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Changed - Modified indicator

Enhanced Definition

A `changed - modified indicator` is a flag or status bit associated with a data item, record, or screen field that signals whether its value has been altered from its original state since it was last read or displayed. Its primary purpose in mainframe environments is to optimize data processing and screen management by identifying only the data that requires further action.

Key Characteristics

- Bit-level Flag: Often implemented as a single bit, such as the Modified Data Tag (MDT) in 3270 data streams, which is part of a field's attribute byte.
- Optimization Mechanism: Reduces network traffic and processing overhead by allowing applications to process only the data that has genuinely changed, rather than re-processing all data.
- User or Programmatically Set: Can be set automatically by the terminal hardware upon user input (e.g., typing into a 3270 screen field) or explicitly by an application program using specific commands or attributes.
- State-Dependent: Its value (on or off) reflects the current modification status, indicating whether the data is "dirty" and needs attention from the application.
- Context-Specific: While the concept is general, its implementation and naming vary across different mainframe subsystems and data structures (e.g., MDT in CICS, specific flags in database records or file control blocks).

Use Cases

- CICS 3270 Screen Processing: The most prominent use is with the Modified Data Tag (MDT) in 3270 terminals and CICS Basic Mapping Support (BMS). When a user types into a screen field, its MDT is turned on, signaling to the CICS application that this specific field's data needs to be read and processed during a RECEIVE MAP operation.
- Database Record Updates: In some database systems (e.g., custom applications or specific IMS/DB implementations), a flag might be set on a record in memory to indicate it has been updated and needs to be written back to disk during a commit cycle.
- File Processing with Record Locking: In batch or online applications processing sequential or VSAM files, an internal indicator might be used to mark records that have been modified and are awaiting a rewrite operation to the file.
- Data Synchronization and Replication: Used in systems that replicate data across different environments to efficiently identify which records or fields have changed and need to be synchronized with target systems, minimizing the data transfer volume.