> dumb question...no I havent read the doc....but quck
> explanation for the difference between a regular return
> and a fast return?  What do you gain by using a fast return?

Ease of execution and speed.  With a fast return, the PIC
assumes that the current values stored in the fast return
shadow registers for STATUS, BSR, and W are correct,
performs a return and restores those values.

Of course, it's up to you to ensure that those values ARE
correct, by only executing fast returns after a call which
caused the appropriate values to be stored in the shadow
registers and ensuring that, while the values in the shadow
registers are important, no other calls occur to overwrite
them.

This becomes important with multiple levels of interrupt.
If a low level interrupt occurs, and your program stores
W, BSR, and STATUS in the shadow registers, then a
high priority interrupt interrupts the low priority one, and
your code again causes the values to be stored in the
shadow registers, a fast return from the high priority
interrupt will work, but if your code then performs a
fast return from the low priority interrupt, the result is
not so certain, because the current state of the shadow
registers is unknown.  In systems with multiple interrupt
priorities, it is crucial that your code perform context
saving for lower priority interrupts without relying on
the fast return "stack" (can it be a stack with only one
level?) to do it for you.

Of course, if you don't have multiple priorities, or if
you do, but want to speed up your service of high
priority interrupts (which is usually why they are
high priority in the first place), the fast return feature
is handy.  It's also much easier to use and doesn't
require the extra registers that "manual" context
saving does.

Mike H.

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