Hardware Reference
In-Depth Information
If your BIOS does not support EDD (drives over 8.4GB), the three possible solutions are as follows:
• Upgrade your motherboard BIOS to a 1998 or newer version that supports >8.4GB.
• Install a BIOS upgrade card, such as the UltraATA cards from
www.siig.com
.
• Install a software patch to add >8.4GB support.
Of these, the first one is the most desirable because it is usually free. Visit your motherboard
manufacturer's website to see whether it has newer BIOSs available for your motherboard that
support large drives. If it doesn't, the next best thing is to use a card such as one of the UltraATA
cards from SIIG (
www.siig.com
)
. I almost never recommend the software-only solution because it
merely installs a software patch in the boot sector area of the hard drive, which can result in
numerous problems when booting from different drives, installing new drives, or recovering data.
The most recent 2.2TB barrier is not a true BIOS barrier in the same way that the previous barriers
were. The issue here is not that the BIOS can't recognize drives 2.2TB or larger; the problem is that it
can't normally
boot
from them. Booting from a 2.2TB or larger drive requires a UEFI (Unified
Extensible Firmware Interface) BIOS, or at a minimum one with an enabled UEFI Boot option.
Drives larger than 2.2TB can be used as data drives even without a UEFI BIOS. Finally, note that
both booting from and recognizing a 2.2TB or larger drive as a data drive also requires that the drive
be formatted using a GPT (GUID Partition Table). The operating system must have GPT support as
well.
CHS Versus LBA
There are two primary methods to address (or number) sectors on an ATA drive. The first method is
called
CHS
(cylinder head sector) after the three respective coordinate numbers used to address each
sector of the drive. The second method is called
LBA
(logical block address) and uses a single
number to address each sector on a drive. CHS was derived from the physical way drives were
constructed (and is how they work internally), whereas LBA evolved as a simpler and more logical
way to number the sectors regardless of the internal physical construction.
For more information on cylinders, heads, and sectors as they are used internally within the drive,
see the
Chapter 9
section, “
HDD Operation
,”
p.
466
.
The process of reading a drive sequentially in CHS mode starts with cylinder 0, head 0, and sector 1
(which is the first sector on the disk). Next, all the remaining sectors on that first track are read; then
the next head is selected; and then all the sectors on that track are read. This goes on until all the
heads on the first cylinder are read. Then the next cylinder is selected, and the sequence starts again.
Think of CHS as an odometer of sorts: The sector numbers must roll over before the head number can
change, and the head numbers must roll over before the cylinder can change.
The process of reading a drive sequentially in LBA mode starts with sector 0, then 1, then 2, and so
on. The first sector on the drive in CHS mode would be 0,0,1, and the same sector in LBA mode
would be 0.
As an example, imagine a drive with one platter, two heads (both sides of the platter are used), two
tracks on each platter (cylinders), and two sectors on each track. We would say the drive has two
cylinders (tracks per side), two heads (sides), and two sectors per track. This would result in a total
capacity of eight (2 × 2 × 2) sectors. Noting that cylinders and heads begin numbering from 0—
whereas physical sectors on a track number from 1. Using CHS addressing, we would say the first
sector on the drive is cylinder 0, head 0, sector 1 (0,0,1); the second sector is 0,0,2; the third sector is
