Hardware Reference
In-Depth Information
LBA, as selected in the BIOS Setup, indicates LBA-assist translation, not pure LBA mode. This
enables software to operate using L-CHS parameters while the BIOS talks to the drive in LBA mode.
The only way to select a pure LBA mode, from the OS to the BIOS as well as from the BIOS to the
drive, is with a drive that is over 8.4GB. All drives over 137GB must be addressed via LBA at both
the BIOS and drive levels, and most PC BIOSs automatically address any drive over 8.4GB in that
manner, as well. In that case, no special BIOS Setup settings are necessary, other than setting the type
to auto or autodetect.
Caution
A word of warning with these BIOS translation settings: If you have a drive 8.4GB or less in
capacity and switch between Standard CHS, ECHS, or LBA, the BIOS can change the
(translated) geometry. The same thing can happen if you transfer a disk that has been formatted
on an old, non-LBA computer to a new one that uses LBA. This causes the logical CHS
geometry seen by the operating system to change and the data to appear in the wrong location
from where it actually is! This can cause you to lose access to your data if you are not careful. I
always recommend recording the CMOS Setup screens associated with the hard disk
configuration so that you can properly match the setup of a drive to the settings to which it was
originally set. This does not affect drives over 8.4GB because in those cases pure LBA is
automatically selected.
The 8.4GB Barrier
Although CHS translation breaks the 528MB barrier, it runs into another barrier at 8.4GB. Supporting
drives larger than 8.4GB requires leaving CHS behind and changing from CHS to LBA addressing at
the BIOS level. The ATA interface had always supported LBA addressing, even in the original ATA-
1 specification. One problem was that LBA support at the ATA level originally was optional, but the
main problem was that there was no LBA support at the BIOS interface level. You could set LBA-
assist translation in the BIOS Setup, but all that did was convert the drive LBA numbers to CHS
numbers at the BIOS interface level.
Phoenix Technologies recognized that the BIOS interface needed to move from CHS to LBA early on
and, beginning in 1994, published the “BIOS Enhanced Disk Drive Specification (EDD),” which
addressed this problem with new extended INT13h BIOS services that worked with LBA rather than
CHS addresses.
To ensure industry-wide support and compatibility for these new BIOS functions, in 1996 Phoenix
turned this document over to the International Committee on Information Technology Standards
(INCITS) T13 technical committee for further enhancement and certification as a standard called the
“BIOS Enhanced Disk Drive Specification (EDD).” Starting in 1998, most of the other BIOS
manufacturers began installing EDD support in their BIOS, enabling BIOS-level LBA mode support
for ATA drives larger than 8.4GB. Coincidentally (or not), this support arrived just in time because
ATA drives of that size and larger became available that same year.
The EDD document describes new extended INT13h BIOS commands that allow LBA addressing up
to 2
64
sectors, which results in a theoretical maximum capacity of more than 9.44ZB (zettabytes, or
quadrillion bytes). That is the same as saying 9.44 trillion GB, which is 9.44×10
21
bytes or, to be
more precise, 9,444,732,965,739,290,427,392 bytes! I say theoretical capacity because even though
