Protecting the input stack from attention interrupts

When a CLIST is executed, it translates each statement into an executable format and places it in a section of storage called the input stack. The input stack is the source from which TSO/E obtains its input (TSO/E commands, CLIST statements).

If you write an attention routine that does not terminate the CLIST, protect the input stack from being erased (flushed) from storage when an attention interrupt occurs. You can protect the input stack by coding a CONTROL statement with the MAIN operand. The MAIN operand indicates that the CLIST is the main CLIST in the invoker's TSO/E environment and prevents TSO/E from flushing the input stack in the event of an attention interrupt.

Attention routine processing depends on whether CONTROL MAIN has been coded, and whether the routine executes a TSO/E command, RETURN statement, or null line.

If CONTROL MAIN has not been coded, the CLIST terminates and the user sees the READY message, indicating that control has returned to the terminal.
If CONTROL MAIN has been coded, and a null line executes in the attention routine, the CLIST continues at the statement or command that was executing when the user entered the attention interrupt.
If CONTROL MAIN has been coded, and a TSO/E command or RETURN statement is issued, the CLIST continues at the statement or command following the one that was executing when the user entered the attention interrupt.

Also refer to z/OS TSO/E User's Guide, for a further explanation of attention interrupt processing.