> Right after finished my embedded systems class in college, a friend of
> mine was taking it from a different prof.  The professor gave them a
> challenge:
>
> In the shortest possible time, reverse the order of the bits in a random
> data register.  Assume this is the time critical element of the
> application, and nothing about the application is as important as
> getting the order of these bits reversed as quickly as possible.
>
> Assume the processor is a 16F876.
>
> My friend came up with the fastest possible means I've yet seen.  What
> say you all?

OK, here's the ugly brute force approach:


;   Subroutine to flip the bits in REG0 and return them in W.
;
     glbsub  flipit, noregs

     movlw   high table      ;set high address of table entry
     movwf   pclath
     movf    reg0, w         ;get the byte value to flip
     movwf   pcl             ;jump to selected table entry, returns from
FLIPIT

table code   <some multiple of 256 in same page as FLIPIT>


However, I think there are far more elegant methods making clever use of
SWAP, XOR, etc.

<soapbox>
In a normal application where this isn't a critical function, I would
prefer the clearest and most undestandable approach.  Usually a few extra
instructions are cheaper in the long run if they decrease the chance that
some green programmer three years later long after you've moved on doesn't
mess it up and spend two days trying to figure out why something obscure
broke.  University professors don't usually appreciate this.  While these
kinds of brain teasers can be good mental exercises and help you think
about using the instruction set in different ways, they also create the
false impression that this sort of programming is valuable or even a good
idea.  Most of the time it's neither.
</soapbox>

That said, my first knee jerk reaction would be to shift one register one
bit at a time into C, then shift the other one in from the same direction.
Yes, I'm quite sure there are faster and smaller methods, but this method
is both cheap to write and maintain.  I can probably write this loop
(including comments of course) or the unrolled version in under a minute.
Most of the time it wouldn't be in my customer's interest to spend 5
minutes creating an optimized version.


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