CICS Components and Transactions
There are three major components associated with CICS namely Management Modules, Control Tables and Control Blocks. These three are very essential in CICS environment and so let us see an overview of the function of each.
We have seen that CICS act as an interface between operating system and application program and this is achieved by management module. In other words Management Modules are programs that act as interface between operating system and application program. Control table is associated with management module and is used to define the CICS environment.
A control block is used to store information about CICS system. For instance we have seen that a transaction handled by CICS is identified by transaction identifier. And also each transaction may have number of task in it. Control blocks have the information about the task in an area called as task control area. Like that control block has many system information stored.
Having got an idea about what CICS is what a transaction is and the components used in CICS now its time to look in brief as how a transaction gets initiated and executed in CICS.
Let us take a look at an example where the Transaction Identifier: EXFOR and Application program: A1 User Makes call to file student for data…
Let use see how the above transaction given in example is initiated and executed by CICS and how the data is presented on the terminal as output in CICS environment. As we have seen before each transaction in CICS has a transaction identifier associated with it.
So as a first step to initiate the transaction one types the transaction identifiers on the terminal. For instance in our example the transaction identifier is EXFOR and it is typed on the terminal. This is taken by CICS and the transaction identifier EXFOR is stored in working storage division.
The CICS service treats the transaction identifier say EXFOR as a call to the associated application program. The application program can be present in storage division or in program library. Suppose in our case if the associated application program say A1 was in program library so A1 is loaded from program library into working storage. Now task is created and the program is started or initiated. Now we must know an important technology called as BMS.
BMS is also called as Basic mapping support. We know the output of any program would appear on the terminal screen. There are many features which are specific to hardware and this may affect the output. In other words to get the output as desired say in formatted way and in order to remove the dependency CICS has a facility called BMS. Now after the program is initiated the next step is to give the control to user. This is done by giving terminal control and also BMS is used by CICS now to get the requirements from user that is from screen.
As per our example user wants to get value from student file. So file control is given by CICS to student file and the information requested by user is retrieved. Now BMS as explained does the primary function of formatting of the data retrieved from student file and present it on the terminal.
This is how transaction flows between user application and system in a CICS environment. There are lot of functions and features available in ICS but the above description gives an overview of CICS.
Author: SriPriya
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