SEAGATE TECHNOLGY, INC.
Technical Suppport 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.

SCSI

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 
card.

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 numberof LBAs.  See your 
controller manual for details.)  After low-level formatting, follow 
the instructions for your DOS version for partitioning and system 
format.

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 LBA's <= Native 
LBA's.  

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 
below 1024.

In order of preference, choose the first that fits your system:

    1.  Does the CMOS have a drive type that matches your drive?

no? 
    2.  Does the CMOS have a drive type that has the same number of 
formatted megabytes?

no?
    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.

no? 
    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.

no?
    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 
chosen.

***  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 
capacity.
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