> The hardware multiply isn't very helpful if you want to do more than
8bit
> multiplications, as it requires that you write everything the standard
way
> instead of utilising the multiply function of the chip.

Huh?

You can build up a larger multiply using a number of 8 x 8 multiplies.
This may seem a little awkward, but it's still faster than a total
software shift and add loop.

Let's say A and B are two 16 bit numbers.  Each is made of two bytes,
A1:A0 and B1:B0.

  A = 256*A1 + A0
  B = 256*B1 + B0

  A * B =
  (256*A1 + A0) * (256*B1 + B0) =
  256*256*A1*B1 + 256*A1*B0 + 256*A0*B1 + A0*B0

This uses the hardware multiplier 4 times and you have to do a few
multi-byte additions, which are also easier on the 18 series due to the
add with carry instruction.  Obviously the multiplies by 256 and 256*256
are performed simply by shifting over one and two bytes.  All in all this
takes a lot fewer cycles than doing the same thing in a shift and add
software loop.


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