On Mon, 30 Jun 1997 08:33:21 -0700 Scott Dattalo <sdattalo@unix.SRI.COM>
writes:
>Kevin Dodd wrote:
>>
>> I have a challange, that without using an additional memory location
>I
>> cannot solve.
>
>>         Pins 4,3,2 are needed to shift right 2 positions for a table
>bias
>
>Try this:
>
>        MOVLW   0
>        BTFSC   PORTC,2
>         IORLW  1
>        BTFSC   PORTC,3
>         IORLW  2
>        BTFSC   PORTC,4
>         IORLW  4

Since the result will be used for table access, it isn't necessary to
preserve the bit order, just make the low 3 bits of W one to one
correspond with the switch inputs.  How about:
        rrf     PORTC,w         ;Port = xxxCBAxx.  Get W =
xxxxCBAx
        andlw   b'00000110'     ;Now  00000BA0
        btfsc   PORTC,4         ;Is C = 1?
        iorlw   b'00000001'     ;Yes, it is.
;W now 00000BAC.  Rearrange the table to account for MSB last.

If speed is more important than code space, make each table entry 2
instructions long and just use a single rotate into W and andlw to get
0000CBA0.  The extra bytes interleaved with the table could be used as
another table by computing an index of 0000CBA1.

Of course, the traditional method requires a RAM location to rotate twice
and is still 4 instructions:
        rrf     PORTC,w         ;Port = xxxCBAxx.  Get W =
xxxxCBAx
        movwf   tmp             ; tmp = xxxxCBAx
        rrf     tmp,w           ;W=xxxxxCBA
        andlw   b'00000111'     ;W=00000CBA