UPGRADING REPAIRING PCs

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Chapter 4—SCSI and IDE Hard Drives and Optical Drives
A BIOS that supports only Standard CHS recognizes only a maximum
of 1,024 cylinders, 16 heads, and 63 sectors per track for any
IDE/ATA drive. Thus, if you install a 6.4GB IDE/ATA drive in a system
with this type of BIOS, it will recognize only 504MB with MS-
DOS and Windows. Non-DOS operating systems such as Novell
NetWare, UNIX, and Linux don’t require translation if they will be
the only operating system on the disk partition.
On systems that provide translation, this BIOS mode is called
Normal because the geometry isn’t changed. Configuring a drive to
use Normal mode is correct for operating systems such as UNIX,
Linux, and Novell NetWare, but not for systems that use MS-DOS
file structures, including MS-DOS itself, Windows 9x/NT/2000/Me,
and OS/2.
The other two modes, Extended CHS and LBA (Logical Block
Addressing), do translate the geometry. Extended CHS is also called
Large mode and is recommended only for >504MB drives that cannot
be operated in LBA mode. Most enhanced BIOSs don’t offer
Large mode, but all offer LBA mode. However, a few (such as older
Acer BIOSs) might call it something different, such as DOS mode or
>504MB mode.
Using LBA Mode
LBA mode can be enabled in two ways, depending on the BIOS. On
most current BIOSs, using the automatic detection option in the
BIOS or during system boot will detect the basic hard drive geometry
and select LBA mode automatically. On some BIOSs, though,
the automatic detection sets up the basic cylinder-head-sectors per
track drive geometry but doesn’t enable LBA mode unless you set it
yourself. Depending on the BIOS release used by a given system,
the LBA mode setting can be performed on the same BIOS configuration
screen used for standard drive configuration, or it might be
located on an Advanced CMOS configuration or Peripheral Setup
screen.
A BIOS that performs LBA translation should enable you to use an
IDE hard drive as large as 8.4GB with MS-DOS. If you find that you
can use a 2.1GB hard disk, but not larger ones, the version of LBA
mode supported by your BIOS is a very early version, and your
BIOS should be updated. Support for drives larger than 8.4GB is
discussed later in this chapter.
When LBA Mode Is Necessary—and When Not to Use It
Use Table 4.3 to determine when to use LBA mode.