The Programmer's Guide to TRSDOS Version 6 - Tim Mann's Home ...

accessed a single-sided diskette, the system will not recognize the second
side of the new diskette until the logging process. When the DCT is
associated with a hard disk drive, this bit may be used to indicate that a
logical cylinder represents two physical cylinders thereby providing support
for twice as many cylinders as limited by the Granule Allocation Table (the
GAT limits the number of logical cylinders to 203 - thus by using this bit,
hard drives to 406 cylinders can be supported as a single logical drive). In
the case of hard drives, this bit is termed the "DBLBIT" bit.
Bit 4 This bit is used to indicate the controller associated with the DCT position
is an "alien" controller. The term, "alien", refers to a controller that
does not return index pulses in its status register. The system uses index
pulse transitions in a finite time period (usually 0.5 seconds) to detect
the presence of a rotating diskette. If a disk drive does not contain a
diskette, or does but the drive door is open, the status obtained on
continuous selection of the drive will not indicate the presence of any
index pulse transitions. By examining the state of the index pulse over a
period of time corresponding to 2.5 possible rotations of a disk, the lack
of an OFF-ON-OFF transition state will indicate that the drive is not
available. If a controller does not return the state of an index pulse in
the controller status byte, then the system will never be able to detect the
availability of the drive if it maintains the state transition examination
in the logging process. This bit should be set when such controllers are
used to inhibit the @CKDRV routine from performing such an examination and
proceed to the configuration logging.
Bits 3-0 This subfield is used for different purposes depending on whether the drive
associated with the DCT is a floppy drive or a hard drive. For floppies, the
field contains the physical drive address (1, 2, 4, or 8) corresponding to
the drive select line (DS0, DS1, DS2, or DS3). Thus, only one of the four
bits will ever be set. Hard drive installations that partition a drive by
head, may use this field to indicate the relative starting head number of
the logical drive partition. This provides support for a drive of up to 16
heads although 4 heads is typical.
4.2.4 CURCYL -
This field is used for different purposes depending on whether the drive associated with
the DCT is a floppy drive or a hard drive. For floppies, the field is used by the disk
driver to store the current cylinder position of the disk drive assigned to the DCT
position. Since a Floppy Disk controller is used to access up to four different drives,
when it accesses a drive, its track register must be loaded with correct information as
to the current track position of the head. The current cylinder position is maintained by
the disk driver in this storage field. The driver can then be use this field to reload
the FDC track register prior to a seek operation and update the field to the cylinder
requested in the seek. Hard disk controllers generally contain their own internal track
register that is not accessible to a software driver. This means that hard disk drivers
do not need to maintain the current cylinder position in this field. The field is thus
available for the storage of other data items as required by the hard disk driver. Other
data items may include the total quantity of heads on the physical drive (as needed by
XEBEC controllers), the complex drive select code (as used by Lobo Drives UniVersal
Controller), or data associated with drive partitioning by cylinder rather than by head.
4.2.5 MAXCYL -
This field contains the highest numbered logical cylinder on the drive referenced from a
starting cylinder numbered "0". Thus, a 35-cylinder drive would be entered as X'22', a
40-cylinder drive as X'27', and an 80-cylinder drive as X'4F'. A typical 153-cylinder ST-
506 compatible winchester drive would have an entry of X'98'. If a hard drive has more
than 203 cylinders but less than 407 cylinders and is to be maintained as a single drive
(or one partitioned by heads), then the system must access it as if each two physical
4-6