MFS control blocks

Users can specify four types of MFS control blocks to format input and output for the application program and the terminal or remote program.

Definitions:
  • Message Output Descriptors (MODs) define the layout of messages that MFS receives from the application program.
  • Device Output Formats (DOFs) describe how MFS formats messages for each of the devices with which the program communicates.
  • Device Input Formats (DIFs) describe the formats of messages MFS receives from each of the devices with which the program communicates.
  • Message Input Descriptors (MIDs) describe how MFS formats messages so that the application program can process them.
  • Message descriptors are both MIDs and MODs.
  • Device formats are both DIFs and DOFs.

Because each MOD, DOF, DIF, and MID deals with a specific message, both a MOD and DOF must exist for each unique message a program sends, and both a DIF and MID must exist for each unique message a program receives.

Overview of MFS components and operation

MFS has the following components:
  • The MFS language utility, which generates control blocks from user-written control statements and places them in a library called IMS.FORMAT
  • The MFS service utility, which is used for maintenance of the control blocks in IMS.FORMAT
  • The MFS message editor, which formats messages according to the control block specifications generated by the language utility
  • The MFS pool manager, which keeps the MFS control blocks that are required by the message editor in the main-storage MFS buffer pool
  • The MFSTEST Pool Manager, which replaces the MFS pool manager when the language utility is being used in test mode

IMS online change also plays an important part in updating the MFS libraries, even though it is not part of MFS. Briefly, online change allows the control block libraries to be modified while the IMS control region is executing.