SX Microcontroller Math Method

32 bit Square Root binary restoring method

Andy David says:

Here's my 32 bit routine as written for the PIC 17c43 [ed: and converted to the Scenix SX] taken from a mail I sent Scott just after I wrote it, hence the comments about the implementations I used.

Looks a lot like Scott's original 16-bit sqrt. As the root is going to be a 16 bit number the last subtract is awkward, so the 24-bit sqrt method wasn't appropriate. I did actually write this myself rather than automatically converting Scott's code to 32 bit. I DID, however, consciously and unashamedly steal two parts - how to carry out the final 17-bit subtraction and how to count iterations - the extra 'counting' bit in the mask was quite a devious idea. This one took a little longer to write than the 24-bit, probably because it needs to iterate more times...

Standard disclaimer applies

;=========================================================================
; brSQRT32
;
; Calculates the square root of a thirtytwo bit number using the
; binary restoring method.
;
; Result in ACCaHI:ACCaLO
; Mask in ACCbHI:ACCbLO
; Input in ACCcHI:ACCcLO:ACCdHI:ACCdLO
;
; Takes between ... and ... cycles (incl. call and return).
; Uses 66 words ROM, 8 bytes RAM including 4 holding the input.
;
;-------------------------------------------------------------------------

brSQRT32:
        mov     W, #$40         ; Initial value for Result is...
        mov     ACCaHI, W       ; ... 01000000 00000000
        clr     ACCaLO          ;

        mov     W, #$C0         ; Initial value for mask is...
        mov     ACCbHI, W       ; ... 11000000 00000000
        clr     ACCbLO          ; (second '1' is loop counter).

Sub_Cmp:
        mov     W, ACCaLO       ; Compare root-so-far with current
        sub     ACCcLO, W       ; ... remainder.
        mov     W, ACCaHI       ;
        sb      C
        movsz   W, ++ACCaHI     ;
        sub     ACCcHI, W       ;
        sb      C               ;
        jmp     brstr           ; (result is negative, need to restore).

In1:    mov     W, ACCbLO       ; set the current bit in the result.
        or      ACCaLO, W       ;
        mov     W, ACCbHI       ;
        or      ACCaHI, W       ;

ShftUp: rl      ACCdLO          ;
        rl      ACCdHI          ;
        rl      ACCcLO          ;
        rl      ACCcHI          ;

        rr      ACCbHI          ; Shift mask right for next bit, whilst
        rr      ACCbLO          ; ... shifting IN MSB from remainder.
        snb     ACCbHI.7        ; If MSB is set, unconditionally set the
        jmp     USet1           ; ... next bit.

        mov     W, ACCbLO       ; Append '01' to root-so-far
        xor     ACCaLO, W       ;
        mov     W, ACCbHI       ;
        xor     ACCaHI, W       ;

        sb      C               ; If second '1' in mask is shifted out,
        jmp     Sub_Cmp         ; ... then that was the last normal iteration.

        mov     W, ACCaLO       ; Last bit Generation.
        sub     ACCcLO, W       ; ... The final subtract is 17-bit (15-bit root
        mov     W, ACCaHI       ; ... plus '01'). Subtract 16-bits: if result
        sb      C
        movsz   W, ++ACCaHI
        sub     ACCcHI, W       ; ... generates a carry, last bit is 0.
        sb      C               ;
        ret

        mov     W, #1           ; If result is 0 AND msb of is '0', result bit
        snb     Z               ; ... is 0, otherwise '1'.
        snb     ACCdHI.7        ;
        xor     ACCaLO, W       ;
        ret

USet1:  snb     C               ; If mask has shifted out, leave. final bit
        ret                     ; ... has been set by iorwf at in1.
        clrb    ACCbHI.7        ; clear bit shifted in from input.

        mov     W, ACCbLO       ; Append '01' to root-so-far
        xor     ACCaLO, W       ;
        mov     W, ACCbHI       ;
        xor     ACCaHI, W       ;

        mov     W, ACCaLO       ; This subtraction is guaranteed not to
        sub     ACCcLO, W       ; ... cause a borrow, so subtract and
        mov     W, ACCaHI       ; ... jump back to insert a '1' in the
        sb      C
        movsz   W, ++ACCaHI
        sub     ACCcHI, W       ; ... root.
        jmp     In1     ;

brstr:  mov     W, ACCaLO       ; A subtract above at Sub_Cmp was -ve, so
        add     ACCcLO, W       ; ... restore the remainder by adding.
        mov     W, ACCaHI       ; The current bit of the root is zero.
        snb     C
        movsz   W, ++ACCaHI
        add     ACCcHI, W       ;
        jmp     ShftUp          ;

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