The general flow of an IMS online system can be described in the following way:
An operator at a 3278 terminal enters data and presses the ENTER key. The IMS online control region retrieves the data and determines the types of message.
If it is an operator command, IMS responds to the command.
If it is a message switch, the message is sent to the logical device.
If there is a valid transaction code in the first eight characters, the message is placed in the message queue to wait for processing. The application program (associated with the transaction code contained in the first eight characters of the message waiting to be processed) is scheduled to execute in a message processing region as soon as a region becomes available. The Message Processing Program (MPP) retrieves the message from the queue and processes it. The processed data, along with other information, is put back into the queue by the MPP. The IMS control region takes the data from the queue and sends it back to the originating device.
A message is data transmitted from a device to a program, or a device to another device, or a device to IMS. There are three types of messages that can be sent to IMS. The first one to eight characters of the message determines which type is being sent.
If the character of the input message is a slash (/), IMS will interpret the message as an operator command. The operator command and any specified keywords should be followed by a period (.) to tell IMS where it ends.
If the message begins with one to eight alphanumeric characters and can be found in the logical terminal (LTERM) code reference table by IMS, it is a message switch. The LTERM code must be followed by a space to tell IMS where the code ends. The message will be sent directly to the physical device associated with the specified LTERM. This is a method of sending messages between devices without the use of an application program.
If the message begins with one to eight alphanumeric characters and can be found in the transaction code reference table, the message is for an application program. The transaction code must be followed by a space to tell IMS where the code ends. This type of message is also called a transaction message.
Each transaction and its associated application program must be defined to the IMS online system. This definition takes place in the IMS system generation. The transaction code is logically associated with a Program Specification Block (PSB) and an application program load module name, which the online system assumes have the same name.
Typically, there is a one-to-one relationship between a transaction and an application program; however, it is possible to have many transactions associated with a single program. The application program can always find out from the first eight characters of the input message the transaction code that caused it to be invoked. Conceivably, a program can be coded to behave in different ways depending on the transaction code that caused the invocation.
Below figure shows various examples of message.
------------------------------------------------------------------------ COLUMNS ****-****1****-****2****-****3****-****4****-****5****-****6****-****7** 1 0 0 0 0 0 0 0 ------------------------------------------------------------------------ /FORMAT CT70000M. (Operator command = /FORMAT ) WRDT0232 Good Morning (LTERM code = WRDT0232 ) CT7POLP1 100001 (Tran code = CT7POLP1 ) |
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