Enhanced Integrated Drive Electronics (EIDE)
The EIDE interface was developed to overcome
many of the limitations of the IDE interface. As we saw
in chapter 7, EIDE provides the capability for
addressing fixed disks with over 540 MB of storage
capacity. EIDE also provides faster data transfers and
the ability to use a CD-ROM drive in an EIDE system.
Small Computer Systems Interface (SCSI)
The Small Computer Systems Interface (SCSI) is
really a systems level interface, not just a disk interface.
SCSI (pronounnced scuzzy) uses a host adapter that plugs
into the computer. The SCSI has eight I/O ports. One
is dedicated as the interface between the host computer
and the adapter. The other seven ports are available for
other device controllers, such as disk drives, CD-ROM
readers, and digital scanners.
The SCSI is a smart interface. When the host
computer requests data from a device connected to the
SCSI, the SCSI will disconnect itself to free up the
computer while it processes the request. The SCSI is
capable of transferring data at up 100 megabits per
FORMATTING FIXED DISKS
Fixed disk systems operate in much the same
manner as the floppy disks and the disk memory set.
Before anew fixed disk drive can be used in a personal
computer, it must be formatted. The formatting of a
fixed disk is performed by two or three separate
operations. These are as follows:
Creating a DOS partition
The low-level format program writes the tracks and
sectors on the disk. Low-level format programs vary
according to the type of drive and controller. Many
controller manufacturers now include the low-level
format program in a ROM on the controller. You can
access this program by using the DOS DEBUG routine.
Refer to the controllers documentation to find the
starting address for the format program.
When you install and format a new freed disk
drive, it is extremely important to enter the defective
tracks from the list supplied by the manufacturer. These
bad tracks are usually listed on a label on the drive, with
another hard copy supplied with the documentation.
When the low-level format program is executed, it
will mark any bad tracks with a checksum error that will
prevent these tracks from being used for data storage.
In addition, the low-level format program will check all
areas of the disk to see if any additional bad tracks are
detected. If you are formatting a new disk, only the
tracks on the manufacturers list should be bad. If you
are reformatting an older disk and find that additional
tracks are listed as bad, the disk is showing signs of
severe damage and should be replaced.
DO NOT run a low-level format program
on an IDE drive. Serious damage could result
by trying to low-level format this type of drive.
There are two additional terms you need to be
familiar with to low-level format or troubleshoot fixed
disks. These are write precompensation and reduced
write current. Write precompensation and reduced
write current are also used in some disk memory sets.
Write Precompensation Write precompensa-
tion is used to prevent problems that can occur when
data is written on the higher numbered cylinders. A
disk is divided into sectors and tracks. Each sector can
store 512 bytes of data. The sectors on the outside of
the disk surface are physically larger than the ones on
the inside of the disk. As data is recorded on the disk,
like poles of magnetic fields are repelled away from
each other and opposite poles are attracted to each other.
As the heads move toward the center of the disk, the
write precompensation circuitry changes the spacing of
the magnetic fields. after the natural attraction or
repelling of the magnetic domains is complete, the
magnetic fields are in the proper place.
Reduced Write Current Reduced write current
also compensates for problems that can arise when
writing on the inner tracks of a disk. As the system
writes on the inner tracks of the disk, less current is
required because the data is more densely packed.
Using the same current on the inner tracks that is
required on the outer tracks would cause the data to run
over each other.
Manufacturers data sheets included with new
drives will indicate what cylinder write
precompensation and reduced write current are