Andrew David (akdavid at ultronics.co.uk), Software Manager, Ultronics Ltd, Cheltenham says:
This division routine uses the standard binary longdivision algorithm. The loop iterates once for each bit in the numerator, so it goes round 24 times. For each loop the numerator (sorry, but I can never remember which is the dividend and the divisor  I guess the divisor is the denominator, but I'd hate to get it wrong) is left shifted 1 bit into the remainder, then the remainder is compared with the denominator. If the remainder is greater than the denominator, the denominator is subtracted from the remainder and a 1 is shifted into the quotient, otherwise a 0 is shifted into the quotient. If you can't see how the routine works, try running through an example on paper, in binary, then think about how you'd write the routine.
;============================================================================= ; DIV24_16u ; ; Divides a 24bit number by a 16bit number. Unsigned. ; ; Inputs: ; 24but numerator in ACCcLO:ACCdHI:ACCdLO ; 16bit denominator in ACCbHI:ACCbLO ; ; Outputs: ; 24bit quotient in ACCcLO:ACCdHI:ACCdLO ; 16bit rem in ACCaHI:ACCaLO ; ; Locals used: ; R5Hi ; ; ; Inputs are not preserved. ; ; No timing analysis performed. ; Andrew David, Software Manager, Ultronics Ltd, Cheltenham ; akdavid at ultronics.co.uk http://www.ultronics.com ; ;============================================================================= DIV24_16u: mov W, #24 ; for 24 shifts mov R5Hi, W ; clr ACCaHI ; clear remainder. clr ACCaLO ; d2416lp:rlcf ACCdLO,f ; build up remainder. rlcf ACCdHI,f ; rlcf ACCcLO,f ; rlcf ACCaLO,f ; rlcf ACCaHI,f ; ; remainder is 16bit, but may have spilled over into carry. sb ALUSTA.C ; check for remainder spill into carry. jmp d2416s ; movfp ACCbLO,WREG ; subwfb ACCaLO,f ; Carry bit is the 17th bit of this subtract! movfp ACCbHI,WREG ; subwfb ACCaHI,f ; setb ALUSTA.C ; bit is known to be zero here. jmp d2416ns ; d2416s: movfp ACCbLO,WREG ; Compare remainder with divisor. mov W, ACCaLOw ; movfp ACCbHI,WREG ; subwfb ACCaHI,w ; sb ALUSTA.C ; jmp d2416ns ; (remainder < divisor), shift in a '0' movfp ACCbLO,WREG ; The remainder is larger, so subtract the sub ACCaLO, W ; ... divisor FROM the remainder and movfp ACCbHI,WREG ; ... shift a '1' into the quot. subwfb ACCaHI,f ; d2416ns:decfsz R5Hi,f ; check all bits jmp d2416lp ; rlcf ACCdLO,f ; shift in final quotient bit. rlcf ACCdHI,f ; rlcf ACCcLO,f ; ret
See also:
file: /Techref/scenix/lib/math/div/24by16ad_sx.htm, 3KB, , updated: 2004/6/10 14:40, local time: 2020/9/25 22:44,

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