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10 bits
Author: Nikolai Golovchenko - John Payson
This code was originally located @ http://www.piclist.com
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;***********************************************
; Square 10 bits
;
; 6 Aug 2000 by Nikolai Golovchenko
; Based on the original version of John Payson
;
; Input:
; SrcH:SrcL
; Output:
; DstH:DstM:DstL
;
; Instructions: 68
; Execution time(including return): 51+5*3+1=67
;
; Description:
; The goal is calculation of Dst = Src^2
;
; Src may be rewritten as:
; Src = SrcH*256 + SrcL
; Let's introduce variables a, ah, al, and b:
; a = ah*16 + al = SrcL
; b = SrcH
; Then
; Src^2 = (256*b+a)^2=65536*b^2+512*b*a+a^2
;
; Sqr4 routine calculates squares for 4 bit data
; (b, al, and ah) using look-up table.
;
; Now we can find b^2 by Sqr4 and multiply it by 65536,
; and calculate 512*b*a.
;
; To calculate a^2 let's expand it in byte halves:
; a^2=(16*ah+al)^2=256*ah^2+32*ah*al+al^2
;
; So,
; Src^2=65536*b^2+512*b*a+256*ah^2+32*ah*al+al^2
;
; This is the algorithm of how to square a 10 bit number
; using Sqr4 look-up table.
;
;***********************************************
Sqr10
clrf DstH ;clear result
clrf DstM
clrf DstL
clrc ;clear carry
;find 32*ah*al
movf SrcL, w ;w = SrcL
andlw 0x0F
btfsc SrcL, 4
addwf DstM, f
rrf DstM, f ;use carry after addition
rrf DstL, f
;DstM:DstL=ah<0>*al*256/2
btfsc SrcL, 5
addwf DstM, f
rrf DstM, f
rrf DstL, f
;DstM:DstL=(ah<0>*al*256/2 + ah<1>*al*256)/2=64*al(ah<0>+2*ah<1>)
btfsc SrcL, 6
addwf DstM, f
rrf DstM, f
rrf DstL, f
;DstM:DstL=32*al(ah<0>+2*ah<1>+4*ah<2>)
btfsc SrcL, 7
addwf DstM, f
;DstM:DstL=32*al(ah<0>+2*ah<1>+4*ah<2>+8*ah<3>)=32*ah*al
;(maximum value = 0x1C20)
;Now add squared al and ah
call Sqr4
addwf DstL, f ;add al^2
skpnc
incf DstM, f ;propagate carry to DstM
swapf SrcL, w
andlw 0x0F ;w = ah
call Sqr4
addwf DstM, f ;carry is reset
;At this point DstM:DstH contains a^2
;Add 512*b*a to Dst
;512*b*a=512*b*a=512*(2*b<1>+b<0>)*<128*a<7>+a<0:6>)=
;=65536*b*a<7>+256*(2*a<0:6>*b<0>+2*a<0:6>*b<1>+2*a<0:6>*b<0>)
movf SrcH, w ;Dst += 512*b*(128*a<7>) = 65536*b*a<7>
btfsc SrcL, 7
addwf DstH, f ;carry is reset
rlf SrcL, w ;w = a<0:6>*2 (a<7> is already used)
clrc
btfsc SrcH, 0
addwf DstM, f ;Dst += 512*b<0>*a<0:6>
skpnc
incf DstH, f
clrc ;Dst += 512*b<1>*a<0:6>
btfsc SrcH, 1
addwf DstM, f
skpnc
incf DstH, f
clrc ;Dst += 512*b<1>*a<0:6>
btfsc SrcH, 1
addwf DstM, f
skpnc
incf DstH, f
;Add 65536*b^2
movf SrcH, w
call Sqr4
addwf DstH, f
retlw 0 ; All done!
Sqr4: ;Look-up table for 4 bit squares
addwf PCL, f
DT 0,1,4,9,16,25,36,49,64,81,100,121,144,169,196,225
;*********************************************** |
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