System message field (3270 or SLU 2 display devices)

Output formats for 3270 or SLU 2 display devices can be defined to include a system message field. If defined in this way, all IMS messages except REQUESTED FORMAT BLOCK NOT AVAILABLE are sent to the system message field whenever the device is in formatted mode. Using a system message field or setting byte 1 bit 5 to B'0' in the DSCA specification prevents an IMS message from destroying a screen format.

When MFS sends a message to the system message field, it activates the device alarm (if any) but does not reset modified data tags (MDTs), move the cursor, or change the protect/unprotect status of the display, except in the event of a multi-segment message. In this case, the status is changed to protected, and the enter key must be pressed to view the next segment or segments of the message. Because IMS error messages are an immediate response to MDTs in input, MDTs remain as they were at entry and you must correct the portion of the input that was in error.

After input from an operator identification (OID) card reader, the device is no longer in formatted mode. Therefore, an IMS message is not sent to a SYSMSG field; it is sent using the default system message format. This is also the case after an XRF (Extended Recovery Facility) takeover because the device is no longer in formatted mode.