Specifications

Windows 137GB Capacity Barrier Version 1.0
5 7-Mar-2003
Background
Hard disc drives and Microsoft disk operating systems (OS) have evolved together in a mutually
supportive way. Many times since the introduction of the original DOS v1 operating system the
fundamental reason for an OS upgrade has to do with the support of larger and larger disc drives.
The operating system elements that deal with storage devices are the kernel, file system and
hardware specific device drivers.
Hard disc drives are engineered to ANSI standards that are followed by the drive and computer
system manufacturers. Called hard drive Interfaces, these standards define communication
protocols, timing between devices, commands and even physical measurements. These Interface
definitions are continuously revised to support new technologies and higher capacities. One of
the very first hard disc drive interfaces was released in 1982 as the ST-506 interface named after
the Seagate Technology 5-megabyte drive. This interface was eventually renamed to the MFM
interface.
Today, the two most common interfaces used in personal computer system are SCSI and ATA.
The ATA interface (also known as IDE, EIDE and ATAPI) will be the focus of this discussion.
The original ATA interface specification has been officially revised six times and is currently
named ATA-6. Prior to ATA-6, the maximum capacity disc drive that could be defined by the
interface was 137GB.
More recently, an evolutionary version of ATA called Serial ATA has entered the marketplace with
many promising enhancements. Serial ATA commands are backward compatible with the
original parallel ATA. Serial SCSI is also under development. The main difference between
serial and parallel has to do with the types of cables.
OS components and ANSI interface specifications are engineered to provide room for growth.
While there have been several file system innovations such as FAT32 and NTFS that can
address capacities up to 2TB and more, Microsoft operating system device drivers that read and
write to ATA interface disc devices follow the same interface specification as the hardware
manufacturers.
Bottom Line
Microsoft operating systems that were released or have Service Packs released prior to August
2002 have a fundamental native maximum limit accessing ATA interface disc drives of 137GB.
Motherboards and hard drive controller cards that have system BIOS released prior to ATA-6
may have the same fundamental 137GB limitation.
Names for this Capacity Limitation
Typically, a disc drives is described and defined by terms which describe its storage capacity.
For example, a disc drive has so many gigabytes. Taking this further into the drive itself, it has
one or more platters with read write recording heads on both sides of the platters. These heads
seek to concentric tracks and access individual sectors that rotate under the heads. These
sectors hold our data and each has 512 bytes. Therefore, one way to say how large is a drive is
based on its total number of sectors.
Operating systems look at these physical sectors from the simple perspective of numbering them
logically from the first one through to the last one. These are called logical blocks and the
method of managing them in an operating system is called logical block addressing or LBA. An
80GB disc drive, for example, has approximately 156 million LBAs.
Prior to ATA-6, the ATA interface defined commands capable of addressing 28 bits worth of
LBAs. Because of this foundation, the early ATA interfaces are said to use 28-bit Addressing. In