SX Microcontroller Math Method

Fast 8 Bit Square Root

by Nikolai Golovchenko

;***********************************************
;
; 8 bit Fast Square root
;
; Input: x (not changed)
; Output: y
; Temporary: Temp
;
; Instructions: 29
; Execution time (includng return):
;  31 cycles
;
; August 9, 2000 by Nikolai Golovchenko
;
;***********************************************
sqrt8fast
        mov     W, x            ;1
        mov     Temp, W
        clr     y
;find bit 3 of result
        mov     W, #$40         ;4
        sub     Temp, W
        rl      y
        sb      y.0
         add    Temp, W         ;restore Temp
;find bit 2 of result
        or      W, #$10         ;9
        sb      y.0
         xor    W, #$40
        sub     Temp, W
        rl      y
        sb      y.0
         add    Temp, W         ;restore Temp
;find bit 1 of result
        mov     W, <>y          ;y = 16 * result<3:2>
        or      W, #$04         ;set bit 1
        sub     Temp, W         ;Temp - (4 * (4 * result<3:2> + 1))
        rl      y               ;store result
        sb      y.0             ;20
         add    Temp, W         ;restore Temp
;find bit 0 of result
        xor     W, #$0E         ;set bit 0 and compensate for set bit 0 in previous substaction
        sb      y.0             ;24
         xor    W, #$08
        rl      Temp            ;shift left the remainder (carry ignored)
        sub     Temp, W         ;Temp - (4 * (4 * result<3:1> + 1))
        rl      y               ;if borrow clear bit 0 and set otherwise
        ret                     ;31
;***********************************************

See also:

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