1747-6.22, Backup Scanner User Manual

9-180 Programming Techniques
Accounting for
Instructions That
Could Cause Problems
During Switchover
Publication 1747-6.22
Some instructions may operate unpredictably when a switchover
occurs if you fail to observe certain programming considerations. For
example, you must consider transferring the control element and data
elements for instructions that are also being executed in the secondary
processor. You also must consider transferring data files which
contain more than one word.
The following instructions are discussed in this section:
• timer
• counter
• logical, arithmetic, compare and move
• diagnostic, sequencing, file arithmetic and logic, file search and
compare, file
• copy and fill
• FIFO and bit displacement
• block transfer
• message
In the remainder of this chapter, we describe the behavior of these
instructions in the SLC 500 backup system.
Important: The primary and secondary systems do not necessarily
execute the same instructions at the same time (the
systems’ program scans are not synchronized). Note
also that the SLC 500 processor executes block transfers
to the program scan asynchronously. Therefore, the
processor interrupts the program scan asynchronously to
access block transfer write (BTW) and block transfer
read (BTR) files.
Timer Instructions
The SLC 500 processors maintain timers by keeping a copy of a
hardware timer in a portion of the three-word structure (timer byte)
used by each timer.
Control byte Timer byte 1
Preset value
Accumulated value
2
3