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Interrupt Enable Flag
After the CCR is pushed on the stack during the interrupt process, the I flag is set so that no further interrupts will be allowed until. . .
the RTI instruction pulls the CCR register off the stack, at which time the I flag is cleared.
Because the I flag is automatically set by an interrupt service routine, the service routine will not be interrupted by another interrupt unless a CLI instruction is placed in the service routine code. If an interrupt must wait for another interrupt to complete before getting control of the processor the interrupt response time can be significantly increased. Interrupt latency issues need to be considered when planning the interrupt interactions.
In general one should keep interrupt routines as short as possible to improve interrupt response times.