UPGRADING REPAIRING PCs

Breaking the 504MB (528-Million-Byte) Drive Barrier 97
Figure 4.3 ATA (IDE) drive jumpers. Many drives now have eight, nine,
or ten jumper pins to allow for special configurations required on some systems
to break the 528-million-byte drive barrier (see the following sections).
Breaking the 504MB (528-Million-
Byte) Drive Barrier
Because IDE was developed in the late 1980s, the combination of
MS-DOS’s limit of 1,024 cylinders, the standard BIOS’s limit of 16
heads, and the IDE interface’s limitation of 63 sectors per track limited
the original size of IDE drives to 504MB (about 528 million
bytes). This limit was merely theoretical until 1994, when IDE
drives larger than this began to appear. A revised version of the
IDE/ATA standard, ATA-2 (also called enhanced IDE) defined an
enhanced BIOS to avoid these limits.
An enhanced BIOS circumvents the limits by using a different
geometry when talking to the drive than when talking to the software.
What happens in between is called translation. For example,
if your drive has 2,000 cylinders and 16 heads, a translating BIOS
will make programs think that the drive has 1,000 cylinders and
32 heads. The most common translation methods are listed in
Table 4.2. These methods are also followed by newer versions of
the ATA specification, such as ATA-3 and above.
Table 4.2 ATA-2 Translation Methods
BIOS Mode Operating System to BIOS BIOS to Drive Ports
Standard CHS Logical CHS Parameters Logical CHS Parameters
Extended CHS Translated CHS Parameters Logical CHS Parameters
LBA Translated CHS Parameters LBA Parameters