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> From: Scott Dattalo <sdattalo@UNIX.SRI.COM>
> To: PICLIST@MITVMA.MIT.EDU
> Subject: Re: Conversion of byte to percentage
> Date: Thursday, May 21, 1998 10:53 AM
>
> adrian aslett wrote:
> >
> > I have a need to convert a byte value into a percentage. I know I have
> > to work with integer values but does anyone know how I should go about
> > doing this.
> >
> > Any ideas will be appreciated ! !
>
> Here's one that'll do it:
>
>
> ;*******************************************************************
> ;scale_hex2dec
> ;  The purpose of this routine is to scale a hexadecimal byte to a
> ;decimal byte. In other words, if 'h' is a hexadecimal byte then
> ;the scaled decimal equivalent 'd' is:
> ;    d = h * 100/256.
> ;Note that this can be simplified:
> ;    d = h * 25 / 64 = h * 0x19 / 0x40
> ;Multiplication and division can be expressed in terms of shift lefts
> ;and shift rights:
> ;    d = [ (h<<4) + (h<<3) + h ] >> 6
> ;The program divides the shifting as follows so that carries are
> automatically
> ;taken care of:
> ;    d =   (h + (h + (h>>3)) >> 1) >> 2
> ;
> ;Inputs:   W - should contain 'h', the hexadecimal value to be scaled
> ;Outputs:  W - The scaled hexadecimal value is returned in W
> ;Memory:   temp
> ;Calls:    none
>
> scale_hex2dec
>
>         MOVWF   temp            ;Hex value is in W.
>         CLRC                    ;Clear the Carry bit so it doesn't
> affect RRF
>         RRF     temp,F
>         CLRC
>         RRF     temp,F
>         CLRC
>         RRF     temp,F          ;temp = h>>3
>         ADDWF   temp,F          ;temp = h + (h>>3)
What happens if result > 0xFF?
>         RRF     temp,F          ;temp = (h + (h>>3)) >> 1
>         ADDWF   temp,F          ;temp = h + ((h + (h>>3)) >> 1)
Again, same issue.
>         RRF     temp,F
>         CLRC
>         RRF     temp,W          ;d = W = (h + (h + (h>>3)) >> 1) >> 2
>
>         RETURN

This is excellent, but will not work for values > 64%.  I like how clean it
is, though.  You could add a line to check if MSB set.  Subtract 128 if
set, and add 50% at the end.