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If you must handle the same data both in the main program and in an interrupt
subroutine, use programming such as that shown below to be sure that data
concurrence is preserved, i.e., mask interrupts while read/writing data that is
also handled in an interrupt subroutine.
(@)INT(89)
100
000
000
(@)INT(89)
200
000
000
Reading and writing common
data words
Masks all interrupts.
Unmasks all interrupts.
Data concurrence can also be a problem if interrupts occur during data transfers
occurring in servicing for Special I/O Units, remote I/O, or Host Link Systems.
For any of these, data can be non-concurrent down to byte units.
Use one of the following methods to preserve data concurrence in the above sit-
uations. The second methods applies to Special I/O Units only.
• Mask interrupts in the main program while moving data transferred to/from
Units to different words and use these alternate words in the interrupt subrou-
tine.
• Use the I/O REFRESH instruction in interrupt subroutines to refresh required
I/O from Special I/O Units and mask interrupts in the main program while read-
ing/writing Special I/O Unit words.
5-23-3 SUBROUTINE ENTER – SBS(91)
N: Subroutine number
00 to 99
Ladder Symbol Definer Data Areas
SBS(91) N
Limitations Subroutine numbers 00 through 07 are used with input interrupts and subroutine
number 99 is used for the scheduled interrupt.
Subroutines and Interrupt Control Section 5-23