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

called TRACKs. Each track is then divided into subareas called SECTORs. Each sector is
uniquely identified by a pattern of information preceding each sector called an ID FIELD.
The division of a surface into sectors may be envisioned as a pie cut up into equal sized
pieces. The process of generating each of the tracks and sectors is termed the formatting
process. The physical length of a sector will be greater on the outer tracks of the
surface than the inner tracks of the surface (similar to the grooves of a phonograph
record). Although the number of sectors per track may vary from one media type to
another, the number of sectors in each track of the same media must always be a constant.
The DOS assigns numbers to every sector, every track, and every surface. Surfaces are
numbered consecutively by one starting from zero. Tracks are numbered consecutively by
one starting from zero at the outermost portion of the disk giving the innermost track
the highest number. A CYLINDER consists of the like-numbered tracks on all surfaces. For
example, on a two-surface media, track zero of surface zero and track zero of surface one
are grouped together into cylinder zero.
Floppy disk drives use a read/write head that is positioned lateral to the disk surface.
The head can step in towards the center of the disk and step out to the circumference of
the disk while the disk rotates on its hub. The rotational speed is 300 rpm for 5-1/4"
floppy disk drives and 360 rpm for 8" floppy disk drives. Hard disk drives rotate at
speeds of 3600 rpm and higher. Because the physical lengths of the sector vary from the
outer to the inner track, the bit density of each sector varies per track. Therefore, the
amount of information stored in all sectors is dependent on the maximum bit density
permitted in its shortest sized sector. Some manufacturers of computer systems are using
a design which keeps the bit density per sector constant by use of a variable speed drive
which maintains a constant linear velocity of the surface across the head regardless of
the track position. This technique promotes a greater capacity for storage but requires a
more precisely controlled drive. If such a drive control were utilized under this DOS, a
suitable translation filter would be needed which would permit the DOS to think that each
track still contained the same number of sectors.
If we concern ourselves with a 5-1/4" double density floppy drive rotating at 300 rpm, we
can calculate that a disk makes one complete rotation every 200 ms (60/300). Since there
are 18 sectors per track, a sector's ID FIELD passes by the drive's head every 11.1 ms.
In a system where the transfer of data to and from the disk is under the control of the
CPU rather than through auxiliary Direct Memory Access (DMA) hardware, the CPU spends its
time handshaking with the controller while transferring each byte of data. If we are
trying to access a series of sectors sequentially (as would be the case with a
sequentially accessed file), there will rarely be sufficient time for the CPU to
establish the handshaking with the controller for the access of the next sector once it
has finished transferring the current sector. Thus, if we number the sectors
consecutively, most likely the ID FIELD of the sector we next want to read has just
passed by the head and we must wait a complete revolution of the disk before getting to
the ID FIELD again. In fact, the worst case would require us to wait just under 211.1 ms
per sector while the time to read an entire track would be 3.8 seconds!
A practical solution to increasing the data transfer is to stagger the sector numbers so
that the next sector to transfer is arriving at the head just after we start looking for
it. If we could read many sectors per single rotation, we could speed up the transfer of
data. This can be done when the disk is formatted. It can also be done when the disk is
accessed by means of a lookup table that translates a logical sector number to a
staggered physical sector number. The process of staggering the sector numbers is termed
INTERLEAVE. An interleave of two means that sequential sector numbers are in every second
physical sector. An interleave of three uses every third position. For a single density
5-1/4 diskette, this pattern would be 0-5-1-6-2-7-3-8-4-9. An 18 sector per track
diskette with an interleave of three would have a pattern of 0-6-12-1-7-13-2-8-14-3-9-15-
4-10-16-5-11-17. The interleave can be precisely calculated with knowledge of the total
time it takes to execute the machine instructions between sector I/O. This is generally a
most difficult task; therefore, interleave patterns are generally derived empirically.
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