BIOS Translation

BIOS Translation

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BIOS translation is one of the most important techniques used to break the 504 MB disk size barrier resulting from the combination of BIOS and IDE/ATA hard disk restrictions. This is often a confusing subject, therefore we have provided more detail regarding the various types of BIOS translations that are in use in PCs. A BIOS that supports the extended CHS and/or LBA modes is referred to as an enhanced BIOS.

The table below is a comparison of the three translation modes, and shows a summary of them in the overall path from the operating system, through the BIOS, to the hard disk controller, to the physical drive:

Interface	Standard CHS	Extended CHS (ECHS) / Large	Logical Block Addressing
Physical Drive Platters to Integrated Disk Controller	Physical Geometry	Physical Geometry	Physical Geometry
Integrated Disk Controller to BIOS	Logical Geometry	Logical Geometry	Logical Block Address
BIOS to Operating System and Applications (through Int 13h)	Logical Geometry	Translated Geometry	Translated Geometry

Notice how, in all cases, the physical geometry is hidden within the hard disk itself. In the case of LBA, the logical geometry is presented to the BIOS only to have it translated for use by the operating system. Communication between the BIOS and the hard disk controller occurs through the use of the logical block address.

Note: When Installing Windows 2000 – Setup Does Not Check for INT-13 Extensions Before Creating System Partition.

When you install Windows 2000, Setup allows you to delete, create, and format the system/boot partition. Windows 2000 does not impose any restriction on the size of the system/boot partition because it can format the partition using the FAT file system (up to 2 gigabytes, or GB), the FAT32 file system (up to 32 GB) or the NTFS file system (up to 2 terabytes) before copying the Setup files. The system partition is defined as the partition containing the files needed for the initial system startup. For Windows 2000, these are the Ntdetect.com, NTLDR, Boot.ini, and sometimes Ntbootdd.sys files.

Windows 2000 supports large system partitions because it has the ability to use BIOS INT-13 extensions to boot the operating system on partitions with more than 1,024 cylinders, or 7.8 GB in size. After Windows 2000 Setup copies files, it restarts the computer to continue with GUI-mode Setup.

To read more about this issue, visit the Microsoft Knowledge Base and Review Article Q240672.