6502-OS/util/parity.s65

.export count_set_bits

;;********************************************************************************
;; @function Count the number of set bits in the `A` register
;; @modifies A, X, Y
;; @param A: Byte to count the set bits in
;; @returns Y: The number of set bits in the byte
;;********************************************************************************
.proc count_set_bits
    ldx #9
    ldy #0  ; number of set bits
@loop:
    dex 
    beq @done
    ror
    bcc @loop
@set:
    iny
    bra @loop
@done:
    rts
.endproc