CICS NUCLEUS

CICS nucleus is constructed by the control programs and corresponding control tables provides unique advantages. It makes CICS system highly flexible and thus easy to maintain also. Following are the important control programs of CICS:

Control Programs
TCP
The fundamental explanation of TCP is as follows:
TCP is known as Terminal Control Program.
TCP is used to receive messages from terminal.
TCP maintains hardware communication requirements.
TCP requests CICS to initiate the tasks.

KCP
The fundamental explanation of KCP is as follows:
KCP is known as Task Control Program.
KCP is used to simultaneously control the execution of tasks and its related properties.
KCP handles all issues related to multitasking.

PCP
The fundamental explanation of PCP is as follows:
PCP is known as Program Control Program.
PCP is used to locate and load programs for execution.
PCP transfers the control between programs and in the end it returns control to CICS.

FCP
The fundamental explanation of FCP is as follows:
FCP is known as File Control Program.
FCP is used to provide application programs with services like read, insert, update or delete records in a file.
FCP manages exclusive control over the records in order to maintain data integrity during record updates.

SCP
The fundamental explanation of SCP is as follows:
SCP is known as Storage Control Program.
SCP is used to control allocation and deallocation of storage within CICS region.


Control Tables
CICS consists of IBMsupplied
CICS control programs and tables. These tables need to be updated accordingly with the application information for successful
execution of CICS application program. Following are the important Control Tables:

TCT
The fundamental explanation of TCT is as follows:
TCT is known as Terminal Control Table.
When we login to CICS terminal an entry is made in the TCT table.
TCT contains the terminal ID's that are connected to current CICS region.
Terminal Control Program along with terminal control table recognizes the incoming data from the terminal.

PCT
The fundamental explanation of PCT is as follows:
PCT is known as Program Control Table.
PCT contains the Transaction IDs and the corresponding program names or program IDs.
TRANSID is unique in PCT table.

PPT
The fundamental explanation of PPT is as follows:
PPT is known as Processing Program Table.
PPT contains Program name or Mapset name, Task Use Counter, Language, Size, Main storage address, Load library address, etc.
Program or Mapset name is unique in PPT table.
CICS receives the transaction and a corresponding program name is allocated to the transaction from the PCT. It checks if the program is loaded or not. If it
is loaded in then the task use counter is increased by one. If program is not loaded, then program is first loaded and the task use counter is set to one. It gets
the load library address from PPT table.

FCT
The fundamental explanation of FCT is as follows:
FCT is known as File Control Table.
FCT contains File names, File type, record length, etc.
All the files used in a CICS program must be declared in FCT and they are opened and closed by CICS itself.

Transaction Life Cycle;

Step 1
The terminal operator initiates the transaction by typing a 1 to 4 character transactionid
and pressing the ENTER key.

Step 2
The TCP periodically checks all terminals for input. When a message is received it does the following:
Instructs the SCP to create TIOA.
Places the message in the TIOA.
Passes the control to KCP.

Step 3
The KCP takes control from the TCP and does the following:
Validates the transactionid
and security.
Instructs the SCP to create a task control area.
Assigns priority to the task based on Terminal priority , Operator priority and Transaction priority .
Adds the task to the queue of waiting programs.
Dispatches waiting programs in the order of priority.
Passes the control to PCP.

Step 4
The PCP takes control from the KCP and does the following:
Locates the program and loads it, if necessary.
Transfers the control to the Application program.

Step 5
The Application program takes control from the PCP and does the following:
Requests the TCP to place the message into the program’s WORKING STORAGE area.
Requests the FCP to retrieve records from the files.

Step 6
The FCP takes control from the Application program and does the following:
Requests a File work area from the SCP.
Informs the KCP that this task can wait until the I/O is complete.

Step 7
The KCP does the following:
Dispatches the next task in the queue.
Redispatches
the old task when I/O is complete.
Transfers the control to FCP.

Step 8
The FCP returns control to the Application program.

Step 9
The Application program does the following:
Processes the file data.
Requests TCP to send an I/O message.
Returns control to PCP.

Step 10
The PCP returns control to KCP requesting it of end the task.

Step 11
The KCP instructs the SCP to free all the storage allocated to the task .

Step 12
The TCP does the following:
Sends the output to the terminal.
Requests the SCP to release the TIOA.