Citation :
PSS ID Number: Q224526
Article last modified on 10-23-2000
:3.5,3.51,4.0,4.0 SP1,4.0 SP2,4.0 SP3,4.0 SP4,4.0 SP5,4.0 SP6,4.0 SP6a
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The information in this article applies to:
- Microsoft Windows NT Server versions 3.51, 4.0, 4.0 SP1, 4.0 SP2, 4.0 SP3, 4.0 SP4, 4.0 SP5, 4.0 SP6, 4.0 SP6a
- Microsoft Windows NT Server versions 4.0, 4.0 SP4, Terminal Server Edition
- Microsoft Windows NT Server, Enterprise Edition versions 4.0, 4.0 SP4
- Microsoft Windows NT Workstation versions 3.5, 3.51, 4.0, 4.0 SP1, 4.0 SP2, 4.0 SP3, 4.0 SP4, 4.0 SP5, 4.0 SP6, 4.0 SP6a
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SUMMARY
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Although Windows NT 4.0 can in theory support partitions of up to 16 exabytes in
size using the NTFS file system, the maximum size of the system partition is
limited to 7.8 gigabytes (GB).
MORE INFORMATION
================
The system partition is defined as the partition containing the files needed for
the initial system startup. For Windows NT, the files are Ntdetect.com, NTLDR,
Boot.ini, and sometimes Ntbootdd.sys.
A boot partition is defined as the partition containing the system files. For
Windows NT, this is the partition containing the %SystemRoot%\System32 folder.
The system partition and boot partition can be on the same partition or on
different partitions. Because there can be multiple operating systems installed
on a single computer, a computer can have multiple boot partitions, but a
computer has only a single system partition.
In some cases, the boot partition must be entirely within the first 7.8 GB of the
drive. If the Boot.ini file uses the multi() syntax for locating the boot
partition, NTLDR uses the INT13 interface to load the HAL, the kernel, and
boot-start device drivers. In this case, these files must reside within the 7.8
GB addresseable range of the INT13 interface. If the Boot.ini file uses scsi()
syntax to find the boot partition, then a file named Ntbootdd.sys should exist
on the system partition. This file is simply a renamed copy of the disk
controller driver. In this case, NTLDR uses the Ntbootdd.sys driver to access
the disk when loading the HAL, kernel, and boot-start device drivers. The
addressable area of the disk is determined by this driver.
When an Intel-based computer first boots, a number of things occur that result in
the operating system being loaded and started. This process, known as the
bootstrap process, has inherent hardware and software limitations beyond which
Windows NT cannot operate. It is these limitations that prevent Windows NT 4.0
from using a partition larger than 7.8 GB as a system partition.
During the bootstrap process, the only mechanism available to Windows NT (or any
other operating system) to access the drive is a set of functions in the BIOS
known as Interrupt 13 (INT13). The INT13 functions allow low-level code to read
from and write to the drive by addressing a specific sector on the drive. When
the INT13 architecture was developed back in the early 1980s, the possibility of
multi-gigabyte hard disks was not taken into consideration. The INT13 functions
define 24 bits to describe a sector on the hard disk. This breaks down to a
maximum of 256 heads (or sides), 1024 cylinders, and 63 sectors. With these
numbers, only 256*1024*63 (or 16,515,072) sectors can be used with INT13
functions. At a standard 512 bytes per sector, this is 8,455,716,864 bytes, or
approximately 7.8 GB. Note that for most modern drives, the computer's BIOS must
support some form of sector translation for the BIOS functions to address the
first 7.8 GB of disk space. The BIOS in virtually all modern computers supports
"Logical Block Addressing," which allows INT13 functions to address the first
7.8 GB of drive space independent of the drive's physical geometry.
The INT13 functions are the only means available to the operating system to gain
access to the drive and system partition until the operating system loads
additional drivers that allow it to gain access to the drive without going
through INT13. Therefore, Windows NT 4.0 cannot use a system partition larger
than 7.8 GB. In fact, the entire system partition must be entirely within the
first 7.8 GB of the physical disk. Windows NT can use a 7.8-GB system partition
only if the partition begins at the start of the physical drive.
NOTE: Partitions other than the system partition are not affected by the these
limitations.
Other operating systems, such as Microsoft Windows 95 OEM Service Release 2,
Microsoft Windows 98, and Microsoft Windows 2000, can boot from larger
partitions because these operating systems were written after the computer
industry defined a new standard for BIOS INT13 functions (the "INT13
extensions" ) and implemented this new functionality on manufactured
motherboards. Because Windows NT 4.0 was written before this new standard was
invented, Windows NT 4.0 is unaware of this new technology and is unable to use
its features.
When you are installing Windows NT 4.0, you can create a system partition with a
maximum size of 4 GB. This occurs because Setup first formats the partition
using the FAT file system. If you want to use an NTFS partition, the partition
is converted to NTFS after the first reboot. The FAT file system has a file
system limitation (unrelated to any BIOS limitations) of 4 GB. When you perform
an unattended installation, use of the ExtendOEMPartition directive in an
Unattend.txt file can expand the system partition to a maximum of 7.8 GB.
In the future, additional limitations may come into play as well. Although the
NTFS file system can address 16 exabytes of disk space in a single partition,
current disk-partitioning schemes store partition information in structures that
limit partitions to 2^32 sectors, or 2 terabytes, in size. The ATA hardware
interface uses 28-bit addressing, which supports drives that are 2^24 sectors,
or 137 GB, in size. These limitations may apply to partitions other than the
system partition as well.
Note that file system limitations and hardware limitations exist independently of
each other, and the most restrictive of the two is the determining factor in the
maximum partition size. Another factor to consider when you are troubleshooting
partitioning problems is that hard disk manufacturers often use "decimal
megabytes" (1 megabyte = 1,000,000 bytes), whereas Windows NT uses "binary
megabytes" (1 megabytes = 1,048,576 bytes). Using both definitions of a megabyte
in calculations can often account for "lost" disk space. Also, this article
assumes a sector size of 512 bytes in all calculations. Although a 512-byte
sector has become a de facto industry standard, it is possible that disk
manufacturers could produce drives with a different sector size. This would
result in a corresponding change in partition limits. Partitions are based on
cylinder, head, and sector calculations, not on byte calculations. Therefore, a
change in bytes per sector causes a change in bytes per partition.
REFERENCES
==========
For additional information about disk partitioning and limitations, please see
the following articles in the Microsoft Knowledge Base:
Q114841 Windows NT Boot Process and Hard Disk Constraints
Q119497 Boot Partition Created During Setup Limited to 4 Gigabytes
Q197667 Installing Windows NT Server on a Large IDE Hard Disk
Q185773 NTFS Corruption on Drives > 4 GB Using ExtendOEMPartition
Q227879 Formatting Using the Compaq Array Configuration Utility
Additional query words: smallbiz
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Keywords :
Version : :3.5,3.51,4.0,4.0 SP1,4.0 SP2,4.0 SP3,4.0 SP4,4.0 SP5,4.0 SP6,4.0 SP6a
Issue type : kbinfo
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Copyright Microsoft Corporation 2000.
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