Section 2: Commands And Instructions; Commands - IBM 3803 Manual

Section 2: Commands and Instructions
Commands
Commands executed by this subsystem fall into one of the following categories:
1. Burst Commands
2. Motion Control Commands
3. Non-Motion Control Commands
Figure 6 lists the subsystem command codes.
Programming Note: The 3803 /3420 subsystem has no interlocking to prevent the
execution of improper sequences of write- and read-type operations that may
result in writing extraneous bits or leaving partial blocks on tape. A voiding these
improper sequences is a program responsibility.
Avoid the following two basic sequences:
1. A write-type operation after a forward read-type operation except:
a. When the block or Tape Mark (TM) read is known to be followed by a
TM.
b. When the block or
TM
read is known to have been followed by ERG when
written or known to have been the last block written before a backward
operation.
For example: RR W* avoid.
W B R W* allowed.
2. A read forward-type operation following write-type operations.
w
R
B
*
For example: R B W R * avoid.
W BRR* avoid.
indicates a write-type operation: Write, Write TM, or Erase Gap (ERG).
indicates a forward read-type operation: Read Forward, Forward Space
Block, or Forward Space File.
indicates a backward read-type operation: Read Backward, Backspace
Block, or Backspace File.
indicates the logical record on which problems may occur.
Because it may be difficult or impossible to ensure the above safe situations, the
use of a write after read forward sequence should be used only in applications
where strict control of format and command sequence exists.
Write is allowable following a backspace. Assume the following tape format with
labels where * is used to denote a TM: VOL HDR
*
DAT A SET
*
EOF
*
HDR
*
DATA SET* EOF
* *.
A rewrite of the last data set involves the following safe
and proper sequence. After processing the next to last EOF and TM, read
forward to verify the HDR label of the last data set, backspace, write a new HDR,
and rewrite the data set. If a new data set is being added, the read forward
verifies the second consecutive TM, and thus, the true end of a data set on this
tape. A backspace, write new HDR, etc., completes the sequence.
Section 2: Commands and Instructions
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