An interesting article, written by Maxtor.
BREAKING THE 137 GIGABYTE STORAGE BARRIER (written and copyright by Maxtor)
This appendix provides information about the 137GB storage barrier. It discusses the history, cause and the solution to overcome this barrier.
A.1Breaking the 137-Gigabyte Storage Barrier Capacity
Barriers have been a fact of the personal computer world since its beginnings in the early 1980s. At least 10 different capacity barriers have occurred in the storage industry over the last 15 years. The most notable barriers seen previously have been at 528 megabytes and then at 8.4 gigabytes. The most recent barrier, which will be surmounted in 2001, is the 137-gigabyte limit or a single ATA drive. The first ATA devices to exceed 137 gigabytes will be four-platter hard disk drives with 40 gigabytes per platter, yielding 160 gigabytes per drive. These drives will be available in the second half of 2001. Later in the same year, capacity will continue to grow to 60 gigabytes per platter, and a three-disk, 180-gigabyte device will be available and shipping. The ANSI NCITS T13 Technical Committee (also known as the ANSI ATA committee) has broken this barrier by incorporating a proposal from Maxtor into the ATA/ATAPI-6 draft standard that defines a method for 48-bit addressing on a single drive, giving more than 144 petabytes (144,000 gigabytes) of storage. In addition, the proposal from Maxtor that was incorporated into ATA/ATAPI-6 defines a method for extending the maximum amount of data that can be transferred per command for ATA devices from 256 sectors (about 131 kilobytes) to 65,536 sectors (about 33 megabytes). This new method is particularly useful for applications that use extremely large files, such as those for A/V or multimedia. The following sections will describe issues surrounding the 137-gigabyte barrier and the solution for breaking it.
A.1.1History
Many of the barriers in the past resulted from BIOS and operating system issues caused by failure to anticipate the remarkable increases in Breaking the 137GB Storage BarrierA-2Maxtor DiamondMax Plus9 60/80/120/160/200GB AT device storage capacity by the people who designed hard disk structures, access routines, and operating systems many years ago. They thought, Who will ever have xxx much storage? In some cases, the barriers were caused by hardware or software bugs not found until hard disks had grown in size beyond a certain point where the bugs would occur. Past barriers often frustrated people trying to add a new hard disk to an older system when they discovered that not all of the designed capacity of the hard disk was accessible. This inability to access the entire drive is referred to as a capacity barrier and it has been seen and overcome many times in the computer and disk drive industry. The 137-gigabyte barrier is the result of the original design specification for the ATA interface that provided only 28 bits of address for data. This specification means a hard disk can have a maximum of 268,435,456 sectors of 512 bytes of data which puts the ATA interface maximum at 137.4 gigabytes.
A.1.2Solving the 137-Gigabyte Capacity Barrier
As described earlier, the issue causing the 137-gigabyte barrier is the 28-bit addressing method of the original ATA specification. A change to expand this method was required to provide more address bits for the interface, allowing significant growth for many years to come. A critical issue in expanding the addressing capability was maintaining compatibility with the existing installed base of products.A new ATA standard, ATA/ATAPI-6, has been in the works for some time, and the latest draft of this standard resolves this issue by increasing the maximum number of bits used for addressing from 28 to 48. This solution increases the maximum capacity of an ATA device to 144 petabytes while maintaining compatibility with current ATA products.
A.1.3How is the Extension Implemented?
The 48-bit Address feature set provides a method to address devices with capacities up to approximately 144 petabytes by increasing the number of bits used to specify logical block addresses (LBAs) from 28 to 48. The feature set also provides a method to increase the number of sectors that can be transferred by a single command from 256 to 65,536 by increasing the number of bits specifying sector count to 16 bits. New commands specific to this feature set have been defined so that devices can implement the new feature set in addition to previously defined commands. Devices implementing the 48-bit Address feature set commands will also implement commands that use 28-bit addressing in order to maintain interoperability with older system components. In addition, 8-bit and 48-bit commands may be intermixed. The 48-bit Address feature set operates in LBA addressing only. Support of the 48-bit Address feature set is indicated in the IDENTIFY DEVICE response data. In a device implementing the 48-bit Address feature set, the registers used for addressing are, in fact, a two-byte deep FIFO. Each time one of these registers is written, the new content written is placed into the most recently written location and the previous content of the register is moved to previous content location. A host may read the previous content of the registers by first setting a bit in the Device Control register to 1 and then reading the desired register.
A.1.4What Do the Drives Need to Meet the Spec?
The challenge to drive manufacturers is to develop and implement new interface chips on drives that can accept and decode the new 48-bit addressing scheme. Many functions of decoding the commands sent to and from the drive are automated in the silicon of the drive interface ASIC, and this is where drive manufacturers must update their designs. Maxtor is the leader in development efforts and is the first to deliver a product with the capacity and drive technology to deliver greater than 137 gigabytes of capacity.Breaking the 137GB Storage BarrierA-4Maxtor DiamondMax Plus9 60/80/120/160/200GB AT
A.1.5What Else is involved?
Effort is required from OS vendors to increase storage device addressing up to 48 bits or more. This increase will be a significant challenge for many OS vendors that have 32-bit code models. Adapting to 48-bit commands will be easy, but most vendors will stop filling data at the 32-bit boundary and pad the upper 16 bits with zeros, leaving that space empty. The BIOS companies will also have to perform some work to recognize the increased capacity of the devices attached to the bus and allow the extended 48-bit commands to pass on to the devices. Boot partitions will also be an issue for the capacity of the drive if the BIOS does not recognize the 48-bit addressing scheme at or before the system boots the OS from the hard drive. Independent software driver efforts for legacy operating systems (Windows NT 4, Windows 98, and so on) will need to be implemented to allow higher-capacity devices to work on installed systems and recognize the maximum available capacity of the drive over the 137-gigabyte limit.
A.1.6What is the Next Barrier?
While it is true that the ATA/ATAPI-6 standard defines a method to provide a total capacity for a device of 144 petabytes, the next limit will be imposed not by the ATA devices but by many of the popular operating systems in use today. This limit will be at 2.2 terabytes (2,200 gigabytes). This barrier exists because many of todays operating systems are based on 32-bit addressing. These operating systems include many flavors of Linux, Mac OS 9.x, and Windows 95, 98, ME, NT 4, 2000, and XP (Windows XP/64-bit also has the limit because of leveraged 32-bit code).This barrier could be real as early as 2004 if current hard drive capacity rate increases continue along the same growth trends.
Appendix A: Terminology
BIOS: (an acronym for Basic Input/Output System design): The BIOS processes and redirects all data as it is being accessed and stored.
FAT: (an acronym for File Allocation Table): The FAT tells the computer where data has been stored on the hard drive.
CHS: (an acronym for Cylinders, Heads, and Sectors): The basic layout components of a hard drive. INT 13h & INT 13h extensions: protocols used for accessing data on hard drives.
Appendix B: Big Numbers
131 kilobytes =131,000 bytes a little more than 30 pages of text
33 megabytes =33,000,000 bytesmore than 8,000 pages of text or 25 300-page books
137 gigabytes =137,000,000,000 bytes more than 100,000 books, or the contents of a good library
2.2 terabytes = 2,200,000,000,000 bytes almost 2,000,000 books, or the about content of the Library of Congress
144 petabytes = 144,000,000,000,000,000 bytes120 billion books - (more than all that man has written)
9.4 zettabytes = 9,400,000,000,000,000,000,000 bytes
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