Re: Emulating ADDWFC on 16F PIC uCONTROLLER
From: Anthony Marchini (a.marchini_at_mericonlokorDot.net)
Date: 12/21/04
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Date: Tue, 21 Dec 2004 17:02:10 -0500
>...
> However, Microchip didn't *have* to add another instruction - my
> disbelief was that the add without carry instruction took preference
> over an add with carry instruction. A straight swap leaves the exact
> same number of instructions - 35 I think. So the limiting the number
> of instructions doesn't cut it as an explanation in my eyes.
> ...
> Regards
>
> Dave Giblin
> Lancashire
> England
Its not a simple decision of just eliminating the instruction. There was
simplicity in not lining up the register with a carry in to the ALU data
path of the chip.
The idea is that this processor is supposed to be doing tremendously
simple operations, not high level math, so if you needed the carry you
can implement it.
Now the 18F versions are designed to do "some Math" so they can do DSP
applications. A full ALU data path complete with a carry in from the
status register makes more sense for these devices.
I think the original "plan" for the pic was a design from 1980 or
something like that. Some teachers kicking around a simple processor
model and then microchip taking off with the 16C54 and the rest is history.
I remember seeing a similar datapath mini micro in a textbook from a
while back. It always reminded me of the PIC.
My details are probably way off...I am loosing my mind faster than most
of my age.
Tony
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