Ed, I have an 16F877 application that **has** to be accurate. Much
of program memory plus external flash is bit-mapped in a chance game

I had a look at some shift-type routines, but found that the remainder
was not always accurate, even with a 0.5 test. Perhaps not quite true,
but the more bits you use for the division, the more bits are in the
remainder fraction, and by the time you work through those it would
sometimes have been quicker to do the division the by subtraction

Can't describe the game itself in detail, but basically there are three
very large bit tables, all the same length. To access a number in each
eg 9,473 => 9473/14 => 676 whole bytes, remainder indicates the
position in the 677th byte

So, anyway, this isn't fast, but it's accurate. I'm sure it could be made
a little faster but it's sufficient for my application as is

Hope it helps (you or someone)

;================================================
;        Divide by subtraction, 
;        Slow but guaranteed accurate remainder
;================================================

div14    bank0
         bcf     res_eq0      ;result=0 flag
         movlw   0x0e         ;divisor
         movwf   dsorl
         clrf    dsorm
         clrf    dsorh
         clrf    dendh
         clrf    resl         ;result low
         clrf    resh         ;result high
         clrf    rem          ;remainder

         movf    dendm,f      ;test dividend for < 14
         btfss   zero
         goto    dloop        ;dendm > 00, continue division

         movlw   0x0e
         subwf   dendl,w
         btfsc   status,c
         goto    dloop        ;dendl > 13, continue division
         movf    dendl,w      ;dendl between 1 and 13, exit
         movwf   rem
         return               ;exit with result=0 + remainder

;------- division loop for num > 13

dloop    btfsc   dendh,7      ;test for < 00 result
         goto    end_div

         bcf     status,c
         movf    dsorl,w      ;subtract divisor from dividend
         subwf   dendl,f

         movf    dsorm,w      ;ripple through high bytes
         btfss   status,c
         incfsz  dsorm,w
         subwf   dendm,f

         movf    dsorh,w
         btfss   status,c
         incfsz  dsorh,w
         subwf   dendh,f

         incfsz  resl,f
         goto    dloop
         incf    resh,f
         goto    dloop

end_div  decfsz  resl,f       ;return with address
         movlw   0x0e         ;in resh / resl
         addwf   dendl,w      ;and bit position in rem
         movwf   rem
         btfss   rem,7
         goto    test_0
         movlw   0x0e         ;correct any underflow
         addwf   rem,f

;------- test result for 0

test_0   movf    resl,w
         addwf   resh,w       ;add resl to resh
         btfsc   zero         ;if resl <> 00
         return               ;else exit with res_eq0 = 0
         bsf     res_eq0
         return               ;exit with res_eq0 = 1

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