068C         ;\  Extensions are not present!
067C F6C101        TEST    CL,01        ;/ If removable drive controller
					;|  functions are present, then
067F 740B          JZ      068C         ;\ Extensions will not be used!

0681 8AE0          MOV     AH,AL        ; Copy Partition Type -> AH
0683 885624        MOV     [BP+24],DL   ; Copy Drive Number to [BP+36]
					;  For a bootable First Entry,
					;         this would be 07E2h.
0686 C706A106EB1E  MOV     WORD PTR [06A1],1EEB ; This is an unusual use of
                                                ;  code in a commercial OS!
                                       ; This instruction actually modifies
                                       ; the code at offset 06A1 from what
                                       ; you see below to a: JMP 06C1 which
                                       ; aborts the Standard INT13 call.

068C 886604        MOV     [BP+04],AH
068F BF0A00        MOV     DI,000A

0692 B80102        MOV     AX,0201     ; Function 02 of INT 13
                                       ;   overwriting the 0B0C in AX!
0695 8BDC          MOV     BX,SP       ; BX = 7C00h (where we copy VBR to).
0697 33C9          XOR     CX,CX
0699 83FF05        CMP     DI,+05
069C 7F03          JG      06A1
069E 8B4E25        MOV     CX,[BP+25]

06A1 034E02        ADD     CX,[BP+02]  ; Normally, [BP+02] = Sector value.
;
; ButCan also be:  JMP 06C1 (if the
                                       ; test for Extended INT 13 above
                                       ; is positive!)
06A4 CD13          INT     13          ; Copy first sector of Volume Boot
                                       ; Record from Cylinder (low 8 bits
; in CH and 2 high bits in CL; 10 bits = 3FFh for max. of 1024 cylinders),
; Head (in DH), Sector (low 6 bits in CL; for max. of 3Fh = 63) and Drive
; Number (in DL). The CHS scheme cannot access a partition beyond the
; 1024th cylinder or LBA 16,450,559 (that's [1024 x 255 x 63] - 1, or:
; C,H,S = 1023, 254, 63). 


I hope to eventually finish commenting 
on the following code... You can still 
email me with specific questions.


06A6 7229          JB      06D1
06A8 BE4607        MOV     SI,0746      ; -> "Missing operating system"
06AB 813EFE7D55AA  CMP     WORD PTR [7DFE],AA55   ; Checking the last two
                                        ;  bytes of newly loaded sector
                                        ;   for "Magic Number" (AA55h).
06B1 745A          JZ      070D         ; See Note at 070Dh for this
                                        ;    seemingly odd usage.

06B3 83EF05        SUB     DI,+05
06B6 7FDA          JG      0692
06B8 85F6          TEST    SI,SI
06BA 7583          JNZ     063F
06BC BE2707        MOV     SI,0727      ; --> "Error loading operating
                                        ;      system"
06BF EB8A          JMP     064B         ; The code at 064B was conveniently
                                        ; used in order to jump all the way
                                        ; back to 063F without having to
                                        ; use a three-byte NEAR jump! See
                                        ; Using SHORT Jumps for more info.

06C1 98            CBW                  ; Convert Byte to Word.
06C2 91            XCHG    CX,AX
06C3 52            PUSH    DX
06C4 99            CWD                  ; Convert Word to DoubleWord.
06C5 034608        ADD     AX,[BP+08]
06C8 13560A        ADC     DX,[BP+0A]
06CB E81200        CALL    06E0
06CE 5A            POP     DX
06CF EBD5          JMP     06A6

06D1 4F            DEC     DI
06D2 74E4          JZ      06B8
06D4 33C0          XOR     AX,AX        ;|  INT 13 Function 0
06D6 CD13          INT     13           ;| "RESET DISK SYSTEM"
06D8 EBB8          JMP     0692

===========================================================================
06DA 0000                               ; Location in Memory of what we're
06DC 00000000                           ;   calling the "Mystery Bytes"
===========================================================================
; SUBROUTINE:
06E0 56            PUSH    SI
06E1 33F6          XOR     SI,SI
06E3 56            PUSH    SI
06E4 56            PUSH    SI
06E5 52            PUSH    DX
06E6 50            PUSH    AX
06E7 06            PUSH    ES
06E8 53            PUSH    BX
06E9 51            PUSH    CX
06EA BE1000        MOV     SI,0010
06ED 56            PUSH    SI
06EE 8BF4          MOV     SI,SP
06F0 50            PUSH    AX
06F1 52            PUSH    DX
06F2 B80042        MOV     AX,4200       ;| ( An INT 13 Extension )
06F5 8A5624        MOV     DL,[BP+24]    ;|
06F8 CD13          INT     13            ;|  Extended READ Function

06FA 5A            POP     DX
06FB 58            POP     AX

06FC 8D6410        LEA     SP,[SI+10]
06FF 720A          JB      070B
0701 40            INC     AX
0702 7501          JNZ     0705
0704 42            INC     DX
0705 80C702        ADD     BH,02
0708 E2F7          LOOP    0701
070A F8            CLC
070B 5E            POP     SI
070C C3            RET

=========================================================================
070D EB74          JMP     0783        ; At first glance, this may appear
                                       ;   to be a wasteful use of bytes,
; but it's necessary because the point of the code here is to get program
; execution to jump from 06B1 to 0783 using only fast 2-byte jumps which
; are limited to +127 bytes forward. See Using SHORT Jumps for more info.
; Apparently, moving the Error Messages you see below wasn't an option.
=========================================================================



Location of Error Messages in Memory
070F                                               49                  I
0710  6E 76 61 6C 69 64 20 70 61 72 74 69 74 69 6F 6E   nvalid partition
0720  20 74 61 62 6C 65 00 45 72 72 6F 72 20 6C 6F 61    table.Error loa
0730  64 69 6E 67 20 6F 70 65 72 61 74 69 6E 67 20 73   ding operating s
0740  79 73 74 65 6D 00 4D 69 73 73 69 6E 67 20 6F 70   ystem.Missing op
0750  65 72 61 74 69 6E 67 20 73 79 73 74 65 6D 00 00   erating system..
0760  00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00   ................
0770  00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00   ................
0780  00 00 00                                          ...

(Note: The unused bytes above allow for longer error messages in other languages.)


Location of the Lonely Code Bytes in Memory

0783 8BFC          MOV     DI,SP       ; Set up Offset (DI) with 7C00.
0785 1E            PUSH    DS          ; (DS = 0000 when executed.)
0786 57            PUSH    DI          ; (DI = 7C00.)
0787 8BF5          MOV     SI,BP       
0789 CB            RETF                ; Use RETF to do a Jump to the OS
                                       ;  Boot Sector code at 0000:7C00;
                                       ; continuing the boot-up process!


Unused and S/N Area
                                      A  B  C  D  E  F
 078A                                00 00 00 00 00 00             ......
 0790  00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00   ................
 07A0  00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00   ................
 07B0  00 00 00 00 00 00 00 00 00 00 00 00 00 00         ..............
        0  1  2  3  4  5  6  7  8  9  A  B  C  D
(The bytes in offset 7B8h through 7BBh, may contain an NT Drive Serial Number if the computer was ever booted up under Windows NT/2000/XP.)

A Sample Partition Table

This shows a sample partition table as it would appear in memory during the boot-up process. See Reference Page for links to complete listings of Partition Identifiers. This Sample Table contains entries in all four records; each record is 16 bytes long. I've underlined the bytes in the first (7BEh through 7CDh) and last (7EEh through 7FDh) of the four entries:

       0  1  2  3  4  5  6  7  8  9  A  B  C  D  E  F
07BE                                            80 01                 ..
07C0  01 00 06 3F 3F C4 3F 00-00 00 81 1E 0C 00 00 00   ...??.?.........
07D0  01 C5 05 3F 7F 47 C0 1E-0C 00 40 0F 08 00 00 00   ...?.G....@.....
07E0  41 48 82 3F 7F 53 00 2E-14 00 00 BD 00 00 00 00   AH.?.S..........
07F0  41 54 83 3F BF 0F 00 EB-14 00 00 91 0B 00 55 AA   AT.?..........U.

And this is how it would be seen in a disk editor that can interpret Partition Table data:

  Partition    Active  Starting Loc   Ending Loc   Relative  Number of
    Type        Boot   Cyl Head Sec  Cyl Head Sec   sectors    sectors
-------------  ------  ------------  ------------   -------    -------
DOS FAT-16      Yes      0   1   1   196  63  63         63     794241
Extended        No     197   0   1   327  63  63     794304     528192
LINUX Swap      No     328   0   1   339  63  63    1322496      48384
LINUX Ext2FS    No     340   0   1   527  63  63    1370880     758016

Note:
The sector must have a 'signature' of 0xAA55. It's located at the very end of the partition table (remember that low-bytes appear first and high-bytes last). The BIOS checks for the signature and if it's not there, you'll see an error message such as "Operating System not found." (The message being dependent upon the BIOS code; most PhoenixBIOS, including those modified for VMWare, display this one. But under BOCHS, you would see: "Boot failed: not a bootable disk" and on a PC using Award BIOS 6.00PG, it actually displays: DISK BOOT FAILURE, INSERT SYSTEM DISK AND PRESS ENTER.)

During boot-up, these locations are later replaced by communications parameters for COM2 thru COM4 and other data. Here's a listing of my own computer's memory after boot-up for these same memory locations:

07BE                                            7A 02                 z.
07C0  AC 02 43 4F 4D 32 20 20 20 20 DC 00 70 00 00 80   ..COM2    ..p..
07D0  7A 02 B2 02 43 4F 4D 33 20 20 20 20 00 00 0D 0E   z...COM3    ....
07E0  00 80 7A 02 B8 02 43 4F 4D 34 20 20 20 20 E8 D2   .z...COM4    ..
07F0  01 D7 07 38 CD 01 3B 0E 00 00 7C 04 18 00 59 04   ......;...|...Y.

 



Major Update: 26 AUG 2004  [26.8.2004].
Updated: 15 SEP 2010  [15.9.2010].
Last Update: 31 OCT 2010  [31.10.2010].


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