Hardware Reference
In-Depth Information
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
capacityandswitchbetweenStandardCHS,ECHS,orLBA,theBIOScanchangethe(trans-
lated) 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 geo-
metry 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 care-
ful. 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
AlthoughCHStranslationbreaksthe528MBbarrier,itrunsintoanotherbarrierat8.4GB.
Supporting drives larger than 8.4GB requires leaving CHS behind and changing from
CHStoLBAaddressing attheBIOSlevel. TheATAinterface hadalways supportedLBA
addressing, even in the original ATA-1 specification. One problem was that LBA sup-
port 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 Spe-
cification (EDD),” which addressed this problem with new extended INT13h BIOS ser-
vices that worked with LBA rather than CHS addresses.
To ensure industrywide support and compatibility for these new BIOS functions, in 1996
Phoenix turned this document over to the International Committee on Information Tech-
nology Standards (INCITS) T13 technical committee for further enhancement and certi-
ficationasastandardcalledthe“BIOSEnhancedDiskDriveSpecification(EDD).”Start-
ing 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. Coin-
cidentally (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
