On 18 May 1994, Peter Knight <P.J.Knight@bradford.ac.uk> wrote:
> I take your point as a general idea, but come on - ADDWF f,d where d is
> *direction*!!! Whats wrong with ADD f,W / ADD W,f?

I agree with your comments about Microchip's syntax being suboptimal, but
I should point out that the ",d" doesn't specify the direction, it specifies
the destination.  While this may seem like a minor point, consider the
SUBWF instruction:

        SUBWF   FOO,W   ; W := FOO - W

        SUBWF   FOO,F   ; F := FOO - W

While discussing subtract instructions, I should point out that the SUBLW
instruction subtracts the contents of W *FROM* the literal, which is not what
people generally expect.

At first this seemed brain-damaged but then I discovered that it is useful.
If you want to take the two's complement of W, for instance, you can use

        SUBLW   0       ; W := 0 - W

If you want to subtract a literal from W, you can use the ADDLW
instruction with the two's complement of the literal:

        ADDLW   0feh    ; W := W - 2

Some assemblers will let you write this in a natural form:

        ADDLW   -2      ; W := W - 2

Cheers,
Eric