I've finally gotten back to PIC programming, after a series of projects using
MIPS 4300i and 4600 RISC chips, and H8/300L microcontrollers.  The PIC is
more fun, IMHO.  I was just writing up some 16Cxx routines I find handy, in
preparation for adding them to my web page, when it occurred to me that they
might make a nice programming challenge.  I'll describe them here, and I'll
put the code on my web page in about a week.

All three routines use only the W and status registers, don't modify any
RAM, and don't use subroutines.


toupper - equivalent to the toupper() library function in C.
    Entry:  character in W
    Exit:   if the character was in the range 'a'..'z'
                return the corresponding upper case character 'A'..'Z'
            else
                return with W *unchanged*

    6 instruction words, 7 instruction cycles (including return)


toascii - convert the low four bits of W into the ASCII representation of
          a hexadecimal digit
    Entry:  W contains a value in the range 00h..0fh
    Exit:   W contains a the character, '0'..'9', 'A'..'F'
            if the input was out of range, the output is unspecified

    6 instruction words, 7 instruction cycles (including return)


ishex - convert the ASCII representation of a hex digit to a binary value
    Entry:  W contains a character
    Exit:   if the character was a valid hex digit ('0'..'9','A'..'F','a'..'f')
                return the corresponding value 00h..0fh, with STATUS.Z set
            else
                return with W *unchanged* and STATUS.Z clear

    18 instruction words, 8 to 17 cycles (including return)

    note - could be made shorter and faster if it didn't accept both cases
           could be made shorter and slower if it called toupper


Hint - all three routines use the same "trick".  Those of you familiar with
the technique may wish to refrain from posting the solutions immediately
to give others a chance to work it out.

Cheers,
Eric

http://www.brouhaha.com/~eric/pic/