sqrt16.a

; https://github.com/TobyLobster/sqrt_test/blob/main/sqrt/sqrt16.a
; adapted and fixed from https://archive.org/details/bbc-micro-machine-code-portfolio/page/99/mode/2up
; and optimised for size, not speed!

byte = $02              ; two byte input value
temp = $04              ; two byte temporary value

* = $0200

sqrt
    ldy #0              ;
    sty temp+1          ; temp = 0
    ldx #0              ;
    sec                 ;
loop
    inx                 ; temp += 1
    stx temp            ;
    lda byte            ;
    sbc temp            ; carry is always set here
    sta byte            ; byte -= temp
    lda byte+1          ;
    sbc temp+1          ;
    bcc finished        ; branch if we fall below zero
    sta byte+1          ;
    iny                 ; root += 1
    inx                 ; temp += 1
    bne loop            ;
    inc temp+1          ;
    bne loop            ; always branch
finished
    rts                 ; y = root