Scott, you really looked too long at comparisons :) After subtraction,
a set carry flag can mean two situations - one number is bigger or
equal to another. But if carry is clear, there is no ambiguity - one
number is definitely bigger. So, immediate return can be made only on
cleared carry. I corrected your routine below...

Best regards,
 Nikolai

Wednesday, November 28, 2001, 9:08:49 AM, Scott Dattalo wrote:
> On Wed, 28 Nov 2001, Vasile Surducan wrote:

>> On Tue, 27 Nov 2001, Drew Vassallo wrote:
>>
>> > >Compare two 16 Bit numbers, require flag when one greater than the other?
>> > >
>> > >Anyone have the solution?
>> >
>> > No, but I'm sure you can find it here:
>> >
>> > http://www.piclist.com/techref/microchip/compcon.htm#16_bit
>>
>>
>>   or here: http://www.geocities.com/vsurducan/electro/PIC/lib.html

> I've been looking at comparisons lately. In fact I've been looking at them
> too much and am starting to get sick of it (but that's another story).

> I saw your page Vasile and noticed that it can be dramatically
> streamlined if your're willing to change the interface ever so slightly.

> ; unsigned char compare_lt_eq_gt(unsigned int a, unsigned int b)
> ;
> ; compare two integers. return 0 if they're equal, 1 if a>b, or 2 if a<b
> ;

     movf   HIGH(b)
     subwf  HIGH(a)     ; first upper bytes
;     skpnc
;      retlw 1           ; b is less than a

      skpc
       retlw 2           ; b > a

; high(a) >= high(b)

;     skpnz
;      retlw 2           ; a is less than b

      skpz
       retlw 1           ; a > b

; high(a) = high(b) = 0

     movf   LOW(b)
     subwf  LOW(a)      ; compare lower bytes

;     skpnc
;      retlw 1           ; b is less than a

     skpc
      retlw 2           ; b > a

; a >= b

;     skpnz
;       retlw 2         ; a is less than b
     skpz
      retlw 1           ; a > b

     retlw  0           ; a and b are equal

> However if you want to return the value in a register (like the example on
> your web page) then:

>     clrf   result

>     movf   HIGH(b),w
>     incfsz result,f    ;put 1 into result, note skip will not happen!
>      subwf HIGH(a),w   ;compare high bytes

>     skpnc
>      return            ; b is less than a, return 1

>     rlf    result,f    ; result == 2
>     skpz
>      return            ; a is less than b, return 2

>     movf   LOW(b),w
>     decfsz result,f    ;put 1 into result again
>      subwf LOW(b),w

>     skpnc              ; b is less than a, return 1
>      return

>     bcf   result,0    ; result now = 0, assume a == b
>     skpnz
>      bsf  result,1    ; result now = 2,  a is less than b

>     return

> A little obscure perhaps, but gets the job done. Note, this won't work on
> the 18c core, but then again on the 18c core you'd make use of the addwfc
> instruction.


> Scott

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