Maxim Integrated DS4830 Optical Microcontroller - Manual (Page 41)

DS4830 User’s Guide
41
When the prefix register is activated by writing a value to it, it retains that value only for the next clock cycle. For the prefix
value to be used properly by the next instruction, the instruction that sets the prefix value and the instruction that uses it
must always be executed back to back. Therefore, writing to the PFX register causes an interrupt exception window on
the next cycle. If an interrupt occurs during an interrupt exception window, an additional latency of one cycle in the
interrupt handling will be caused as the interrupt will not be serviced until the next cycle.