Chapter 2. Operation Modes And Considerations; Buffers; Current Record Buffer; Previous Record Buffer - IBM 3741 Data Station Reference Manual

)
Entering, updating, verifying or searching of data on a
disk is performed by the data stations using different
modes of operation. The sequence of operations per-
formed by the control unit within the data station is
directly dependent upon the mode in which the station is
operating. Each mode of operation is unique, and, there-
fore, the use of buffers, keys, and information displayed
is different in each mode.
BUFFERS
Buffers are temporary storage areas used for transferring
data between the keyboard, display, and disk. The buffers
are the current record buffer, previous record buffer,
hold buffer, and program buffers.
Current Record Buffer
The current record buffer receives data characters as they
are keyed. Records read from disk are also placed in this
buffer. Data displayed on the display screen is from the
current record buffer.
Previous Record Buffer
The previous record buffer contains the record that was
previously displayed in the current record buffer. Data
is written on the disk from this buffer. The previous record
buffer is the buffer from which characters are obtained
by the control unit for duplicating into duplicate fields
in the current record buffer.
Hold Buffer
The hold buffer contains the next record to be verified
when in verify mode. This buffer contains the search mask
during search on content or search on sequential content
operati ons.
Chapter 2. Operation Modes and Considerations
Program Buffers
The 3741 has 10 program buffers numbered 1 through 9
and A. The program buffers usually contain programs
that control record format and processing. For offline
field totals operations, the program buffers will also
contain some field totals control statements that are
requ ired in order to perform the field totals operations.
See Chapter 6. Offline Field Totals for more information.
Program buffers are also used to contain operator guidance
prompting message records. For information on operator
guidance, see Operator Guidance in Chapter 3. Program
Information.
POWER ON INFORMATION
When the 3741 is turned on, the current record, previous
record, and hold buffers are filled with blanks, and the
program buffers are filled with N's. Program buffer 0 is
selected. The status I ine displays the machine-not-ready
disk status (N), the read index mode (X), program level 0,
and current disk address 00000. All other positions on
the status line are blank.
Closing the cover on the diskette that has been inserted
changes the machine status to wait (W). The volume label
is read but not displayed, and the accessibility field in the
volume label is checked. The accessibil ity field is used to
allow (field is blank) or inhibit (field contains any char-
acter) use of a disk. If the accessibility field is not blank,
the disk cannot be processed and an A error is displayed.
Then the 3741 waits for the diskette to be removed.
A write test is made; if not successful, ?? is displayed in
positions 9 and 10 of the status line. If?? is displayed,
other disks should be inserted to see if the?? disappears.
If the?? continues to be displayed, call I BM for service
since the machine cannot write records on disk.
If all tests are successful, sector 08 is read and displayed.
Sector 08 contains the first data set label (a data set
label defines required information about a data set. See
Index Track Data Set Labels in this chapter for more
information). Data entry or other data station operations
can now be performed.
Operation Modes and Considerations 5