Andy David says:
Here's my 24bit routine as written for the 17c43 [ed: and converted to Scenix SX] taken from a mail I sent Scott just after I wrote them, hence the comments about the implementations I used.I did a few pencilandpaper runs through the algorithm to be sure I understood the steps involved. This 24bit resembles these 'manual' steps of shifting along the input string 2 bits at a time and subtracting  as the result is 12bit, the final subtraction isn't awkward as in the 32bit sqrt. I can't remember the performance figures for this routine, but I've just counted 348 min, 406 max (inc. call & return). A downside of this method is that the loop counter has to be a seperate variable, so it uses the same amount of ram as the 32bit sqrt. This one is easier to follow and compare to an example on paper than the method I've used for the 32bit sqrt.
I've added a 16bit square root routine (again for the 17c43) that uses successive approximation to find the square root, the binary restoring method I'm fairly sure would be quicker. This is the only 16bit sqrt routine I have specifically for the 17cxx PICs, I've only included it as the original poster requested a 16bit routine. If you really want speed speed speed, I'd rewrite the 32bit routine to be 16bit.
Standard disclaimer applies
;========================================================================== ; brSQRT24 ; ; Calculates the square root of a twentyfour bit number using the ; binary restoring method. ; ; Result in ACCaHI:ACCaLO ; Input in ACCcLO:ACCdHI:ACCdLO ; Test ACCbLO:ACCcHI ; Counter in ACCbHI ; ; 44 words long, uses 8 bytes RAM (inc. 3 holding 24bit input). ; ; brSQRT24: clr ACCaHI ; clear result clr ACCaLO ; clr ACCcHI ; clear test bytes clr ACCbLO ; mov W, #12 ; initialize counter mov ACCbHI, W ; (6 cycle intro, 9 incl. call) ShftUp: rl ACCdLO ; Shift input up 2 places. rl ACCdHI ; (39 cycles per loop if bit is 0) rl ACCcLO ; (33 cycles per loop if bit is 1) rl ACCcHI ; rl ACCbLO ; rl ACCdLO ; rl ACCdHI ; rl ACCcLO ; rl ACCcHI ; rl ACCbLO ; rl ACCaLO ; Shift rootsofar up by two and append rl ACCaHI ; ... '01'. rl ACCaLO ; rl ACCaHI ; clrb ACCaLO.1 ; setb ACCaLO.0 ; SubTest: mov W, ACCaLO ; sub ACCcHI, W ; mov W, ACCaHI ; sb C ; movsz W, ++ACCaHI ; sub ACCbLO, W ; snb C ; jmp Set1 ; mov W, ACCaLO ; Restore the remainder. add ACCcHI, W ; ... (the current bit is 0). mov W, ACCaHI ; snb C ; movsz W, ++ACCaHI ; add ACCbLO, W ; jmp Set0 ; Set1: setb ACCaLO.1 ; Set0: rr ACCaHI ; rr ACCaLO ; clrb ACCaHI.7 ; BitLoop: decsz ACCbHI ; jmp ShftUp ret
See also:
file: /Techref/scenix/lib/math/sqrt/sqrt24_sx.htm, 4KB, , updated: 2004/6/10 14:40, local time: 2019/9/20 18:12,

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