The Communications between Terminals and CICS

 

The Communications between Terminals and CICS

1.       Define the terminals in the CICS. Each terminal receives a Resource Definition called a Terminal Control Table Terminal Entry (TCTTE) that is stored in the CSD. Each TCTTE contains:

l         A unique 4-character identifier for the terminal

l         The type of terminal

l         The terminal status

l         A pointer to the buffer (TIOA) containing any pending input or output messages

l         An area reserved for use by CICS during processing.

If the Autoinstall process is used, the TCTTE is built dynamically and is defined in the CICS region when the user logs on and a VTAM session is established. The advantage to this process is that terminals are only defined when in use. This means that storage space is decreased since CICS does not need to build TCTTEs for every terminal.

2.       When input is received from a terminal, the message is stored in a control block created dynamically by the CICS Terminal Management module. This control block is called a Terminal Input/Output Area (TIOA), storing messages exchanged between a terminal and an application program. The TIOA contains information about the length of the message, the message itself, and a pointer to the TIOA holding the previous message in the dialog.

3.       In order to associate each message with the appropriate terminal, the addresses of a terminal’s TIOA are stored in the TCTTE for that terminal. Since transactions are initiated form terminals. To do this, the address of the TCTTE for the initiating terminal is stored in the TCA that has been set up for the task. In addition, the address of the TCA is stored in the TCTTE of the initiating terminal.

4.       When a message is sent to a terminal, Terminal Management creates a TIOA to hold the message and transmits it to the terminal.