Example: Record format *VBS

This is an example of record format *VBS, which consists of graphics with explanations.

Variable-length, blocked, spanned (*VBS).

Figure 1. Variable-length, blocked, spanned (*VBS)

This depicts three blocks
all connected. Block 1, is comprised of XX YY, VV WW, All of Record A, VV
WW, and lastly the 1st segment of Record B. Block 1 then points to Block 2.
Block 2 is comprised of XX YY, VV WW, and the middle segment of Record B.
Block 2 then points to Block 3. Block 3 is comprised of XX YY, VV WW, the
last segment of Record B, VV WW, and all of Record C. Connected to the last
segment of Record B is TT CC and the actual data from the last segment of
B.

Figure 2. Parts of record B
This diagram depicts the
part of Record B. The 1st part is comprised of TT CC and the 1st part of Record
B data. The 2nd part is comprised of TT CC and the 2nd part of Record B data.
The last part of Record B is comprised of TT CC and the last part of Record
B data.

This diagram depicts the
part of Record B. The 1st part is comprised of TT CC and the 1st part of Record
B data. The 2nd part is comprised of TT CC and the 2nd part of Record B data.
The last part of Record B is comprised of TT CC and the last part of Record
B data.

XX and YY make up the BDW for each data block. XX is the length of all records in each block plus the length of the BDW (4 bytes). YY represents currently reserved fields and must be 00. XX should be the actual length of the data block that is written.

Note that logical record B spans across three actual data blocks on the tape.

Each segment of record B has its own by mapping by including the SDW. TT and CC make up the SDW. TT is the length of the record plus the length of the SDW (4 bytes). CC is the segment control character. The first byte of the CC defines which part of the record the segment is. The values of the control character can be:

00 binary - Complete logical record
01 binary - First segment of a multi-segment record
10 binary - Last segment of a multi-segment record
11 binary - Middle segment of a multi-segment record

The second byte of the control character is reserved, and should be 0. TT should be the actual length of record data segment plus the length of the SDW (4 bytes).

From a user's point of view, a logical view of record B is the the same as other records defined above.

Figure 3. Logical view of record B
Record B in this diagram
is comprised of VV WW and the 1st, middle, and last portions of Record B data,
concatenated.