Basic Groups; Figure 23. Structure Of A Basic Group - Seagate STD124000N Product Manual

Chapter 6
Page 48
Entities are defined as logical objects that contain processed records.
Entities, unprocessed records and separator marks are collected into groups .
Groups are processed through a series of transformations including randomizing,
interleaving, blocking, generating and inclusion of ECC, and translation of data bytes
to channel bits before recording to tape.
Each group is recorded on a set of tracks . User data, separator marks and
associated information are recorded in the main data zone of each track. Group
content, track location and track content information is recorded in the headers of
fragments, which are in the main data zone.

Basic Groups

Data from the host system to be recorded to tape is grouped into basic groups of
384,296 bytes each. For location purposes on the tape, each individual basic group
is identified by a number beginning at 0, running consecutively thereafter. Within
each basic group, the individual bytes are also identified, again with a consecutively
running number beginning at 1 and running through 384,296.
Data and separator marks transferred from the host system are grouped into an
individual basic group according to the following structure:
Byte No.
. .
1, 2, 3, .
Entities or
Unprocessed
Records

Figure 23. Structure of a Basic Group

As seen in Figure 23, in addition to entities or unprocessed records, the structure of
a basic group includes a group information table (GIT) and a block access table
(BAT) . The group information table always occupies the last 35 bytes of the basic
group.
. . . 384,261
384,296 bytes
Tape Formats
384,262
384,296
Block
Group
Access
Information
Table
Table
35 bytes
DAT Drives