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

exceeds this capacity, then it must be divided into more than one drive in order to
address its total capacity.
The DOS also limits the number of files per logical drive to 256 (of which two are taken
up by the BOOT/SYS and DIR/SYS files). Although data base applications may find the most
practical arrangement is a single volume, the typical use of even a 5 megabyte drive will
find the file slots filled before all of the space is allocated - thus space is wasted
[It is possible and highly practical for the hard disk integrator to consider combining
individual static files into members of a partitioned data set to free up multiple file
slots. PRO-PaDS is a utility program capable of creating and maintaining such files].
Therefore, even with the smaller 5 megabyte drive, there exists a rationale for
partitioning.
Once the decision is made to divide a drive, the question arises as to how to go about
such a division. There are three methods of partitioning. One is to divide the drive by
cylinder. For example, Take a 306 cylinder, four head, 10 megabyte drive. This can be
divided into two drives with the first logical drive using cylinders 0-152 while the
second uses cylinders 153-306. The DOS actually uses logical cylinder numbers 0-152 for
both partitions and the hard disk driver must recognize that it needs to translate the 0-
152 for the second partition into the range 153-306. Obviously, one can divide up the
drive into partitions smaller than 5 megabytes. A second method is to divide the drive so
that all of the cylinders are included in a single logical volume, but volumes use
different heads. Thus, the previously mentioned drive could be divided into two, three,
or four logical drives. A third method would be to translate the drive's physical
parameters into quantities acceptable to the system while staying within the maximum
number of 256 sectors per logical cylinder.
There are advantages and disadvantages to each method. First, our discussion of floppy
configurations pointed out a use for addressing as much capacity in a single cylinder
prior to having to step the drive. This means that we would lean towards divisions by
cylinder. However, if we are alternately selecting different partitions, the drive must
be stepped a great distance to get to each partition. Another problem is that a head
crash would essentially wipe out all drives since a single head is used on all
partitions. Of course, if the drive physically has more than 406 cylinders, it must be
partitioned by cylinders (or translation) to address the higher cylinders.
Partitioning by head provides less sectors per physical cylinder; however, since hard
drives today usually use very fast buffered seek, the stepping time to advance a track is
minimal. A head crash will also only wipe out a single logical drive.
Translation methods can be useful with drives whose parameters do not lend themselves to
the DOS limits (a 39 sector per track drive, for instance). A drawback to translation
methods is the difficulty in keeping logical cylinders referencing a physical cylinder.
The important point in any method, is that the driver must be written to do the
conversions as the operating system's reference is to logical cylinder and sector within
that cylinder when it issues an I/O request. The driver may make use of the CURCYL byte
and FLAG-2, bits 3-0 for storage of partition specific data. The driver can also
establish its own table when these DCT fields do not provide sufficient space to store
the quantities needed by the driver.
Let's take a look at a few examples. The number of file slots identified assumes that all
logical drives are considered to be data drives. Subtract 14 from the number for each
SYSTEM drive. In the first, case we will use an ST-506 type drive which has four heads
and 153 cylinders. This will be the division of a 5 megabyte drive partitioned by head.
Figure 4-8 illustrates the DCT parameters to divide the drive into two logical drives of
2.5 megabytes each. Notice that we are using 8-sector granules (2K). Since we can have at
4-14