"Why do Western Digital Caviar EIDE hard drives
8.4 GB and larger all have the same parameters?"
Drives 8.4 GB and larger all use the same
parameters: 16383 cylinders, 16 heads, and 63 sectors. This is due to a limitation
designed into the original IDE interface. Hard drives identify themselves to the system
BIOS using the "Identify Drive Data." The original design of the IDE interface
allowed for values up to 16383 x 16 x 63 (around 8.4 GB) to be used in the Identify Drive
Data. In order for drives to identify themselves as being larger than 8.4 GB, a new method
was needed. This new method was added on to the Identify Drive Data, and is known as the
"Extended Functions."
The extended functions communicate drive size in a
more straight forward manner, simply telling the BIOS how many sectors are available on
the drive (the capacity can then be calculated by multiplying the number of sectors times
512 bytes). Once the Extended Functions are used, the Cylinders, Heads, and Sectors on the
drive are no longer important. All hard drive manufacturers agreed that drives larger than
8.4 GB would report the maximum number of cylinders, heads and sectors, which is 16383 x
16 x 63. The actual size of the drive must be determined by the Extended Functions. Some
manufacturers may list larger parameters with their drives, but these drives will still
only be able to report 16383 x 16 x 63 in the Identify Drive Data.
In order to make use of drives larger than 8.4 GB,
both the system BIOS and the Operating System must be able to interpret Extended
Functions. Older Operating Systems such as DOS 6.22 and Windows 3.1 are not able to
interpret Extended Functions, so they will never be able to use drives larger than 8.4 GB
even with the use of EZ-Drive. Newer OS's, such as Windows 95, 98, and NT are capable of
handling larger drives. Windows 95 and 98 require that the system BIOS also support the
Extended Functions, while NT only requires that Service Pack 3 be installed. Other OS's
such as OS/2 may require special patches to access drives larger than 8.4 GB. Consult your
software manufacturer for specifics.
If your system BIOS does not support drives larger
than 8.4 GB (and most systems that are currently in use do not) then you can add this
support by using EZ-Drive to install your large drive.
This page was last modified on November 09,
1999.
Copyright 1999 Western Digital
Corporation. All rights reserved.