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this one is from: [[http://www.geocities.com/oneelkruns/asm1step.html]] (defunct page)

Returns the 8-bit square root in $20 of the 16-bit number in $20 (low) and $21 (high). The remainder is in location $21. 

<code>
sqrt16
	LDY #$01 ; lsby of first odd number = 1
	STY $22
	DEY
	STY $23 ; msby of first odd number (sqrt = 0)
again
	SEC
	LDA $20 ; save remainder in X register
	TAX ; subtract odd lo from integer lo
	SBC $22
	STA $20
	LDA $21 ; subtract odd hi from integer hi
	SBC $23
	STA $21 ; is subtract result negative?
	BCC nomore ; no. increment square root
	INY
	LDA $22 ; calculate next odd number
	ADC #$01
	STA $22
	BCC again
	INC $23
	JMP again
nomore
	STY $20 ; all done, store square root
	STX $21 ; and remainder
	RTS
</code>

\\
And here a version for a 24bit input number: (use page zero locations for best performance; anyway is a bit slow!)

<code 6502acme>
;-----------------------------------
;   Square Root of a 24bit number
;-----------------------------------
; by Verz - Jul2019
;-----------------------------------
;
;  load the 24bit input in square
;  16bit result in  sqrt & remainder
;-----------------------------------

square    byte 0,0,0     ; input number
storage   byte 0,0,0     ; temporary data
sqrt      byte 0,0       ; result
remainder byte 0,0       ; result remainder

sqrt24
        LDY #$01        ; lsby of first odd number = 1
        STY storage
        DEY
        STY storage+1   ; msby of first odd number (sqrt = 0)
        sty storage+2
        sty sqrt
        sty sqrt+1
again
        SEC
        LDA square     ; save remainder
        sta remainder             
        SBC storage    ; subtract odd lo from integer lo
        STA square
        LDA square+1
        sta remainder+1
        SBC storage+1   ; subtract odd mid from integer mid
        STA square+1
        lda square+2
        sbc storage+2   ; subtract odd hi from integer hi
        sta square+2
        BCC nomore    ; is subtract result negative?
        INC sqrt      ; no. increment square root
        bne sqnxt
        inc sqrt+1
sqnxt   LDA storage     ; calculate next odd number
        ADC #$01        ; +1+C(=1)        
        STA storage
        BCC again
        lda storage+1
        adc #$00
        sta storage+1
        bcc again
        INC storage+2
        JMP again
nomore
        RTS
</code>