Data
Record
The data record provides data and error checking
information. Data is recorded
in
records
of
512 bytes,
plus sequencing. and checking bytes.
~
Forward
Tape
Motion
Format
Last
Data
Record in
Previous
File
Record
Format
Header
Record
,""
,,""
.".."'"
.".."'"
,."
.".."
Data
Record
No.1
Format
Record
c"
......
,,~·
Forward Tape Motion
~~
__
~I~~I~~I~~~~I
_____
51_2B_yte_sof_Da_ta
____
~~~
__
~
Record Type (hex):
81
Header
Record
42 End
of
Data
24
Bad
Record
18 Data Record
29
Bytes
of
Hex
00
The first 3 bytes are read
to
establish synchronization,
The
next
byte indicates the record type, followed by 512
bytes
of
data.
The
CRC
(cyclic redundancy check) is contained in the
next
two
bytes. This
CRC
tests the 512 bytes
of
data
and the record type byte
for
an
error. The controller
counts 513 bytes after it detects
E7
and then reads the
next
two
bytes
as
CRC
bytes. The last
two
bytes
of
a's
in the data record generate extra byte counts in case
any
of
the 513 bytes are missed.
A format record and a header record, which indicates
the end
of
marked tape,
follow
the last data record
in
the last file.
4·36
a a No
a a
Data
Data
Record
a
512 Bytes
of
Data
/
"'-
\
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