;******************************************************************* ;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) RRF temp,F ;temp = (h + (h>>3)) >> 1 ADDWF temp,F ;temp = h + ((h + (h>>3)) >> 1) RRF temp,F CLRC RRF temp,W ;d = W = (h + (h + (h>>3)) >> 1) >> 2 RETURN
For the more general case:
C = A*B/255
There is another trick that you can attempt. Recall the power series for division:
N N / / e \ / e \2 / e \3 \  =  *  1   +    + ...  v + e v \ \ v / \ v / \ v / /
If, N=A*B, v+e = 255. If you let v=256 and e=1 then the series simplifies to:
A*B A*B / / 1 \ / 1 \2 / 1 \3 \  =  *  1 +  +  +  + ...  256  1 256 \ \256/ \256/ \256/ /
Or just keeping the first two terms:
A*B A*B / 1 \  ~=  *  1 +   255 256 \ 256 /
If A & B are 8bit quantites, then the second term in the series would produce 0 (you'd be dividing the product A*B by 256 twice). However, as Harold notes, you may use the second term to round the result. Specifically, if you treated the multiplication (conceptually) as floating point then you'll end up with a fractional component that's greater than 0.5 if A*B is greater than 2^15. Or stated differently, if the most significant bit is set in the product, then increment the result.
See also:
file: /Techref/microchip/scale.htm, 2KB, , updated: 1999/9/28 17:29, local time: 2014/4/18 19:02,

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