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Auto-Detecting Large ATA (IDE) Disc Drives


The original IBM PC BIOS had a capacity limitation of 528MB. The original interrupt for block devices (floppy drives and hard disc drives), called INT13, had a maximum definition of 1024 Cylinders, 256 Heads, and 63 Sectors per track. Using 512 bytes per sector, a grand total of 8.4 Gbytes is possible using CHS translation. Concurrently, ATA (IDE) disc drives have utilized CHS default translation parameters that also have a maximum of 8.4 Gbytes (16,383 Cylinders, 16 Heads, and 63 Sectors per track). ATA drives greater than 8.4 Gbytes rely on total sectors to determine the actual capacity of the drive, not CHS.

Later BIOS's would use LBA assisted translation to access capacities beyond 528 Mbytes up to 8.4 Gbytes. Today disc drives with capacity points beyond 8.4 Gbytes do not use CHS translation in BIOS. Instead, they use LBA auto-detection to address the drive's full capacity.

What To Do

  1. Select automatic drive detection if available. Choosing this option automatically configures the computer for your new drive.

  2. If your BIOS does not provide automatic drive detection, select "user-defined" drive settings and enter the CHS values. CHS values support drive capacities up to 8.4 Gbytes.

  3. Verify that LBA mode is enabled. Many BIOS use the logical block addressing (LBA) mode to access drives with capacities greater than 528 Mbytes. Most BIOSs automatically set this mode during the auto-detection process.

We recommend you use DiscWizard to evaluate your BIOS and determine whether your system needs additional BIOS support. For more information, see the DiscWizard Homepage.