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ST9 USER GUIDE

The difference between them is explained below.

3.4.2.1 Global Interrupt Enable Flag

In both modes, when an interrupt request is acknowledged, the Interrupt ENable (IEN) bit is

cleared, preventing the interrupt service routine from being interrupted again until it is finished.

If required, you may prefer to keep it cleared for the duration of the routine or to set IEN at

some place in the routine using the ei instruction. In the first case, if an interrupt request of a

sufficient priority level is received, it will only be serviced as soon as the service routine cur-

rently running returns. In the second case, the same interrupt request is serviced as soon as

both it occurs and the IEN bit is set. In short, interrupt service routines may be re-interrupted

or not, at will.

3.4.2.2 Concurrent Mode versus Nested Mode

Selecting either Concurrent Mode (automatically chosen on reset) or Nested Mode changes

the way the Current Priority Level is managed.

In Nested Mode, the CPL is automatically set to the priority of the current interrupt service rou-

tine, and is reset to the previous value on return. This allows you handle the interrupt request

according to priority at all times, since during the execution of the service routine for a given in-

terrupt, only those interrupts whose priority is strictly higher than that of the one currently being

served will be taken into consideration. Then if, as must normally be done, the IEN bit is set

during the current service routine, it will be interrupted at once if an interrupt request of higher

priority occurs. If the IEN remains cleared for the whole duration of the service routine, those

interrupt requests will be served first after the current routine has returned.

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