As Father Erik intones, from the
Bible : Chapter 3 of the Book Of Intel

Idle Mode
An instruction that sets PCON.0 causes that to be the last
instruction executed before going into the Idle mode, the internal
clock signal is gated off to the CPU but not to the Interrupt, Timer,
and Serial Port functions. The CPU status is preserved in its
entirety; the Stack Pointer, Program Counter, Program Status Word,
Accumulator, and all other registers maintain their data during Idle.
The port pins hold the logical states they had at the time Idle was
activated. ALE and PSEN hold at logic high levels.

There are two ways to terminate the Idle. Activation of any enabled
interrupt will cause PCON.0 to be cleared by hardware, terminating
the Idle mode. The interrupt will be serviced, and following RETI, the
next instruction to be executed will be the one following the
instruction that put the device into Idle.


A short review of interrupt programming may be called for.

Steve