As mentioned in the "Exec Tasks" chapter, it is sometimes necessary to disable interrupts when examining or modifying certain shared system data structures. However, for proper system operation, interrupts should never be disabled unless absolutely necessary, and never for more than 250 microseconds. Interrupt disabling is controlled with the Disable() and Enable() functions. Although assembler DISABLE and ENABLE macros are provided, we strongly suggest that you use the system functions rather than the macros for upwards compatibility and smaller code size. In some system code, there are nested disabled sections. Such code requires that interrupts be disabled with the first Disable() and not re-enabled until the last Enable(). The system Enable() and Disable() functions are designed to permit this sort of nesting. Disable() increments a counter to track how many levels of disable have been issued. Only 126 levels of nesting are permitted. Enable() decrements the counter, and reenables interrupts when the last disable level has been exited.
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