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

6. Disk File Access and Control
6.1 GENERAL FILE STRUCTURES
The primary reason we make use of computer systems is to aid us in managing large volumes
of data. Our computers utilize the Disk Operating System (DOS), the fundamental purpose
of which is to make an easier job of handling the storage of that data. We usually want
rapid access to data; therefore, the random access disk storage device is the selected
storage medium due to its inherent speed in accessing data. These devices take two forms,
floppy disks with either one or two heads which use a single diskette with corresponding
one or two surfaces, and winchester hard disk drives which consist of one or more
platters with each platter consisting of two surfaces. The hard disk drive may use either
a fixed or removable media.
Regardless of the disk drive type, each surface is divided into concentric circles of
storage area called tracks. Each track is then subdivided by a fixed number of subareas
called sectors. 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 is 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. The sectors in each track are numbered starting
from zero. Thus, each track contains like numbered sectors - regardless of track number
or surface. Therefore, each sector on a disk is designated unique by its respective
sector, surface, and track numbers.
Data is stored in these sectors. Obviously, if your program had to keep track of all the
sectors your data was occupying, you would have to make the program necessarily complex
[if this is not obvious, you will become a believer after reading the section on file
access]. The DOS alleviates you of this task by totally managing the storage space. It
does this by associating an 8-character name with the storage areas assigned to a
logically connected set of data called a file. Thus, the name becomes a FILENAME. The DOS
also permits a 3-character extension to be affixed to that name to better classify the
type of file: data, text, command program, etc. This extension is termed the FILE
EXTENSION. You can attach a unique PASSWORD and access level such as EXECute only or READ
only to each file in order to provide a greater degree of protection to the information
contained in the file. Furthermore, the file can be placed on any of up to eight disk
storage devices. Each disk drive is assigned a DRIVE number from zero to seven.
Therefore, to uniquely reference a file, we put together the NAME, EXTENSION, PASSWORD,
and DRIVE and refer to the result as a FILE SPECIFICATION. The term, file specification,
is rather long so we shorten it to "filespec".
In order to assign space on a disk for storage of file data, the DOS groups together a
quantity of sectors into a GRANULE. The size of the granule varies according to the
capacity of the media. This variation in size was discussed in the GRANULE ALLOCATION
TABLE section. The DOS assigns space dynamically to a file. This means that space is
reserved for the file only when the file needs it. The process whereby the system looks
for additional space is termed the ALLOCATION process. The DOS would prefer to allocate
granules that are connected sequentially to each other. The sequential connections are
only logical in nature, not physical connections. The DOS prefers to access a disk drive
device in a particular order to optimize the transfer of data. Since the time to step the
head from one cylinder to another is greater than the time to access a sector in the
cylinder where the head is positioned, it is far preferable to access all sectors of a
cylinder before stepping to another cylinder. If we look at sequential access of a file,
we then would want to conceptualize a sequential connection to start from track zero,
surface zero, sector zero incrementing the numbers like the odometer in a car as it
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