In addition to the character device and system
functions, many of which are similar to calls found in the
BIOS and XBIOS, GEMDOS provides a number of unique
functions involved with the disk filing system. Disk drives
are unique among the I/O devices because they are random-
access devices. Each disk contains a fixed amount of storage
space and can retrieve information from anywhere on the
disk. Rather than reading the information as one continuous
stream, the disk starts at the beginning and works its way to
the end each time. Disk drives are also unique because they
are divided into both physical storage units (tracks and sec
tors) and logical storage units (such as directories and files).
A blank disk contains a magnetic medium with nothing
significant recorded on it. When you format the disk, the
drive controller encodes information on the disk that divides
it into tracks and sectors. On the ST, each sector holds 512
bytes of data. There are normally nine sectors per track, and
80 tracks per side of the disk. This means that single-sided
disks can hold 368,640 bytes (512 x 9 x 80) of data, and
double-sided drives can store twice that amount.
Although nine tracks per sector and 80 sectors per side
is the default format for disks, it is possible to format a disk
with ten or even eleven sectors per track, and 81 or 82 tracks
per side. Because of variations in manufacturing tolerances
from drive to drive, the default values are the safest ones to
use, particularly for disks that will be used on more than one
disk drive.
Dividing the disk into tracks and sectors in effect gives
you hundreds of thousands of little boxes, each of which
may hold one character of information. It would not be prac
tical, however, to expect the user to keep track of what infor
mation is in each box. Can you imagine having to tell the
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