CICS - Nucleus
The five CICS system components described above constitute a convenient grouping of CICS system programs, each performing its own specialized functions. The core of CICS known as CICS Nucleus, which consists of CICS control programs and control tables supplied by IBM.
The CICS kernel is built by control programs and the corresponding control tables. It offers unique advantages. This makes the CICS system extremely flexible and therefore easy to maintain. Here are the important CICS control programs -
TCP is known as the terminal control program.
TCP is used to receive messages from the terminal.
It maintains the hardware communication requirements.
It asks CICS to start the tasks.
KCP is known as the task control program.
PCP is known as a program control program.
FCP is known as a file control program.
FCP is used to provide application programs with services like reading, inserting, updating or deleting records in a file.
It maintains exclusive control over records in order to maintain data integrity during updatesur recordings.
SCP is known as the storage control program. It is used to control the allocation and deallocation of storage in a CICS region.
CICS consists of CICS programs and control tables supplied by IBM. These tables must be updated accordingly w with the application information for successful execution of CICS application programs. Here are the important control tables -
TCT is known as the terminal control table.
When we connect to a CICS terminal, an entry is made in the TCT table.
TCT contains the terminal IDs connected to the current CICS region.
The terminal control program as well as the terminal control table recognize incoming terminal data.
PCT is known as the program control table.
PPT is known as the Processing Program Table. PPT contains program name or card game name, task usage counter, language, size, primary storage address, upload library address, etc.
Program or card game name is unique in a PPT table.
CICS receives the transaction and a corresponding program name is assigned to the transaction from the PCT. It checks whether the program is loaded or not. If loaded, the task usage counter is increased by 1. If the program is not loaded, the program is loaded first and the user counter is loaded.isation of the task is set to 1. It obtains the address of the loading library of the PPT table.
FCT is known as the File Control Table.
It contains file names, file type, length of recording, etc.
All files used in a CICS program must be declared in FCT and they are opened and closed by CICS itself.
When a TP02 transaction ID is entered on the CICS terminal, it first checks if there is a program associated with this transaction ID in Table PCT. If it finds one, it checks the PPT table to find the location of the program to run it.
If the program is already one available in memory, it starts executing that particular program; if not, it loads the program into memory from secondary storage, then comstart to execute it.
The transaction lifecycle includesnd the following steps -
The terminal operator initiates the transaction by typing a1 to 4 character transaction ID and pressing the ENTER key.
TCP periodically checks all terminals for contribution. When a message is received, it does the following -
The KCP takes control of the TCP and does the following -
Validate transaction ID and security.
Asks the SCP to create a task control area.
Assigns task priority based on terminal priority (defined in TCT), operator priority (Defined in SNT) and transaction priority ( Defined in PCT).
Adds the task to the waiting programs queue.
Send waiting programs tos the order of priority.
Pass control to the PCP.
The PCP takes control of the KCP and does the following -
The application program takes control of the PCP and does the following -
The FCP takes control of the application program and does the following -
Request a File work area from the SCP.
Informs the KCP that this task can wait for I / O to complete.
The KCP does the following -
Sends the next task to the queue.
Redistributes old task when I / O is complete.
Transfers control to the FCP.
The FCP returns control to the Application program.
The Application program performs the following operations -
Processes the data in the file.
Instructs TCP to send an I / O message
Return control to the PCP.
The PCP returns control to the KCP asking it to complete the task.
The KCP asks the SCP to release all the storag e allocated to the task (except TIOA).
TCP does the following -