8.3.1-2

        segment .data
data    dq      0xfedcba9876543210
sum     dq      0

        segment .text
        global  main
main:
        push    rbp
        mov     rbp, rsp
        sub     rsp, 16

;   Register usage
;
;   rax : bits being examined
;   rbx : carry bit after bt, setc
;   rcx : loop counter, 0-63
;   rdx : sum of 1 bits
;
        mov     rax, [data]
        xor     ebx, ebx
        xor     ecx, ecx
        xor     edx, edx
while:
        cmp     rcx, 64
        jnl     end_while
        bt      rax, 0
        setc    bl
        add     edx, ebx
        shr     rax, 1
        inc     rcx
        jmp     while
end_while:
        mov     [sum], rdx
        xor     eax, eax
        leave
        ret