from by Nikolai
Golovchenko
Don't know how to explain its operation, but what it does is approximate calculation of 32768/x using linear interpolation between points where x=1,2,4,8,16,32,64,128,256.
The routine can be extended for other number of bits in x.
;test
cblock
x, yh, yl
endc
clrf x
; movlw 129
; movwf x
again
call Reciprocal8
nop
incf x, f
goto again
;****************************************************
;
; Approximate calculation of 32768/x
; Input: x
; Output: y = 256 * yh + yl ~= 32768/x
;
; To calculate reciprocal, linear interpolation
; is used between points, where x=1,2,4,8,16,32,64,128
;
; Size: 19 instructions
; Execution speed(worst case including return):
; 6+4*8-1+4+4+2=47
;
; 4 Aug 2000 by Nikolai Golovchenko
;****************************************************
Reciprocal8
movf x, w
skpnz
retlw 1 ;error - division by zero
movwf yh
clrf yl
setc
Reciprocal8a
rrf yl, f
rlf yh, f
skpc
goto Reciprocal8a
clrc
rrf yh, w
clrf yh
subwf yh, f
Reciprocal8b
rrf yh, f
rrf yl, f
skpc
goto Reciprocal8b
retlw 0 ;ok
;****************************************************