Seeing as I have spent half a day chasing this bug, I thought I would share
it with you all.

If you write an interrupt handler on a 16C84, then you need to manually
store the W register and STATUS register to ensure they main program doesn't
go wrong. No problem so far. You also may need to store the FSR if used by
the main program and interrupts, as in a FIFO handling situation... Easy...

So at the start of the handler I store W and STATUS, and at the end I
restore STATUS and W.

I forgot that re-loading W would of course change the Z flag !!!
So you must restore STATUS, then restore W, and then set or clear Z
depending on the stored value of STATUS. What a mess...

ARGGGGGGGGGGGGGGGGG! half a day wasted... You learn something new every day.

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