Relationships between MFS control blocks

Several levels of linkage exist between MFS control blocks.

You must understand these linkages to design an application environment properly.

The following figure shows the interrelationships between MFS control blocks. The subsequent illustrate the four levels of linkages, which are then summarized in the final figure.

Figure 1. Control block interrelationships
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The following figure shows the highest-level linkage, that of chained control blocks.

Figure 2. Chained control block linkage
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Notes to the previous figure:

  1. This linkage must exist.
  2. If the linkage does not exist, device input data from 3270 devices is not processed by MFS. For other devices, the MID name can be provided by the operator.
  3. This linkage is provided for application program convenience. It provides a MOD name to be used by IMS if the application program does not provide a name by way of the format name option of the DL/I ISRT or PURG call. This MOD name is also used if the input is a message switch to an MFS-supported terminal.
  4. The user-provided names for the DOF and DIF used in one output-input sequence are normally the same. The MFS language utility alters the name for the DIF to allow the MFS pool manager to distinguish between the DOF and DIF.

    The direction of the linkage allows many message descriptors to use the same device format if desired. One common device format can be used for several application programs whose output and input message formats as seen at the application program interface are quite different.

The following figure shows another level of linkage that exists between message fields and device fields. The dots show the direction of reference, not the direction of data flow, in the MFS language utility control statements; that is, the item at the dotted end of a line references the item at the other end of the line.

References to device fields by message fields do not need to be in any particular sequence. An MFLD does not need to refer to any DFLD. In this case, MFLD defines space in the application program segment that is to be ignored if the MFLD is for output and padded if the MFLD is for input. Device fields do not need to be referenced by message fields. In this case the fields are established on the device, but no output data is transmitted to them and any input data from them is ignored.

Figure 3. Linkage between message fields and device fields
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The following figure shows a third level of linkage, which exists between the LPAGE and the DPAGE.

Figure 4. LPAGE and DPAGE relationships
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A MOD LPAGE must refer to a DPAGE in the DOF. However, not all DPAGEs must be referred to from a given MOD.

If no MID LPAGE is defined, the defined MFLDs can refer to fields in any DPAGE. However, input data for any given input message from the device is limited to fields that are defined in a single DPAGE.

If one or more MID LPAGEs are defined, each LPAGE can refer to one or more DPAGEs. All DPAGEs must be referred to by an LPAGE. When input data is processed as defined by a particular DPAGE, the LPAGE referring to it governs the message editing.

The following figure shows a fourth level of linkage that is optionally available to allow selection of the MID based on which MOD LPAGE is displayed when input data is received from the device.

Figure 5. Optional message descriptor linkage
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Notes to the previous figure:

  1. The next MID name provided with the MSG statement is used if no name is provided with the current LPAGE.
  2. If a next MID name is provided with the current LPAGE, input is processed using this name.
  3. When the format definition includes 3270 or SLU 2 devices, all MIDs must refer to the same DIF. The same user-provided name must be used to refer to the DOF when the MOD is defined.

The following figure summarizes the previously explained MFS control block linkages.

Figure 6. Summary of control block linkages
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Control block linkages are fundamental to MFS functions but there are a few device-oriented conditions that could affect application design.