Radio Shack TRS-80 Model II Reference Manual page 174

Model II
TRSDOS
Diskette Organization
Model II uses single-sided, double-density diskettes. Each diskette contains
77 tracks, numbered 0-76.
Each track contains 26 sectors, numbered 1-26. Each sector contains 256
bytes, except for track 0 sectors, which contain 128 bytes. The total capacity
of a diskette is:
(76
*
26
*
256)
+
(1
*
26
*
128) = 509,184 bytes
Disk Space Available to User
Sector 26 of each track is reserved for System use, giving the user 25 sectors
per track. On System diskettes, 65 tracks are available for the user; on
non-System diskettes, 75 tracks are available.
Details: Track 0 is reserved by the System. Another track (usually track 44) is
reserved by the System for the diskette directory. On Operating System
diskettes, ten additional tracks are used for System files.
Unit of Allocation
The only unit of disk space allocation is the "granule". A
TRSDOS
granule is
defined as 5 sectors. Therefore the smallest non-empty file consists of 5
sectors; i.e., one granule.
NON-SYSTEM
DISKETTE
TRACKS GRANULES SECTORS BYTES
75 375 1875 480;000
-
1
5
25
6400
-
- 1 5
1280
- - -
1 256
Table 4.1
210
Space Available to User