Atari ST series Technical Reference Manual page 120

computer the letter you wrote with your word processing
program is stored at locations 40,658 to 41,949? To avoid this
problem, GEMDOS goes farther than merely dividing the
disk into tracks and sectors. It also divides it into logical
units known as files and directories. This filing system allows
you to give a name to a collection of storage locations on the
disk. Thereafter, whenever you wish to read or write to that
collection of data, you can refer to it by that name, and
GEMDOS takes care of keeping track of the actual physical
storage locations to which the name refers.
To implement this filing system, GEMDOS must store
several internal data structures on each disk, allowing it to
keep track of the disk organization, filenames, and locations
occupied by each file. These data structures are nearly identi­
cal to those used by MS-DOS, and as a result, it is possible
to read MS-DOS format disks on the ST. The first of these
structures, known as the Boot Sector, is located on the first
sector on the disk (Track 0, Sector 1). The boot sector per­
forms a couple of different functions. First, it lets GEMDOS
know how the disk is organized so it knows how to read the
data stored on it. The ST supports single-sided 3V2-inch
floppy drives, double-sided 3V2-inch floppy drives, 5y4-inch
floppies, and hard drives, and these drives can be formatted
with varying numbers of sectors per track, and tracks per
side. If it weren't for the boot sector, GEMDOS wouldn't
know where to begin looking for the data. Much of this in­
formation is the same as that stored in the BIOS Parameter
Block, which may be retrieved with the BIOS Getbpb() com­
mand. The second function of the boot sector is to allow
disks to be bootable—that is, to take control and run a certain
program as soon as the computer is turned on. It does this
with a program fragment called the boot code. The boot code
is usually just a tiny program that loads another larger pro­
gram from disk into the computer memory and turns control
over to that program.
The organization of the Boot Sector is shown in Table 6-1.
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CHAPTER 6