Initiation of a CICS transaction is a very complex process involving the control programs and controls tables mentioned above. A simplified version of the process is given below. A transaction identifier and some data are entered through a terminal. Terminal Control program (TCP), with the help of Terminal Control Table (TCT) recognizes the incoming data from the terminal. Storage Control Program (SCP) acquires the storage for the Terminal Input Output Area (TIOA). TCP places the data from the terminal into TIOA and sets the pointer to the TCT entry of the terminal. If there is no task associated with this terminal, TCP passes control to Task Control Program (KCP), which realizes the transaction identifier in the data in TIOA. SCP acquires the storage for the Task Control Area (TCA), in which KCP prepares control data for this task. KCP through Program Control Table (PCT) tries to find the application program associated with the transaction. If Processing Program Table (PPT) entry of the application program does not show the resident address of the program, KCP passes control to Program Control Program (PCP), which fetches the application program from the load library and places it in the main storage. KCP passes control to the application program, which starts its processing—that is, the transaction has been initiated.

There are many different ways of initiating a CICS transaction, the above method being one among them. The following are five different ways of initiating a CICS transaction, each of which is discussed in detail in the later chapters:

  • By a transaction identifier entered at the terminal with Enter key.
  • By a transaction identifier associated with a terminal for pseudo-conversation.
  • By the START command, which initiates the transaction, specified in the parameter.
  • By Automatic Task Initiation (ATI).
  • By a 3270 attention Identifier, any PF or PA keys could be defined in PCT to initiate a transaction.

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