SEAGATE TECHNOLOGY, INC.
Technical Support Bulletin Board
(408)438-8771 [300-9600 HST, MNP 3/5, N-8-1]
About Choosing a Drive Type in an AT
The drive types for SCSI, RLL, and ESDI interface drives are
generally easy to determine, especially the SCSI drives.
Almost all SCSI drives use DRIVE TYPE 0 or NONE, as the host adapter
bios and the drive communicate together to establish the drive
geometry. The low-level formatting routines are accessed on the
host adapter through DEBUG. After the low-level format, follow the
instructions for your DOS version for partitioning and system
format. Note: SCSI drives from the Seagate Wren and Swift families
are already low-level formatted at the factory.
RLL / ESDI
RLL and ESDI drives are usually not represented at all in the
internal drive tables and consequently the controllers for these
drives have onboard a ROM BIOS which either contains its own
internal list of choices for the interface or else provides the
ability to dynamically configure (define) the controller to the
specific geometry of the drive. In the case of the ESDI interface,
the controller gets parameters directly from the drive with a mode
sense equivalent command. Unlike the SCSI, the CMOS drive type
should start at 0 or NONE at the start of the installation (low
level format through DEBUG - consult your controller manual for
instructions), but it may be reset to DRIVE TYPE 1 by the controller
Many of the older AT's only provided 14 (MFM only) or so drive types
to choose from in the CMOS. The middle-aged AT's usually have up to
46 (still usually only MFM) types. Some newer AT's have drive types
which begin to include direct support for the popular RLL and ESDI
drives. If you have this newer kind of CMOS then by all means pick
the one that matches the drive and DISABLE the controller Bios.
(Note: This may also disable the controller's caching feature).
Likewise, most new machines have a "User Definable" or "Custom"
drive type that can be created and saved in the CMOS, thus providing
a standard drive type. "User Definable" drive types will usually
not work with most non-MS/PC-DOS applications.
A special note on ESDI and other drives that have more than 1024
cylinders. Since DOS cannot access cylinders above this 1024 limit,
a translation scheme may be elected in the controller's bios. As
the number of Logical Block Address (LBAs) is defined as
CYLINDERS*HEADS*SECTORS PER TRACK, translations that equal the same
number of LBAs with the cylinder count below the 1024 limit will be
devised. The controller bios will need to be ENABLED in order to
utilize translations schemes. (e.g. Many popular controllers
increase the number of sectors and/or heads and decrease the # of
cylinders to achieve an equivalent number of LBAs. See your
controller manual for details.) After low-level formatting, follow
the instructions for your DOS version for partitioning and system
AT / IDE
This idea of translation schemes bring us to the AT or IDE (Imbedded
Drive Electronics) interface. These drives are intelligent in that
they can use the geometry that represents their true physical
parameters or else they can "mimic" other drive geometries (or
translations) that equal or are very close to, but NOT exceeding,
the same number of logical blocks. Note: Translate LBAs <= Native
Many AT/IDE drives have physical cylinder counts that are greater
than 1024. Therefore, for DOS users, it is necessary to utilize the
translate feature by using a geometry that keeps the cylinder count
In order of preference, choose the first that fits your system:
1. Does the CMOS have a drive type that matches your drive?
2. Does the CMOS have a drive type that has the same number of
3. Does the CMOS have a "custom" or "user definable" drive type
option you can use? If so, use a translation geometry to keep the
cylinder count below the DOS 1024 limit.
4. Do you have the Disk Manager program to provide a software
driven solution? The Disk Manager will run automatically to perform
the partitioning and system format.
5. Pick the drive type that comes closest to, but not
exceeding, the formatted capacity of your drive. The final
formatted capacity of the drive will be equal to the drive type
*** Warning! ALL AT drives from Seagate are already low-level
formatted at the factory.
MFM (ST412 interface)
Finally, the MFM drives and their associated drive types are next.
If the internal drive type table lists the exact geometry, great. If
not, then check to see if a "Custom" or "User Definable" CMOS option
is available. Also, some AT 16-bit MFM controllers provide an
onboard BIOS which will allow the unique geometry of the drive to be
dynamically configured (our Seagate ST21M/22M MFM controllers have
this VALUABLE feature). Otherwise, a drive type match that is close
but not exceeding either the cylinder or head values is the only
choice left. An exact match in the head count is definitely
preferred when getting a "close" match.
When there is no direct match in the internal drive type tables, a
partitioning program may be needed to provide a software driven
translation solution in order to achieve full capacity. Keep in
mind that the drive will only format out to the capacity of the
chosen drive type when not using partitioning software. In the
event that the ST412 Interface drive has more than 1024 cylinders, a
partitioning program will be needed in order to achieve full
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