On Sun, 1 Dec 2002, Bob Axtell wrote:

> In standard binary arithmetic, the SUB function can
> be substituted by the NEGate function followed by
> an ADD.  NEG is also not part of the 12C08 instructions,
> so to do that, complement and increment the result by 1.
> The carry will set if (negate of reg) + (limit) overflows, which
> corresponds to subtracting limit from reg > w.
>
> In the following PIC12C508A code, if limit is set to d'10', reg
> will count to 9 then go to 0.
>
> inccmp  macro    reg, limit
>          incf    reg,f
>          comf    reg,w
>          incf    w
>          addwf   limit,w
>          btfss   status,c
>          clrf    reg
>         endm

This clearly will not work. First, W is not a register (you'll need an
'addlw 1' instead of an 'incf w') and second, the original poster was
comparing "reg" to a constant "limit". One other subtle issue is that the
original macro would return if reg was greater than limit.

Fortunately, Eke's problem is solved with a 'subwf':

inccmp  macro    reg, limit
     incf   reg,F
     movlw  limit+1
     subwf  reg,W
     skpc
      return        ; reg is less than or equal to LIMIT
     clrf  reg      ; reg is greater than LIMIT

  endm

This changes the comparison from an A-B to a B-(A-1) and looks at the
opposite state of the carry flag.

Scott


>
> --Bob
>
> ----- Original Message -----
> From: "Eke Neehr" <ake.neehr@TELIA.COM>
> To: <PICLIST@MITVMA.MIT.EDU>
> Sent: Sunday, December 01, 2002 10:01 AM
> Subject: [EE]: PIC 12C508A
>
>
> Hello!
>
>
> For a Pic 16F84 device I have this
> macro routine:
>
> inccmp  macro    reg, limit
>                incf         reg, f
>                movf       reg, w
>                sublw      limit
>                btfsc       STATUS, C
>                 return
>                clrf          reg
>                endm
>
> but for the 12C508A device the instruction
> sublw don4t exist.
> Can any kind person tell me what I have
> to take for instructions to get the same function!
>
> Regards
> EkeN
>
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