Sector; Index Cylinder - IBM 5110 User Manual

84
Sector
A sector is a portion of a cylinder. All sectors on a single cylinder are
the same size, and the number of sectors on a cylinder depends on
the number of bytes per sector (see Diskette Types and Formats in
this chapter).
Track 00
I
I
I
Index Cylinder
Cylinder 0 is called the index cylinder and is reserved for information
describing the diskette and its contents. It contains information about
the diskette, such as volume and owner identification. The index
cylinder also contains information associated with each file on the
diskette. This includes the name of each file and the addresses
associated with the file extents. An extent is the maximum space a file
can occupy. The address at the beginning of this space is called the
beginning of extent (BOE). The address at the end of this space is
called the end of extent (EOE). A file may not use all of the space
allocated for it by the BOE and EOE addresses; therefore, another
address for end of data (EOO) exists.
Actual space being used for data
BOE EOO
I
I
Area allocated for the file (extent)
EOE
~
The EOO address is used to identify the next unused area within the
extent or to indicate that data has been written to the EO.E address.
(See the diskette initialization function in the IBM 5110 Customer
Support Functions Reference Manual, SA21-9311, for a complete
description of the index cylinder.)