At 12:58 AM 3/19/00 -0500, you wrote:




This is what I use for my 17Cxx programmer that also has the i/o lines
swapped. This is optimized for space but is considerably slower than
other solutions. However it is up to you to decide your priorities.

;**************************************
;SPIN
;enter byte to reverse in abuff.
;exit with reversed byte in bbuff
;KnownZero must be zero of course. Code will leave knownZero as 0

;(Do NOT use with interrupt driven code that may use KnownZero!)


               bsf KnownZero,3  ;count = 8
yyr6           rrf abuff
               rlf bbuff
               decfsz KnownZero
               goto yyr6

Jim


>Nickolei's solution edited out for brevity.
>
>>
>> On Friday, March 17, 2000 Andy Baker wrote:
>> > Dear All,
>>
>> > Another one of those algorithm challenges (apologies if this is a well
known
>> > one):
>>
>> > How do I reverse the bits in an 8 bit register in as few instructions as
>> > possible?
>>
>> > i.e. you start with a byte 'abcdefgh' and end up with 'hgfedcba'.
>>
>> > The reason I'd like to know is that I've got my data lines to an LCD
display
>> > reversed on my PCB,, but don't want to redesgin the PCB as it is
otherwise
>> > OK.
>>
>> > Cheers,
>>
>> > Andy
>
>Actually Andy your specification is somewhat vague. Does the few instructions
>refer to actual instructions executed or total instruction size of the code.
>
>For example the absolute shortest execution time is simply to have a 256
>byte lookup table and jump to the appropriate entry. No more than 6
>instructions executed taking 10 cycles but the total code cost is 260
>instructions in memory.
>
>Let's see if I can come up with a quick compromise that utilizes the
>swapf instruction and a 16 byte lookup table:
>
>swapbyte:
>        movf    byte,W          ; get the low nybble
>        andlw   15              ; And mask it off
>        call    reverse         ; reverse the nybble
>        movwf   out             ; Save it
>        swapf   out,F           ; Swap to high position
>        swapf   byte,W          ; Get the high nybble
>        andlw   15              ; Mask
>        call    reverse         ; reverse
>        iorwf   out,F           ; Merge with other nybble
>        return
>
>reverse:
>        addwf   PCL,F           ; jumptable
>        retlw   15              ; reverse of 0
>        retlw   14              ; reverse of 1
>        ...
>        retlw   0               ; reverse of 15
>
>Let's see the mainline code takes 14 cycles. The reverse takes 4. So
>Nickolei's code is faster.
>
>BAJ
>
Regards,

Jim Robertson
NEWFOUND ELECTRONICS
________________________________________
Email: newfound@pipeline.com.au
http://www.new-elect.com
MPLAB compatible PIC programmers.
________________________________________