CICS Program Invocation
User Enters
TRANS-ID
CICS finds
TRANS-ID in PCT
to get program name
CICS finds
program name
 in PPT
CICS loads program
into memory/
starts program
CICS runs program
in address space
TRANS-ID = Transaction Identifier
PCT = Program Control Table
PPT= Processing Program Table
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CICS is able to handle simultaneous execution of many tasks.  This ability to handle many simultaneous tasks is known as multitasking.
For CICS a task is the running of a set of programs as a unit to form a transaction.
To start a transaction, a four character code known as the TRANS-ID is entered.  In an interactive session a user types this TRANS-ID in at a terminal and presses the Enter key.
The TRANS-ID is sent to the Program Control Table (PCT) which contains a translation of the four character code to a program name.
Assuming that the TRANS-ID is valid, CICS checks the Processing Program Table (PPT) to find where the program resides.  The program may already reside in memory, in which case execution begins.  If the PPT shows that the program is on disk, the executable version of the program, known as the LOAD MODULE, is retrieved from disk and loaded into memory.  Once inside of the CICS memory area, known as the ADDRESS SPACE, the program runs and the transaction completes.