An
Examination of the
NTFS Volume Boot Record
Of Microsoft® Windows™ 2000
(NT5.0) and Windows™ XP (NT5.1)
[ It's also found in SPCMDCON.SYS and
other System files; see Introduction below. ]
Web Presentation and Text are Copyright©2004, 2007, 2009
by Daniel B. Sedory
NOT to be reproduced in any form without Permission of
the Author !
Note:
If something you read here is very important to you, it's best to confirm it
with your own experiments; especially since the code
may be changed by Microsoft at any time.
This has already occurred. We refer to:
http://support.microsoft.com/kb/320397
(“Windows
may not start and you may receive an "NTLDR is missing" error message
if Windows is not up-to-date and there are too many files in the root folder”)
in which you'll find out
why Microsoft had to change the Boot Record code for Windows 2000 with
Service Pack 4 (SP4) and Windows XP with Service Pack 2 (SP2).
Note: Installing Windows XP SP2 or Windows 2000 SP4, will not correct
this issue on existing volumes. To update the boot code on existing volumes,
you must obtain and run the utility "bcupdate2.exe" (MD5 hash value
is: "2e2ed06d9ae54c8d959a534a83284705") from Microsoft, or manually
change the boot code using an updated copy. The service pack updates will only
write the new boot code for new volumes; they cannot correct old volumes.
You can
contact us here if you have a specific
question about this page.
Introduction
This page
examines the NTFS Boot Record for Windows™ 2000 (NT5) and
Windows™ XP (NT5.1). Although Windows 2000 and XP can both
use the MS-FAT32 File System, I'd advise anyone interested in serious computing
to run these operating systems in NTFS formatted partitions (rather
than FAT32*) for a number of reasons.
Some of those being for better performance, less wasted space and more security.
One or more FAT32 partitions can also reside on the same drive if required.
For example, if a drive includes a Linux partition or some other OS that cannot
write to an NTFS partition, then a FAT32 partition could act as a communications
buffer for sharing data with both the NTFS and Linux OSs.
[There is, however, at least one
free application that allows NTFS users to acquire files directly
from a Linux partition (e.g., explore2fs by John Newbigin,
http://www.chrysocome.net/explore2fs
(new site) and a number of Linux versions that can read
files directly from NTFS partitions by using kernel drivers from the
linux-ntfs project.
Unfortunately, the linux-ntfs.org website has remained damaged since about
AUG 2007! You can check its earlier contents at the
WayBackMachine
archive.
Many will probably like to review their documentation file here:
ntfsdoc.pdf
(Note: This is not an exhuastive paper, and it contains at least one error:
We will point out below that the NTFS "Backup Boot Sector"
is not part of the NTFS Volume, yet their paper incorreclty states:
"Hidden at the end of the volume, is a copy of the boot
sector (cluster 0)."; it's actually 1 sector beyond the end of
the Volume).
But
there doesn't appear to be any reliable (and simple) methods
of writing files under Linux to an NTFS
partition, nor any free programs for writing
to Linux from Windows;
that we're aware of -- beware some "demo" downloads by commercial
companies which are in fact "read only". We were hopeful this dismal
situation would soon change. Is
the future here? We are presently testing an NTFS disk under a
64-bit Fedora
Core 10 install. We used GParted 0.4.3 (with Libparted 1.8.8) to create
a new NTFS formatted partition, and it used the command
mkntfs -Q -v (that's mkntfs 2.0.0 with libntfs 10:0:0
from the linux-ntfs project's ntfsprogs) to do so.]
__________________
*There's
one odd quirk about maintaining FAT partitions under either Windows
2000 or XP you should know about: If you delete a file
directly (without sending it to the Recycle Bin; using
SHIFT+DEL keys for example), it can be very difficult, if even possible,
to recover that file! For some odd reason, these OSs zero-out
the 'high bits' (upper 16 bits) of any FAT file's
starting cluster number! This affects any file deleted directly
that's located beyond a starting cluster of 65,535.
[Note:
You can find copies of the Boot Record code which we are examining here, in
many different Windows™ System files; see “Where's
the Code?” for more information on the names and locations of these
files.]
All
of the NTFS Boot Code:
Although
Microsoft's NTFS Boot Record is often described as being a single
sector (see the proof below!), the truth is that it would never be able to
boot up a Win 2000/XP OS without the next six sectors (so far)
that comprise most of an NTFS partition's Boot Code. So, even
though an NTFS partition's “Boot Record” is technically only
1 sector in length, we should never forget about the necessity of the “Bootstrap
Code” which follows it. Furthermore, Microsoft states that
the OS itself “allocates
the first 16 sectors for the boot sector and the bootstrap
code1.” Thus,
Microsoft could have created a bootstrap code that was more than twice
the size it is now. (I wonder if there are any Assembly programmers working
on a way to enhance the present bootstrap code?)
The
NTFS Boot Record really is only one sector long!
We carried
out an experiment which proves that only the first
sector of an NTFS partition is the NTFS Boot Record:
In a disk editor under real DOS, every byte of the “Bootstrap
Code” for a Win XP NTFS partition (on a second drive)
was filled with zero bytes! Upon booting up our Master
drive's Windows 2000 OS, we found that all the files in that Win XP partition
could still be accessed (both read and write) and used without any problems.
So, the "Bootstrap code" really is just that: It's only used
to boot up any existing OS in that partition! Even though the phrase "Boot
Record" isn't very descriptive for a sector that's only used to access
files in the partition, there's an historical precedent
for using it this way: The first sector of every floppy disk is
also called a "Boot Record" even though
it will never be bootable unless you add the correct system
boot files to it! You can, of course, remove most of the boot code
from either the NTFS or a floppy's Boot Record, since their BPBs
are the only thing that an OS needs in order to access their files!!
If you look for them, you'll eventually find many floppy disks with a
"Boot Record" that contains only a BPB
and a very short string of code that displays a message saying that you
can not boot-up with that diskette if
you try to boot your system with it!
NOTE: If you create a new NTFS partition with a Win 2000/XP
OS, the "Bootstrap Code" will always be appended to the
Boot Record; though it might be possible to change this 'boot code,' to
make it only display a message saying that the partition isn't bootable,
there's no practical reason for doing so, and it might lead to serious
problems if you ever tried to install a Win2000/XP OS in that partition!
|
Our examination
will mostly be limited to just the code in the first sector. Once we
began examining the code, we found that the “boot sector” actually
loads all 15 of the remaining “bootstrap code” sectors
into Memory; even though the last eight sectors are nothing
but zero-bytes!
Take this link
to see: The NTFS Boot Record as
viewed in a Disk Editor. [ Opens
in a new window, so you can refer to it later! ] Includes color-highlighting
of the code, BPB, Error
Messages, etc.
The NTFS
Boot Record is loaded into the usual Memory location of 0000:7C00
by the MBR code (see the Windows 2000/XP
MBR page for more info). Locations 7C0Bh through 7C53h
are filled with the NTFS BPB (Bios Parameter
Block), and the next 303 bytes (7C54h through 7D82h)
contain this sector's executable code (the main
program and various subroutines) which eventually load the N.T.L.D.R.
(or “Bootstrap”) sectors into Memory (try not to confuse
these sectors with the NTLDR file that's found in the
root directory of Win NT/2000/XP/2003 OS partitions).
__________________
1
The Microsoft
Windows 2000 Professional Resource Kit, Chapter
32 - Disk Concepts and Troubleshooting, section "Disk
Sectors Critical to Startup" under the heading "NTFS
Boot Sector" above Table 32.12.
[
Note: Be aware that many of the Hex numbers
in Microsoft's tables and even the text on this page (and others) do
not use what we'd call normal Hex representation!
They use the symbol "0x" in a nonstandard manner, saying
for example, that: "0x3F000000" is
"little-endian" for decimal 63. Although they do describe this usage
(you have to look hard for it), it's still incorrect
according to all programming languages (such as C or C++)
that use "0x"; including Microsoft's own compilers!
Preceding any value with "0x" should always mean a normal Hexadecimal
number.
For example,
if you search for "0x" on this page, the first instance is: "0x55AA"
in the main section "Disk Sectors Critical to Startup" in
the first paragraph under "Master Boot Record" where this
value is called a "signature word or end of sector marker."
Yet the very next paragraph, states a disk signature may occur at offset "0x01B8";
which is inconsistent with the first instance of "0x"! If that offset
was stored anywhere on a hard disk, it appears Microsoft would have
written "0xB801" instead. In "Table
32.2 Partition Table Fields" in the row labeled "0x01C6"
under the "Byte Offset" column, you'll find: "0x3F000000"
under the "Sample Value" column along with the explanation: "The
offset from the beginning of the disk to the beginning of the volume, counting
by sectors." It isn't until you finally read the notes under this
table, that its author(s) state: "Numbers larger than
one byte are stored in little endian format, or reverse-byte ordering. ....
For example, the sample value for the Relative Sectors field in the previous
table, 0x3F000000, is a little endian representation
of 0x0000003F." After reading that, your very next thought should
be: How are we supposed to know when it does or doesn't
mean that? The symbol "0x" by itself does not indicate
that. Furthermore, a portion of the note which wasn't quoted above, read:
"Little endian format is a method of storing a number so that the least
significant byte appears first..." and with that we certainly agree,
but then their statement adds: "...in the hexadecimal number notation."
In the "number notation"? This last phrase
is just plain wrong! It had already been agreed that when you prepend
"0x" (or append an "h") these symbols indicate hexadecimal
rather than decimal-based numbers. Apparently, some Microsoft author(s)
decided to make things a bit easier for themselves by using it differently,
rather than considering how this might confuse their readers. The term "Little-endian"
should only apply to the actual byte-order on a storage medium or in Memory.
Rather than confuse people, these authors should drop the "0x" hex
indicator, writing a string of bytes with or without spaces between them and
let you know that's how the bytes would appear in a disk editor.
Although this
insanely inconsistent usage seems to have begun only in this small part of
Microsoft's world, we have found other authors copying it; usually
when discussing a Boot Record's signature Word 0xAA55 (which is the
only correct way to state that using "0x"). It appears the Microsoft
authors writing these pages used the symbol "0x" in whatever
way suited them at the moment; such as how a particular type of CPU represents
a Hex number for a given length of bytes in one sentence, but then in the
very next sentence, an offset for locating some byte(s) on a disk is written
as a normal Hex number. Imagine the confusion if a stock analyst wrote
in a tiny footnote: 'In this report, $100.00 of shares are really worth
only $000.01 (1 penny); unless I decide otherwise!' then
listed his inflated data that way? Microsoft texts should
not be written like Alice in Wonderland. We're supposed to be dealing
with helpful facts, and Microsoft's statement about some
special little-endian use of 0x only makes the material more confusing
for students trying to understand it! "0x" should
only prepend pure Hex numbers (as understood by humans);
independent of their storage medium's endianness! Afterall,
we are human beings; there's no intel™ chip inside
my brain! But even if there were, it still wouldn't know for sure when
"0x" menat a "little-endian number" and when it didn't
without an indicator of some kind. We don't believe this is an unreasonable
expectation. We do see how such a usage might make things easier for Microsoft®
to document their OS, but a standard should be followed.
So, to avoid
any further confusion, all our Hex Bytes, Words, Double
Words, etc. in the tables below are shown as plain Hex values (without
any prepended symbols) in the columns marked "Hex Numbers";
they will be the same Hex numbers no matter which processor
you use or how the number is stored in Memory or on a hard disk, unless we
need to make their byte-order clear.
NOTE:
Due to the confusion such pages have caused for new students in x86 Assembly
classes, we've created a new page dealing with the correct way to refer to
the Boot Record Signature as a hexadecimal Word on a PC: AA55H.html.
]
Although
it actually makes sense in this case (after examining
the code for the NTFS Boot Record), to call the first sector
the “boot sector” and the rest
of it the “bootstrap code,” this isn't true of all operating
systems. For example, the first sector of the MSWIN4.1
Boot Record's three sectors, cannot be thought of as having
a separate function from the code in its third sector (so the phrase
“boot sector” does not apply to the MSWIN4.1
Boot Record).
The
NTFS Boot Record's “Backup Sector”
The Win 2000/XP
OSs make a "backup" of each NTFS volume's Boot
Record which they store in the very last sector of its partition!
[ Note: I said "partition" not
volume. This is why an NTFS partition's
Total Sectors count in the MBR/EBR's Partition Table
is always 1 sector more than the "Total
Sectors (in Volume)" count found
in its Boot Record. Although the words partition
and volume are often thought of as being synonymous (we
may even use them as such here!), they are definitely not
the same thing. ]
|
The NTFS Bios Parameter
Block
0 1 2 3 4 5 6 7 8 9 A B C D E F
00B0 00 02 08 00 00 .....
0010 00 00 00 00 00 F8 00 00 3F 00 FF 00 3F 00 00 00 ........?...?...
0020 00 00 00 00 80 00 80 00 AE 39 D7 00 00 00 00 00 .........9......
0030 04 00 00 00 00 00 00 00 8D EF 00 00 00 00 00 00 ................
0040 F6 00 00 00 01 00 00 00 FC 5D E1 A4 99 E1 A4 B4 .........]......
0050 00 00 00 00 .....
|
The following
table describes what the values in the "Hex Dump" above mean. Offsets
for each field are taken from the beginning (00h) of the Boot Record (which
is located at Absolute Sector 63 for the first partition on most hard disks).
Microsoft®
came up with an entirely new Extended BPB for its NTFS File System.
Although it does contain some fields that are used for the same items as previous
versions, beginning with offset 28h those are completely cast
aside in favor of the much larger (LONG LONG Word) fields that are needed
by NTFS!
The green colored fields contain data that may
vary from one computer to the next, whereas, the data in
the red and gray fields
should never vary between NTFS Systems. The data in the
yellow colored fields might possibly be different under some
circumstances, but normally should remain the same as shown below:
| Offset
| Length
| Hex
Numbers
| Decimal
Equiv. or
Meaning
| Description |
| 0Bh |
Word |
0200 |
512 |
Sector
Size (in bytes) |
| Reference |
" If maximum compatibility is desired, only the value 512
should be used. There is a lot of FAT code in the world that is
basically "hard wired" to 512 bytes per sector and doesn’t bother
to check this field to make sure it is 512. [So]...values
[other than 512] are not recommended." |
| 0Dh |
Byte |
08 |
8 |
Sectors
per Cluster; so
Cluster size here = 4 KiB |
| Reference |
" Number of sectors per allocation
unit. This value must be a power of 2 that is greater than 0. Values
of 1, 2, 4, 8 are quite common."
[ I've seen
a value of 4 ( for a Windows XP partition of less than
2GB ), but a value of 8 ( for Windows 2000 and
XP ) is probably the most common. ] |
| 0Eh |
Word |
0000 |
0 |
Reserved
Sectors; I've never seen anything but a zero here for NTFS
(does your BPB have anything else here?). |
| After
loading into Memory, this location becomes: 7C0Eh, and
it's used to store how many sectors of the Boot Record are to be
read into Memory; it begins with a value of 16 (decimal) and counts
down to zero. (See line: 7C6C
below.) |
| 10h |
Byte |
00 |
N/A |
Must
always be zero for NTFS volumes (originally the number of
FATs for a FAT partition). |
| 11h |
2
Bytes |
0000 |
N/A |
Must
always be zero for NTFS volumes (originally the Max.
Root Directory Entries under FAT12/16). |
| 13h |
2 Bytes |
0000 |
N/A |
Most often set to
zero, but not used by NTFS (orig. used for the FAT12/16
small sectors count; must be zero for FAT32). |
| 15h |
Byte |
F8 |
"Fixed
Disk" |
Media
Descriptor ID.
( Win 2000/XP do not use it, but other code might
so it's set to F8). |
| 16h |
2
Bytes |
0000 |
N/A |
Must
be set to zero for NTFS volumes (originally for FAT12/16
"Sectors per FAT" value). |
| 18h |
Word |
003F |
63 |
Sectors
per Track |
| 1Ah |
Word |
00FF |
255 |
Number
of Heads (Sides) |
| 1Ch |
Double
Word |
0000003F |
63 |
Number
of "Hidden
Sectors" (Cyl=0 Head=0) |
| Under Vista and 7 |
00000800 |
2048 |
(Cyl=0 Head=20h Sect=21h) |
| Reference |
Note: Under Windows™ Vista and Windows™ 7, the number of "Hidden" or Reserved Sectors for the first partition has been increased to 2048 (0x800) rather than 63.
" Count of hidden
sectors preceding the partition that contains this [NTFS] volume. This
field is generally only relevant for media visible on interrupt 0x13.
This field should always be zero on media that are not partitioned.
Exactly what value is appropriate is operating system specific."
[ Although I'm not exactly
sure what all the implications are of this quote from Microsoft,
I can tell you that this value is definitely used while reading
the boot sectors into Memory! See line: 7CCF
in the Subroutine below. This value is supposed
to be the number of physical sectors on the disk preceding the first
sector of this volume; it's obvious why the first entry in a disk's
partition table has a 63 here, but it will also be a 63 for each Logical
volume on a disk since each Logical volume in an Extended Partition
is preceded by its own Extended MBR. So, this value varies only
for an NTFS partition that is a 2nd, 3rd or 4th Primary Partition.
Note: If you see a value of 29 (1D
hex) in this field, it's most likely because the volume was created
on a Dynamic Disk.]
Is
it safe to change this value for booting a Windows OS? & Can
Win 2000/XP boot from an Extended partition?
|
| 20h |
4
Bytes |
00000000 |
N/A |
Most
often set to zero, but not used by NTFS (originally used
for the Total Number of Sectors in a FAT32 volume). |
| After
loading into Memory ( and carrying out the instructions in the
subroutine at offsets 7C7B and following ),
offset 7C20h is used to store the Total Number of Sectors
in the partition we're trying to boot ; see line: 7CA5
for the reference. |
| 24h |
4
Bytes |
80008000 |
First
byte:
80h
is the drive number ! |
An NTFS OS always
fills these bytes with:
"80008000" |
| According
to most documentation, this area is not supposed to be used by
NTFS, but after examining the code in the Boot Record,
it seems clear that the first byte of these four bytes is used to
set the drive number! (See offsets: 7C7B
and 7CAF). |
| 28h |
Long
Long
Word
(8 Bytes) |
00000000
00D739AE |
14,105,006 |
Total Sectors
( in the Volume )
Read following note! |
As we mentioned above, this value will always be 1 sector less than the
total number
of sectors in the volume's partition table entry, because an NTFS
"Backup Sector"
is not considered part of the NTFS Volume. |
| 30h |
Long
Long
Word
(8 Bytes) |
00000000
00000004 |
4 |
Starting
Cluster Number for the $MFT File in this partition (
Logical Sector 32; if the Sectors
per Cluster value is an 8 ). |
| 38h |
Long
Long
Word
(8 Bytes) |
00000000
0000EF8D |
61,325 |
Starting
Cluster Number for the $MFTMirror File in this partition. |
| 40h |
Signed
(+/-)
Double
Word |
000000F6 |
-10
=>
1024 bytes |
Clusters
(or bytes) per
File Record Segment
(FRS). |
|
The Number of Clusters
for each MFT record, which can be a negative number
when the cluster size (8 sectors in this case)
is larger than the MFT File Record size! The value F6 hex
(which appears as 246 decimal in many utility programs)
is in fact a negative 10 (-10 decimal). However,
you obviously can not have a negative number of
clusters! So, a negative number here is instead an indicator
that the record length for any MFT File Record should be:
2 to
the power of (-1
times
"any negative
value"); which turns the result into a positive
number. For our example values above, that would be: 2 ^ (-1
x -10) = 2 ^ (10) = 1024 bytes! This
number agrees perfectly with the MFT Record sizes I've seen on real
disks. It is most unfortunate that I couldn't find any official Microsoft®
documentation for this, yet; and simply had to come to
this conclusion as others have. The
Linux-NTFS Project now has a new note (c) about this on
their page for the:
$Boot
File . |
| 44h |
Double
Word |
00000001 |
1 |
Clusters
per Index Block (or Record). |
|
An Index Record is used by NTFS structures
such as a Directory. |
| 48h |
Long
Long
Word
(8 Bytes) |
B4A4E199
A4E15DFC
|
- |
NTFS
Volume Serial Number. |
| |
NOTE: When you use the DIR command and others inside of a CMD/DOS-box
under Windows 2000/XP, it will show a Volume Serial Number composed
of only the last four bytes of this Long Long Word! Example:
C:\>dir
Volume in drive C is Win2000
Volume Serial Number is A4E1-5DFC
Obviously, we must look into this further to find out how all these
bytes are being used by the Operating System!
It is interesting
to note how the (3rd and 4th) and (6th and 7th)
bytes repeat here!
Do you have a Serial Number where these two sets of bytes are
not the same? |
| 50h |
Double
Word |
00000000 |
0 |
Checksum. |
| |
Has
anyone ever seen anything other than zeros in this field? I wonder if
it's ever used. |
_______________
* Detailed Notes Reference:
Microsoft Extensible Firmware Initiative / FAT32 File System
Specification
FAT: General Overview of On-Disk Format
Version 1.03, December 6, 2000. [This PDF Document's Information
gives the title as: "FAT: General Overview of On-Disk Format" and "Microsoft"
is listed as the Author with the Subject as: "FAT on disk format". The date
is given as: 12/11/2000 9:51:19AM, Created by Microsoft Word 9.0 with a file
size of: 168,960 bytes.]
References for just the NTFS BPB Table Entries
:
This data was checked against the
Information found in both:
The
Microsoft
Windows 2000 Professional Resource Kit, Chapter
32 - Disk Concepts and Troubleshooting, section "Disk
Sectors Critical to Startup" heading "NTFS Boot Sector"
Table 32.13 "BPB and Extended BPB Fields on NTFS Volumes"
and the Boot Record display field names from Power Quest's
Partition Info program for Windows NT/2000/XP (PartInNT.exe; 09/16/2002
2:04AM 859,136 bytes MD5 sum: 1f74ca66c42ff49f114b8c471d3ef88a). See
below for an example screenshot.
The NTFS “$Boot”
Sectors
As
stated above, it's been my experience that both the Windows 2000 and Windows
XP Boot Record (a single sector) plus the “Bootstrap Code”
consists of seven sectors beginning with the one we're examining in detail
on this page. But the code itself shows that all 16 sectors of the NTFS Boot
Record "area" are loaded into Memory. In NTFS file system
terminology, the first 16 sectors of an NTFS partition are known
as the $Boot (which occupies Clusters 0 through 1 on most systems; those
with 8-sector clusters).
The Second
sector always begins with the 16 Hex bytes:
05 00 4E 00 54 00 4C 00 44 00 52 00 04 00 24 00
N T L D R $
|
which is mostly
Unicode for the 5 characters: NTLDR. The Third through
Sixth sectors don't really have any outstanding features, and
the Seventh sector ends with 138 zero bytes (300 in the original
code prior to Windows™ XP SP2). On a newly formatted NTFS volume, the $Boot
is immediately followed by the $MFT (Master File Table)
which usually contains a number of sectors full of FF
bytes (often part of $Bitmap) before you can find something which is
humanly intelligible! Most NTFS volumes will also contain some Metadata
(system files) in the middle of the partition; such as, $MFTMirr (a
backup copy of the first four $MFT records) and $LogFile
(which is the NTFS journal
and can be many megabytes in size). NOTE: On volumes that have had many
files added and deleted, and especially those which have come close to running
out of space, the Mast File Table may have been relocated.
An Examination of
the Assembly Code
Here's a disassembled copy of the code (;
with comments) after being loaded into memory by the MBR at 0000:7C00 (An
asterisk '*' in front of any Assembly Instruction listed below means
that the instruction can not be disassembled correctly by MS-DEBUG):
7C00 EB52 JMP 7C54 ; Jump over BPB (Bios Parameter
; Block) to beginning of code.
7C02 90 NOP
; 7C03 thru 7C0A 'NTFS ' 8-byte OEM ID or System Name.
;
; 7C0B thru 7C53 Bios Parameter Block (BPB) See above for a detailed
; explanation of each parameter field in the BPB.
; (The data here is from a different partition than the
; one we used for the Sample "Hex Dump" above!)
7C03 4E 54 46 53 20 20 20 20 00 02 04 00 00 NTFS .....
7C10 00 00 00 00 00 F8 00 00 3F 00 FF 00 3F 00 00 00 ........?...?...
7C20 00 00 00 00 80 00 80 00 F3 90 3B 00 00 00 00 00 ..........;.....
7C30 BF F6 04 00 00 00 00 00 1E 72 07 00 00 00 00 00 .........r......
7C40 F6 00 00 00 02 00 00 00 DB 05 66 38 2B 66 38 1A ..........f8+f8.
7C50 00 00 00 00 ....
7C54 FA CLI ; Disable maskable Interrupts.
7C55 33C0 XOR AX,AX ; Zero out both the Accumulator
7C57 8ED0 MOV SS,AX ; and Stack Segment Registers.
7C59 BC007C MOV SP,7C00 ; Set Stack Pointer to 0000:7C00
7C5C FB STI ; Enable Interrupts again.
; NOTE: Since the next two instructions change the (DS) Data Segment
; Register from 0000 to 07C0, all references below to Data offsets
; (such as [0024]) are still to the same bytes in the BPB above!
7C5D B8C007 MOV AX,07C0 ;|-> This is important to know!
7C60 8ED8 MOV DS,AX ;|Set DS to 7C0 hex (with Segment=
;| 07C0, an offset such as [0024]
;| is the same as 0000:7C24), then
7C62 E81600 CALL 7C7B ; "Get Drive Parameters" and try
; to determine Total Sectors!
7C65 B8000D MOV AX,0D00 ;|
7C68 8EC0 MOV ES,AX ;|Set Extra Segment to D00 hex.
7C6A 33DB XOR BX,BX ; Zero-out the Base Register.
7C6C C6060E0010 MOV BYTE PTR [000E],10 ; 16 decimal -> 0000:7C0E
;
7C71 E85300 CALL 7CC7 ; Read all Boot Sectors into Memory
; beginning at 0D00:0000 (see below)
7C74 68000D * push 0D00 ; Segment where Code Sectors were
; stored by subroutine above.
7C77 686A02 * push 026A ; (Offset in new Code Segment.)
7C7A CB RETF ; Use FAR RETURN instruction to
; jump to 0D00:026A (or 0000:D26A)
; which is where execution resumes
; with the NTLDR Section!
; Subroutine to
Calculate Total Sectors in
; the Boot Partition (using only CHS)
7C7B 8A162400 MOV DL,[0024] ; -> 80h (which is the First HDD)
; See BPB chart for Offset 24h.
7C7F B408 MOV AH,08 ; Function 08h of INT13
7C81 CD13 INT 13 ; "Get Drive Parameters"
; After executing INT 13 Function 08h:
; AH = 00h and CF is cleared (set to 0) if successful.
; +-------------------------------------------------------+
; | CH = Low 8 bits of Maximum Cylinder Number |
; |CL (bits 7-6) = High 2 bits of " " " |
; +-------------------------------------------------------+
; CL (bits 5-0) = Maximum Sector Number.
; DH = Maximum Head Number.
; DL = Number of drives on the system.
7C83 7305 JNB 7C8A ; Jump if successful, CF=0 (NoCarry)
; we have an easy partition here!
7C85 B9FFFF MOV CX,FFFF ; Otherwise: Set CX = FFFFh and
7C88 8AF1 MOV DH,CL ; DH = FFh which will
; result in a count of 00FC0000h or
; 16,515,072 sectors being placed into
; [0000:7C20] and following.
7C8A 660FB6C6 * movzx eax,dh ; Move with Zero Extend DH -> EAX
7C8E 40 INC AX ; Maximum Head number + 1 = Total Heads
7C8F 660FB6D1 * movzx edx,cl ; CL = Maximum Sector number (bits 5-0)
7C93 80E23F AND DL,3F ; Pick up only bits 5 through 0 and
7C96 F7E2 MUL DX ; mutiply by the number of Heads.
; EAX now equals [Sectors * Heads].
7C98 86CD XCHG CL,CH ; Put the low bits of Maximum Cylinder
; Number at the end of the CX register,
7C9A C0ED06 * shr ch,06 ; and shift bits 7-6 of CL next to them
; so that CX now equals Max Cyl number!
7C9D 41 INC CX ; CX + 1 = Total Cylinders.
7C9E 660FB7C9 * movzx ecx,cx ; Move with Zero Extend CX -> ECX
7CA2 66F7E1 * mul ecx ; EAX = [Sectors * Heads] * Cylinders
; = Total Number of Sectors.
7CA5 66A32000 * mov [0020],eax ; Store Total Number of Sectors
; in [0000:7C20] (4-byte Word).
7CA9 C3 RET
; NOTE: A 4-byte Word for Total Sectors allows for almost 2048 Binary GB!
; ( FFFFFFFFh = 4,294,967,295 sectors = 2,199,023,255,040 bytes! )
; However, this subroutine can return a Maximum of only about 8.45 GB;
; [ 63 sectors * 255 heads * 1024 cylinders (2 + 8 bits -> 3FFh) =
; 16,450,560 sectors = 8,422,686,720 bytes (or only about 7.8 Binary GB)].
; (Also see my Disk Terms page for a reality note about a 'bug' in MS-DOS!)
; So, if there's an error in determining the Sector Count, this subroutine
; returns a count of: 63 * 256 * 1024 = 16,515,072 sectors; which is not
; really possible using only CHS notation and standard BIOS functions.
;
Subroutine to test for INT13 Extensions
7CAA B441 MOV AH,41 ; Function 41h of INT 13:
7CAC BBAA55 MOV BX,55AA ; (with BX = 55AAh) is a check for
; the "Installation of the INT 13
; BIOS Extensions" in Memory.
7CAF 8A162400 MOV DL,[0024] ; -> 80h (for First drive).
7CB3 CD13 INT 13
7CB5 720F JB 7CC6 ; Return
7CB7 81FB55AA CMP BX,AA55
7CBB 7509 JNZ 7CC6 ; Return
7CBD F6C101 TEST CL,01
7CC0 7404 JZ 7CC6 ; Return
7CC2 FE061400 INC BYTE PTR [0014]
7CC6 C3 RET
;
Subroutine to Read all 16 Sectors of Boot Code into Memory
7CC7 6660 * pushad ; Push all "Double" Registers onto the
; stack: EAX, ECX, EDX, EBX, original
; ESP, EBP, ESI, and EDI.
7CC9 1E PUSH DS
7CCA 06 PUSH ES
7CCB 66A11000 * mov eax,[0010] ; "Must always be zero for NTFS!"
7CCF 6603061C00 * add eax,[001C] ; Number of "Hidden Sectors"
7CD4 663B062000 * cmp eax,[0020] ; Apparent Sectors in partition.
7CD9 0F823A00 * jb 7D17 ; Can't have more "Hidden Sectors"
; than the total you start with!
7CDD 1E PUSH DS
7CDE 666A00 * push 00000000
7CE1 6650 * push eax
7CE3 06 PUSH ES
7CE4 53 PUSH BX
7CE5 6668100001 * push 00010010
00
7CEB 803E140000 CMP BYTE PTR [0014],00
7CF0 0F850C00 * jnz 7D00
7CF4 E8B3FF CALL 7CAA ; Check for Extended INT13 code.
7CF7 803E140000 CMP BYTE PTR [0014],00
7CFC 0F846100 * jz 7D61 ; Extensions are not installed.
; -> "A disk read error..." etc.
7D00 B442 MOV AH,42 ; Function 42h of INT 13:
; " Extended Read "
7D02 8A162400 MOV DL,[0024] ; -> "80h" (first hard drive).
7D06 16 PUSH SS
7D07 1F POP DS
7D08 8BF4 MOV SI,SP
7D0A CD13 INT 13
7D0C 6658 * pop eax
7D0E 5B POP BX
7D0F 07 POP ES
7D10 6658 * pop eax
7D12 6658 * pop eax
7D14 1F POP DS
7D15 EB2D JMP 7D44
7D17 6633D2 * xor edx,edx
7D1A 660FB70E18 * movzx ecx,word [0018] ; [7C18] = Sectors per Track
00
7D20 66F7F1 * div ecx
7D23 FEC2 INC DL
7D25 8ACA MOV CL,DL
7D27 668BD0 * mov edx,eax
7D2A 66C1EA10 * shr edx,10
7D2E F7361A00 * div word [001A] ; [7C1A] = Number of Heads
7D32 86D6 XCHG DL,DH
7D34 8A162400 MOV DL,[0024] ; [7C24] = Hard Drive Number
7D38 8AE8 MOV CH,AL
7D3A C0E406 * shl ah,06
7D3D 0ACC OR CL,AH
7D3F B80102 MOV AX,0201 ; Function 02h of INT 13:
7D42 CD13 INT 13 ; "Read Sector into Memory"
7D44 0F821900 * jb 7D61 ; -> "A disk read error..." etc.
7D48 8CC0 MOV AX,ES
7D4A 052000 ADD AX,0020 ; Add another 200h (512 bytes) to
7D4D 8EC0 MOV ES,AX ; the Segment pointer in ES.
7D4F 66FF061000 * inc dword [0010] ;
7D54 FF0E0E00 DEC WORD PTR [000E] ; Decrement the Count of how
; many Sectors are left to read
7D58 0F856FFF * jnz 7CCB
7D5C 07 POP ES
7D5D 1F POP DS
7D5E 6661 * popad
7D60 C3 RET
7D61 A0F801 MOV AL,[01F8] ; Contains 83h (Pointer value for
; all Languages to Message Area.)
; -> "A disk read error occurred"
7D64 E80900 CALL 7D70 ; DISPLAY MESSAGE
7D67 A0FB01 MOV AL,[01FB] ; Contains C9h (for English string).
; -> "Press Ctrl+Alt+Del to restart"
7D6A E80300 CALL 7D70 ; DISPLAY MESSAGE
7D6D FB STI
7D6E EBFE JMP 7D6E ; Endless Loop -> Lock-up System,
; so a reboot is necessary!
7D70 B401 MOV AH,01 ; An alternate method to add 100h to
; the AX register for the Message
7D72 8BF0 MOV SI,AX ; Offsets in AL -> Error Messages.
7D74 AC LODSB ; Load Error Message strings 1 byte
; at a time and INC SI pointer.
7D75 3C00 CMP AL,00 ; Look for Zero-byte terminator in
; the Error Message strings.
7D77 7409 JZ 7D82 ; Return (done)
7D79 B40E MOV AH,0E ; Function 0Eh of INT 10:
; "Teletype Output" (One Character)
7D7B BB0700 MOV BX,0007 ; (BH=Page Zero; BL=Normal display)
7D7E CD10 INT 10
7D80 EBF2 JMP 7D74 ; Do it all over again . . .
7D82 C3 RET
Location of Error Messages and
Message Offsets
in Memory
0 1 2 3 4 5 6 7 8 9 A B C D E F
7D83 0D 0A 41 20 64 69 73 6B 20 72 65 61 64 ..A disk read
7D90 20 65 72 72 6F 72 20 6F 63 63 75 72 72 65 64 00 error occurred.
7DA0 0D 0A 4E 54 4C 44 52 20 69 73 20 6D 69 73 73 69 ..NTLDR is missi
7DB0 6E 67 00 0D 0A 4E 54 4C 44 52 20 69 73 20 63 6F ng...NTLDR is co
7DC0 6D 70 72 65 73 73 65 64 00 0D 0A 50 72 65 73 73 mpressed...Press
7DD0 20 43 74 72 6C 2B 41 6C 74 2B 44 65 6C 20 74 6F Ctrl+Alt+Del to
7DE0 20 72 65 73 74 61 72 74 0D 0A 00 00 00 00 00 00 restart........
7DF0 00 00 00 00 00 00 00 00 83 A0 B3 C9 00 00 55 AA ..............U.
|
Free Boot Record Tools
See my Free Tools
page for a link to
PQ's Partition Info Tools (Power Quest's Partition Information viewer
will list all the fields in an NTFS Bios Parameter Block). The following is
a display similar to what you'll see using PQ's Partition Info program
for NT (after clicking on the "Boot Record" button):
Entries 1, 2 and
23 are in the first sector of the Boot Record, but are not part
of the BPB or Extended BPB. All of the red colored
boxes show entries which must be zero in order for this to
be an NTFS Boot Record. Note the eight-byte Serial
Number in the pink box. Although documentation from Microsoft
indicates that the data in the 'white' boxes (5, 8, 14 and 15) is unused
by NTFS systems, I've seen nothing but zeros and the '80 00 80
00' entry in box 15.
Updated: June 6, 2007 (2007.06.06).
Updated: Jan. 29, 2009 (29.01.2009); Feb. 1, 2009 (01.02.2009);
March 1, 2009 (01.03.2009);
March 8, 2009 (08.03.2009).
Last Update: August 16, 2009 (16.08.2009).
You can write to me using this: contact
page (opens in a new window).
MBR and Boot Records Index
The Starman's
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