Hardware Reference
In-Depth Information
As you can see, the ATA interface uses different-sized fields to store CHS values. Note that the ATA
limits are higher than the BIOS limits for cylinders and sectors but lower than the BIOS limit for
heads. The CHS limits for capacity according to the ATA-1 through ATA-5 specification are as
follows:
Max. Values
-----------------------------
Cylinders 65,536
Heads 16
Sectors/Track 255
=============================
Total Sectors 267,386,880
-----------------------------
Total Bytes 136,902,082,560
Megabytes (MB) 136,902
Mebibytes (MiB) 130,560
Gigabytes (GB) 136.9
Gibibytes (GiB) 127.5
When you combine the limitations of the BIOS and ATA CHS parameters, you end up with the
situation shown in
Table 7.19
.
Table 7.19. Combined BIOS and ATA CHS Parameter Limits
As you can see, the lowest common denominator of the combined CHS limits results in maximum
usable parameters of 1,024 cylinders, 16 heads, and 63 sectors, which results in a maximum drive
capacity of 528MB. This became known as the 528MB barrier (also called the 504MiB barrier), and
it affects virtually all PCs built in 1993 or earlier.
CHS Translation (Breaking the 528MB Barrier)
Having a barrier limiting drive capacity to 528MB or less wasn't a problem when the largest drives
available were smaller than that. But by 1994, drive technology had developed such that making
drives larger than what the combined BIOS and ATA limitations could address was possible. Clearly
a fix for the problem was needed.
Starting in 1993, the BIOS developer Phoenix Technologies began working on BIOS extensions to
work around the combined CHS limits. In January of 1994, the company released the “BIOS
Enhanced Disk Drive (EDD) Specification,” which was later republished by the T13 committee (also
responsible for ATA) as “BIOS Enhanced Disk Drive Services (EDD).” The EDD documents detail
several methods for circumventing the limitations of older BIOSs without causing compatibility
