
See “Locate Record” on page 4-13 for a complete description.
Note:
Performance can be significantly affected by the way that Locate Record
CCWs are used in the channel program. For example, two different
channel programs can read two consecutive records on a track. One
channel program has one Locate Record CCW followed by two read CCWs;
the other has two Locate Record CCWs, each followed by one read CCW.
While the two channel programs do the same function, the second
always
takes longer, because orientation is reset and must be reestablished before
the second record can be transferred.
The Locate Record Extended command provides all of the functions described in
the base Locate Record command except Orient, and Read Data. The Locate
Record Extended command also provides other functions. While the base Locate
Record command uses a 16-byte parameter list, the Locate Record Extended
command uses a variable length parameter list.
See “Locate Record Extended” on page 4-27 for a complete description.
Formatting
Format write commands format user data records. The format write commands
establish a count area, and format the key and data areas of a record. These
commands, if not immediately followed by another format write command, always
cause the remaining portion of the track to be erased. Use only format write
commands to change the format of a record (the length of the key or data areas of
a record).
See page 4-82 for a list of the format write commands.
File Protection
A file mask protects the integrity of data on the device from multiple users. The file
mask limits the write operations in a channel program. It can also limit channel
programs to operations on a single track or on a single cylinder.
The file mask is zero at the start of a channel program. During a Define Extent or
a Set File Mask command, the file mask is set equal to the value in the mask byte
parameter. The format of the file mask is in the description of the Define Extent
command. See “Mask Byte – Byte 0” on page 4-8.
End-of-File
A record with a data length of zero is an end-of-file (EOF) record.
When a read command other than Read Track or Read Multiple CKD processes
the data area of an end-of-file record, no data transfer occurs. Also, the command
is terminated with channel end, device end, and unit exception status. The control
unit does not present unit exception for a Read Track or Read Multiple CKD
command. Update write commands outside the Locate Record or Locate Record
Extended domain or in a Locate Record or Locate Record Extended domain with
CKD conversion mode active are terminated with channel end, device end, and unit
exception status when they try to update the data area of an end-of-file record.
To calculate the size of an end-of-file record with the track capacity formula for the
device, use a data length of 1 byte. See Appendix A, "Device Characteristics" on
3-4
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