Attention Identifier (AID) Key

Enhanced Definition

An Attention Identifier (AID) key is a special key on an IBM 3270 terminal (or its emulator) that, when pressed, signals an event or request to the host mainframe application. It causes the terminal's input buffer contents, along with cursor position and modified field indicators, to be transmitted to the z/OS application for processing.

Key Characteristics

- 3270 Terminal Specific: Primarily associated with IBM 3270 display devices and their emulators, which operate in a block-mode fashion.
- User-Initiated Event: Activated by the end-user pressing a designated key, such as Enter, Clear, Program Function (PF) keys (PF1-PF24), or Program Attention (PA) keys (PA1-PA3).
- Data Transmission Trigger: An AID key press is the primary mechanism that triggers the transmission of user-entered data from the terminal to the host application.
- Application Event Indicator: The host application receives an AID byte (or similar indicator) that identifies which specific AID key was pressed, allowing it to determine the user's intended action.
- Synchronous Interaction: Facilitates a synchronous, request-response interaction model where the application processes input and then sends a new screen or response.
- Standard Key Set: Includes commonly used keys like Enter (default input submission), Clear (resets screen), PF keys (for specific functions), and PA keys (for system attention or interruption).

Use Cases

- Navigating Menus and Panels: Users press Enter or a PF key to select options, move between panels, or execute commands within TSO/ISPF or CICS applications.
- Submitting Data Input: After entering data into fields on a screen (e.g., an order entry form), pressing Enter transmits the data to the application for validation and processing.
- Executing Application Functions: PF keys are widely used to trigger specific actions, such as PF3 for Exit, PF7 for Page Up, PF8 for Page Down, PF12 for Cancel, or PF1 for Help.
- Online Transaction Processing: In CICS, an AID key press initiates or continues a transaction, allowing the CICS program to receive input and respond, driving the application flow.
- Error Recovery or Interruption: Less common, but PA keys can sometimes be used to interrupt a long-running process or signal a system-level attention, though Clear is more often used to reset a screen after an error.

Related Concepts

AID keys are fundamental to the 3270 data stream protocol, acting as the signal for the terminal to send its buffer contents to the host. In CICS (Customer Information Control System), an AID key press is the primary driver for transaction processing, causing a CICS program to receive input via commands like EXEC CICS RECEIVE MAP or EXEC CICS RECEIVE. Similarly, TSO/ISPF relies on AID keys for command execution and panel navigation. COBOL programs interacting with 3270 terminals (often via BMS maps in CICS) explicitly check the AID byte to determine which key was pressed and branch their logic accordingly.

Best Practices:

Consistent Key Assignments: Establish and adhere to consistent assignments for PF keys across all applications (e.g., PF3 for Exit, PF1 for Help) to enhance user experience and reduce training requirements.
Clear On-Screen Instructions: Always provide clear prompts and indicators on the screen to inform users about the functions assigned to available PF keys and other AID keys.
Robust Error Handling: Implement comprehensive error handling in application code to gracefully manage unexpected or invalid AID key presses, guiding the user back to a valid state.
Minimize PA Key Reliance: Avoid using PA keys for primary application logic, as they are generally intended for system-level attention and can be less predictable in application flow than PF or Enter keys.
Optimize Data Transmission: Design screens and application logic to minimize unnecessary data transmission with each AID key press, especially in high-volume environments, to improve network efficiency and application responsiveness.