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Tuesday, November 6, 2012

Pseudo-conversational Programs

Pseudo-conversational coding technique 
CICS allows many users to log on and use CICS concurrently. Computer main storage is finite. If numerous users are running transactions, that demand considerable main storage, a Short-On-Storage(SOS) condition could occur, causing delays.

Pseudo-conversational programming is a coding technique, that results in the program being loaded into memory when required and released during the operator “think time”. Resources are not held.

Shared Code, Re-entrancy
CICS employs several task-management techniques to efficiently utilize computer memory. One such technique is shared-code. If two passengers are to book their railway tickets at a time, there will be two instances of the BOOK transaction viz, task X and task Y. On CICS, task X and task Y will share the same program-code, and merely have their own separate copies of working storage areas in memory. This significantly reduces the computer memory used.

Shared-code may be shared, after a user is completely finished with it(serially reusable), or may be shared, even if the first user is not finished(reentrant).

You see, CICS allows multi-tasking. Two or more tasks X and Y can run simultaneously. Behind the scenes, time is sliced into equal slots, say 2 seconds. Task X is run for 2 seconds, then it is interrupted, and task Y is run for 2 seconds, it is interrupted, and again task X is run for 2 seconds and so forth. CICS switches back-and-forth between the two tasks in a round-robin fashion.

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This is possible only, if CICS applications are re-entrant. On the CICS environment, IBM uses the term quasi-reentrant. If a mid-execution stop can occur anywhere in the program, the program code is said to be completely reentrant. If execution can stop only at I/Os or system calls, the program code is said to be quasi-reentrant. Making CICS programs reentrant is generally done automatically, when you supply the RENT option while compiling CICS programs.

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