The IMS message format service (MFS) is a facility of the IMS Transaction Manager environment that formats messages to and from terminal devices, so that IMS application programs do not deal with device-specific characteristics in input or output messages. In addition, MFS formats messages to and from user-written programs in remote controllers and subsystems, so that application programs do not deal with transmission-specific characteristics of the remote controller.
MFS uses control blocks you specify to indicate to IMS how input and output messages are arranged.
- For input messages, MFS control blocks define how the message sent by the device to the application program is arranged in the program’s I/O area.
- For output messages, MFS control blocks define how the message sent by the application program to the device is arranged on the screen or at the printer. Data that appears on the screen but not in the program’s I/O area, such as a literal, can also be defined.
In IMS Transaction Manager systems, data passing between the application program and terminals or remote programs can be edited by MFS or basic edit. Whether an application program uses MFS depends on the type of terminals or secondary logical units (SLUs) your network uses.
The advantages of using MFS are as follows:
- MFS simplifies developing and maintaining terminal-oriented applications by performing common application functions and providing independence from specific devices or remote programs.
- MFS improves online performance by using control blocks for online processing.
MFS Control Blocks: There are four types of MFS control blocks that you specify to format input and output for the application program and the terminal or remote program:
Message Output Descriptors (MODs) define the layout of messages MFS receives from the application program.
Device Output Formats (DOFs) describe how MFS formats messages for each of the devices the program communicates with.
Device Input Formats (DIFs) describe the formats of messages MFS receives from each of the devices the program communicates with.
Message Input Descriptors (MIDs) describe how MFS further formats messages so that the application program can process them.
MFS edits input data from a device into an IMS application message format using the message definition that the MFS application designer writes as input to the MFS language utility program. An input message consists of all segments presented to an IMS application program when the program issues a DL/I GU or GN call.
The format of input messages is defined to the MFS Language utility. Each message consists of one or more segments; each segment consists of one or more fields:
MESSAGE SEGMENTS FIELDS
MFS edits output segments created by an IMS application program into a device format suitable for the device or remote program for which the message is destined. Normally, the output segments from the IMS program contain no device-related data. All information needed for output to a device or remote program is provided when the message format is defined to the MFS Language utility program. For a remote program with DPM, specific device-dependent information is provided by the remote program without interpretation by MFS.
An output message consists of all segments presented to IMS with an ISRT call between a GU call to the I/O PCB and either a PURG call, another GU call to the I/O PCB, or normal program termination.