Exec can provide a task with its own task-local "interrupt" called an exception. When some exceptional event occurs, an Exec exception occurs which stops a particular task from executing its normal code and forces it to execute a special, task-specific exception handling routine. If you are familiar with the 680x0, you may be used to using the term "exceptions" in a different way. The 680x0 has its own form of exception that has nothing to do with an Exec exception. These are discussed in more detail in the "Task Traps" section of this chapter. Do not confuse Exec exceptions with 680x0 exceptions. To set up an exception routine for a task requires setting values in the task's control structure (the Task structure). The tc_ExceptCode field should point to the task's exception handling routine. If this field is zero, Exec will ignore all exceptions. The tc_ExceptData field should point to any data the exception routine needs. Exec exceptions work using signals. When a specific signal or signals occur, Exec will stop a task and execute its exception routine. Use the Exec function SetExcept() to tell Exec which of the task's signals should trigger the exception. When an exception occurs, Exec stops executing the tasks normal code and jumps immediately into the exception routine, no matter what the task was doing. The exception routine operates in the same context the task's normal code; it operates in the CPU's user mode and uses the task's stack. Before entering the exception routine, Exec pushes the normal task code's context onto the stack. This includes the PC, SR, D0-D7, and A0-A6 registers. Exec then puts certain parameters in the processor registers for the exception routine to use. D0 contains a signal mask indicating which signal bit or bits caused the exception. Exec disables these signals when the task enters its exception routine. If more than one signal bit is set (i.e. if two signals occurred simultaneously), it is up to the exception routine to decide in what order to process the two different signals. A1 points to the related exception data (from tc_ExceptData), and A6 contains the Exec library base. You can think of an exception as a subtask outside of your normal task. Because task exception code executes in user mode, however, the task stack must be large enough to supply the extra space consumed during an exception. While processing a given exception, Exec prevents that exception from occurring recursively. At exit from your exception-processing code, you should make sure D0 contains the signal mask the exception routine received in D0 because Exec looks here to see which signals it should reactivate. When the task executes the RTS instruction at the end of the exception routine, the system restores the previous contents of all of the task registers and resumes the task at the point where it was interrupted by the exception signal. Exceptions Are Tricky. ---------------------- Exceptions are difficult to use safely. An exception can interrupt a task that is executing a critical section of code within a system function, or one that has locked a system resource such as the disk or blitter (note that even simple text output uses the blitter.) This possibility makes it dangerous to use most system functions within an exception unless you are sure that your interrupted task was performing only local, non-critical operations.
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