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TAPE FORMATS
The third level of ECC provides recovery across frames. Only the frames in the group are covered by the
third level of ECC. Each group is considered independently of all other groups. The Main Data area of the
ECC frame contains the actual error correction bits.
The third level ofECC can correct any two tracks that are bad in a group. These tracks may be of the same or
different azimuth angles and may be in the same or different frames.
6.8.4 Amble Frames
Amble frames are only written after the last frame in a group or after another amble frame. The amble frame
contains a valid Header, but the remainder of the Main Data area is set to zero. The logical frame number of
an amble frame is zero.
6.9 Indexing
The indexing concept allows variable-sized host records to map onto fIxed-size groups. All groups except the
Vendor Group contain an index. Each index is divided into two parts:
o
The Block Access Table (BAT), which describes the contents of the group and has an entry for each
record and save-set mark.
o
The Group Information Table (GIT), which contains a list of counters and pointers that describe the
characteristics of the group. For the DDS-DC format, the defInitions of three of the fIelds in the
GIT are extended.
6.9.1 Group Information Table (GIl)
The GIT is included in each group. It is in a fIxed location at the end of the group and is the same size
regardless of the contents of the group (bytes 126601 through 126632). It contains a list of counters and
pointers that describe the characteristics of the group. Its entries are two or four bytes wide.
For the DDS-DC format, the Record Count, Number of Records in Group, and Previous Record Group Count
fields are extended as described in the following text.
Group Count: The Group count is the number of user data groups written since the beginning of the current
partition, beginning with one and including the current group.
Block Access Table Count: The Block Access Table Count is the number of entries in the Block Access
Table.
Record Count: The Record Count is the number of records written since the BOT of the current partition
and includes any completed records in the group.
For the DDS-DC format, this 4-byte field specifies the sum of the values in the Number of Records in Group
of the GIT of all groups written after the logical beginning of tape (LBOT) through the current group.
6-13
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