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About Drives

Bios Limitations

An important issue when upgrading your computer is whether your current BIOS will support a larger capacity drive. Often manufacturers have programmed the BIOS with a limitation. If this is the case, your system may not recognize the full capacity of your drive. For example, this means if you try to install a 20gb hard drive, your system will only recognize the drive as an 8.5gb.

If your current hard drive is an 8gb or smaller, you may have BIOS limitations. If your system has a drive larger than 8.4gb, then your BIOS should be able to recognize drives up to 60gb. You may verify your BIOS limitations at the Manufacturers web site.

There is usually a solution to overcome the BIOS limitations. You can obtain an upgraded BIOS from the notebook manufacturer and "Flash" your BIOS or you can try visiting the manufacturers web site to download and install the latest BIOS upgrade available for your computer.

If there aren't any BIOS upgrades available, you can use "Drive Overlay Software" to override the BIOS limitations.

Below are some detailed BIOS limitations:

33.8-Gbyte Limitation
If your drive is greater than 33.8 Gbytes, your system BIOS may freeze or lockup at Power On Self Test (POST). If the BIOS does not freeze, it may show the wrong capacity for the drive. Similar to the 8.4 Gbyte limit below, there are three methods to overcome this limitation:

  • A third party device driver such as Disk Manager.
  • An intelligent ATA Host Adapter (e.g., Promise Technology)
  • A system BIOS upgrade (contact the system BIOS manufacturer)

If the system BIOS can display the full capacity of your drive, a FAT32 file allocation system (i.e., Windows 95 OSR2, Windows 98, Win Me) or an NTFS file allocation system (e.g., WinNT) is required to show partition sizes over 2.1 Gbytes.

8.4-Gbyte Limitation
If your drive is larger than 8.4 Gbytes, the capacity may exceed the limits of your system BIOS and operating system. Most system BIOSs cannot support ATA drives this large. DOS and Windows operating systems limit the drive capacity to 8.4 Gbytes per physical drive and 2 Gbytes per partition. Because of these limitations, a 32-bit file allocation table (FAT32) is required to acheive full capacity of your drive, beyond 8.4 Gbytes. To acheive full capacity of your drive, you will need BIOS support for drives greater than 8.4 Gbytes and a Windows operating system that supports FAT32.

This support is available by way of the following methods:

  • A third party device driver, such as Disk Manager
  • An intelligent ATA Host Adapter
  • A system BIOS upgrade

3.27-Gbyte (6322 Cylinder) Limitation
Some computers have a BIOS that does not properly handle a cylinder value over 6322. If you are in the CMOS attempting to set the cylinder value higher than 6322 (for a 3.27 Gbyte+ drive) and your computer hangs, your computer may have a BIOS with this limitation. To by pass the limitation, you have two options:

  • Set the cylinder value to 1024 or less and use Disk Manager to provide support for the whole drive.
  • Contact your computer manufacturer for a BIOS upgrade.

Invalid BIOS Information
Some computers have a BIOS that may display invalid information in the CMOS. This issue may show up in one of two ways:

  • The CMOS will display invalid drive parameters. However, the BIOS is translating the drive correctly.
  • The CMOS will display the drive parameters and capacity correctly. However, the BIOS is not translating the drive correctly.

To ensure your drive is translated to its full capacity, you will need to check the actual drive size. This can be done when creating partitions on the drive.

2.1-Gbyte (4096 Cylinder) Limitation
Some computers have a BIOS that does not properly deal with the "13th bit". The 13th bit is needed to provide support for a drive having 4096 or more cylinders. The chart below displays the corresponding cylinder values in decimal, hex, and binary values.

Decimal Hex Binary Size

1023 = 3FF = 10 bits = 528 Mbytes
2047 = 7FF = 11 bits = 1.0 Gbytes
4095 = FFF = 12 bits = 2.1 Gbytes
8191 = 1FFF = 13 bits = 4.2 Gbytes
16383 = 3FFF = 14 bits = 8.4 Gbytes

If you have added a new drive and your system locks right after turning the power on or when you are in the CMOS, there may be several causes. Verify that the data cable is properly attached to your drive, pin 1 is correct, and the cable is not off-shifted a row of pins. If your new drive is larger than 2.1 Gbytes and the CMOS is set to "AUTO", you may have a BIOS with a 4096 or greater cylinder limitation. In this case, power off your system, remove your new drive, and follow the instructions that Disc Manager provides. When configuring CMOS, Do NOT use AUTO; choose one of the following:
1) USER DEFINABLE set to 1024 cyls 16 hds 63 sects, or
2) Drive type.

Another option is to contact your computer manufacturer to get a BIOS upgrade that will support more than 4096 cylinders.

528-Mbyte Limitation
Using the traditional ATA (IDE) interface limits the system to a maximum drive capacity of 528 Mbytes. The cause of this limitation is Int 13h (BIOS) and ATA (IDE) field sizes for the CHS (Cylinder, Head, and Sector) entries.

Because the system must perform a translation between the CHS parameters recognized by the drive and those established in the Int 13h code, parameters are limited to the smaller of the field sizes allowed for each parameter by the BIOS and the ATA (IDE) register set. The chart below displays the BIOS, ATA (IDE), and limiting field size.

Specification Int 13h BIOS ATA (IDE) Limitation
Maximum Sectors/Track 63 255 63
Number of heads 255 16 16
Number of Cylinders 1,024 65,536 1,024
Maximum Capacity 8.4 Gbytes 136.9 Gbytes 528 Mbytes

The maximum system drive capacity in a combined BIOS/ATA (IDE) setup is determined by the limiting field size 528 Mbytes. Currently, computers are being shipped with a BIOS that implements Extended Int 13h or "Logical Block Addressing" (LBA), both of which are solutions to the 528-Mbyte limitation.

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