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Table 6-1. Organization of the Boot Sector
Byte
Number
0-1
If disk is bootable, a BRA.S instruction to boot code
starting at byte 31 is stored here. If disk is not bootable,
two 0 bytes are stored here.
2-7
Reserved for OEM use (unused on ST)
8-10
Volume serial number 24 bits long (used to determine is
a disk was changed)
11-12
Number of bytes per sector (must be 512 under current
GEMDOS)
13
Number of sectors per cluster (must be two under
current GEMDOS)
14-15
Number of reserved sectors at the start of media
(including the boot sector)
16
Number of File Allocation Tables (FATs) on disk
17-18
Number of entries in the root directory
19-20
Total number of sectors on disk
21
Media Descriptor (not used on ST)
22-23
Number of sectors used by each File Allocation Table
(FAT)
24-25
Number of sectors per track (normally 9)
26-27
Number of disk heads (one for single-sided, two for
double-sided)
28-29
Number of hidden sectors (not used on ST)
30-509
Boot code (if any)
510-511
Checksum
Directory Blocks
GEMDOS also requires the root directory to keep track of
named files. This directory contains a number of 32-byte di
rectory entries, each one having information about a specific
file, such as its name, its file attributes, the time and date of
its creation, its length, and the file's starting cluster number.
The starting cluster number helps GEMDOS find the first
sector used to store the file's data.
When GEMDOS allocates disk space for a file, it does so
in increments called clusters. The current cluster size is two
sectors, which means that even if a file is only 10 bytes long,
it still occupies 1024 bytes (two sectors) on disk. Once GEM
DOS knows the starting data cluster for a file, it can find the
rest of the file by using the File Allocation Table (FAT).
The root directory of a floppy disk has space for 112 di
rectory entries. The format of each 32-byte entry is shown in
Table 6-2.
GEMDOS File System Functions
Description
113
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