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8.4 GB Barrier

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Disk drive technology, as always, is advancing towards higher capacity devices. However, limitations on addressing drives with capacities larger than 8.4 GB could possibly slow down acceptance of these drives in the marketplace. Addressing limitations are caused mainly by the BIOS (basic input/output system) of computer systems. As a result, many new systems may show only 7.9 GB or 8.4 GB of capacity when drives larger than 8.4 GB are used as mass storage devices.

Executive Summary

Addressing more than 8.4GB of capacity on a hard disk drive is a problem for most PCs because of BIOS limitations. The best way to solve the problem is to use a BIOS that utilizes Interrupt 13 extensions and an LBA (logical block address) method of addressing. If a system's BIOS can’t be upgraded, OnTrack Disk Manager driver software (available on Quantum’s World Wide Web) can be used to move beyond the 8.4GB limitation.

Traditional Addressing

In the original IBM PC AT, the BIOS passed addresses to the disk service interrupt (INT 13) using three parameters: cylinder, head and sector (CHS). Since conventional INT 13 functions use 24 bits to represent addresses, this traditional CHS method permits 8.4GB ( 224 * 512 bytes/sector) to be addressed.

Extended INT 13 and LBA Addressing

To go beyond the 8.4GB limit, extended INT 13 functions, which use 32 bits to represent addresses, were created. Although a BIOS relying on these extended routines could conceivably use CHS methodology to pass the address limitation, this is not recommended. Because hard disk drives traditionally "range check" a CHS address, they could possibly reject values outside the 8.4GB range.

As a result, BIOS developers are encouraged to use the LBA addressing method, which passes a single parameter to represent the address. Since an ATA hard disk drive can accept a single address of 28 bits, the LBA method permits addressing of 137.4GB (228 * 512 bytes/sector).

New Systems

PC manufacturers have the ability to program the BIOS in their new systems. These manufacturers should select a BIOS that conforms to the "Enhanced BIOS Service for Disk Drives" report from ANSI1, which recommends the use of LBA addressing.

Installed Base of Systems

Many recent vintage PCs allow their BIOS to be upgraded. To address a hard disk drive capacity greater than 8.4GB, the ideal solution is to upgrade the PC’s BIOS with one that uses the addressing mode described above. PC manufacturers are the appropriate contacts to determine if BIOS upgrades are possible.

If the BIOS cannot be upgraded, the 8.4GB limitation can be overcome with driver software from a number of independent vendors. Quantum has worked closely with OnTrack to modify their Disk Manager software (Revision 9.x and higher) to ensure full addressing of Quantum hard disk drives larger than 8.4GB. This driver is available at no charge from Quantum's web site at (choose "Software and Utilities," and then choose the most current version of OnTrack Disk Manager software).

Operating System Limits

It should be noted that neither the correct BIOS nor drivers such as Disk Manager can overcome limitations inherent in operating systems. For example, in Windows 3.1x and early versions of Windows 95, 16 bits are available for cluster addressing in the file allocation table (thus the term, FAT16). Since each address specifies a cluster of up to 32KB, the maximum partition size is 2.1GB. So while hard disk drives larger than 8.4GB can be used with these FAT16 operating systems, multiple partitions no larger than 2.1GB each are required.

This limitation is overcome in later OEM versions of Windows 95, with 32 bits available for addressing in the file allocation table (FAT32). FAT32 will also be included in Windows 98. Unfortunately, Microsoft has not made available in the retail market an upgrade of early versions of Windows 95 from FAT16 to FAT32. To determine which FAT version of Windows is running on a given PC, you may use the following method(s):

For Windows 95 or Windows 3.1

  • On the menu bar, select "Start"
  • Select "Settings"
  • Then select "Control Panel"
  • Find the "System" icon or script and double click
  • In the "General" page of "Systems Properties", the system section will have the number 4.00.950B if FAT 32
  • The system section will have the number 4.00.950(A) if FAT 16

For DOS (under Windows)

  • At the "C" prompt, type "ver"
  • For FAT 32 OS, the line will read [Version 4.00.1111]
  • For FAT 16 OS, the line will read [Version 4.00.950]

Another operating system limitation is presented by Windows NT. Specifically, the first partition, or boot partition, cannot be larger than 4GB. Because of this, under Windows NT, a hard disk drive larger than 8.4GB must have at least two partitions.

See Also