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View code
48 by 24 bit
Author: Nikolai Golovchenko - Scott Dattalo - Tony Kubek
This code was originally located @ http://www.piclist.com
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***********************************************************************
; DIVIDE_48by24 - Routine to divide a 48 bit number with a 24 bit number
; result upto 48 bits !
;
; Formula: Dividend = Dividend/Divisor
;
; Format: Little endian, Ram = msb, Ram+x = lsb ( where
x=bytesize-1)
; Ram used: Dividend 6 bytes ( 48 bits )
; Divisor 3 bytes ( 24 bits )
; Temp 3 bytes
; BitCount 1 byte for loop counting ( and temp. saving of
carry )
;
; Divisor is preserved
; Dividend holds result
; Returns with zero in W if failed ( division with zero )
; Else returns with one in w
;
; Based on pseudo-code from Nikolai Golovchenko [golovchenko@mail.ru]
;
;
DIVIDE_48by24
; Test for zero division
MOVF Divisor,W
IORWF Divisor+1,W
IORWF Divisor+2,W
BTFSC STATUS,Z
RETLW 0x00 ; divisor = zero, not possible to calculate return with zero in w
; prepare used variables
CLRF Temp
CLRF Temp+1
CLRF Temp+2
MOVLW D'48' ; initialize bit counter
MOVWF BitCount
CLRC ; clear carry at entry
DIVIDE_LOOP_48by24
; a) first time carry cleared
; b) every other bit carry contains the result of
; last subtraction ( division ).
; shift in carry as lowest bit
RLF Dividend+5,F
RLF Dividend+4,F
RLF Dividend+3,F
RLF Dividend+2,F
RLF Dividend+1,F
RLF Dividend,F
; shift in highest bit from dividend through carry in temp
RLF Temp+2,F
RLF Temp+1,F
RLF Temp,F
; save carry status in bitcounter temporarily ( possible overflow from temp msb )
BTFSC STATUS,C
BSF BitCount,7 ; set bit ( carry was set, temp>24bit)
; subtract 24 bit divisor from 24 bit temp
MOVF Divisor+2,W ; get lsb
SUBWF Temp+2,F ; subtract
MOVF Divisor+1,W ; get middle byte
SKPC ; if overflow ( from prev. subtraction )
INCFSZ Divisor+1,W ; incresase source
SUBWF Temp+1,F ; and subtract from dest.
MOVF Divisor,W ; get top byte
SKPC
; if overflow ( from prev. subtraction )
INCFSZ Divisor,W ; increase source
SUBWF Temp,F ; and subtract from dest.
; BTFSC STATUS,C ; save carry status again to use for negative check
; BSF BitCount,7 ; yes carry was set.
; if was carry set ( saved in top bit of bitcount ):
; a) Temp overflowed ( i.e. temp > divisor )
; b) Result was positive from subtraction ( i.e. temp > divisor )
; if carry was cleared:
; a) temp < divisor
; BTFSC BitCount,7
; GOTO DIVIDE_SKIP_48by24 ; carry was set, subtraction ok, continue with next bit
;Scott Dattalo says:
;...[the lines above which are commented out can be] reduced to:
skpc
btfsc BitCount,7
goto DIVIDE_SKIP_48by24
; result of subtraction was negative restore temp
MOVF Divisor+2,W ; get LSB of divisor
ADDWF Temp+2,F ; add it to the lsb of temp
MOVF Divisor+1,W ; middle byte
BTFSC STATUS,C ; check carry for overflow from previous addition
INCFSZ Divisor+1,W ; if carry set we add 1 to the source
ADDWF Temp+1,F ; and add it if not zero in that case Product+Multipland=Product
MOVF Divisor,W ; MSB byte
BTFSC STATUS,C ; check carry for overflow from previous addition
INCFSZ Divisor,W
ADDWF Temp,F ; handle overflow
DIVIDE_SKIP_48by24
; CLRC ; clear carry
; carry should now reflect if divison was possible ( i.e. temp > divisor )
; so if it was set, set carry to be shifted in as lowest bit of Dividend
; BTFSC BitCount,7
; BSF STATUS,C ; restore carry
; BCF BitCount,7 ; clear bit used for saving carry
; DECFSZ BitCount,F ; decrement loop counter
; GOTO DIVIDE_LOOP_48by24 ; another run
;Scott Dattalo says:
;...[the lines above which are commented out can be] reduced to:
rlf BitCount,W ;Restore the carry
bcf BitCount,7 ;clear temporary carry
decfsz BitCount,F
goto DIVIDE_LOOP_48by24
; finally shift in the last carry
RLF Dividend+5,F
RLF Dividend+4,F
RLF Dividend+3,F
RLF Dividend+2,F
RLF Dividend+1,F
RLF Dividend,F
NOP
NOP
RETLW 0x01 ; done |
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