Once a handler receives a request signal for the TSR, it checks the various active flags maintained by the handlers that monitor system status. If any of the flags are set, the handler ignores the request and exits. If the flags are clear, the handler invokes the TSR, usually through a near or far call. Figure 19.1 illustrates how a timer handler detects a request and then periodically scans various active flags until all the flags are clear.
A TSR that changes stacks must not interrupt itself. Otherwise, the second execution would overwrite the stack data belonging to the first. A TSR prevents this by setting its own active flag before executing, as shown in Figure 19.1. A handler must check this flag along with the other active flags when determining whether the TSR can safely execute.