BSR - x86-64 instruction reference

bsrBit scan reverse 
Stores the index of the most significant set bit of the source in the destination, or sets ZF when the source is zero.
386 and later
Intel's reference: BSR 
CFPFAFZFSFDFOF ???●?–?
● set from the result
? undefined afterwards
– not changed
Forms  bsr ,  163264 
Operands reg A general-purpose register matching the operation’s size, for example al, ax, eax or rax. The byte registers ah, bh, ch and dh cannot be used with a REX prefix, which is required by spl, bpl, sil, dil and r8–r15. 
r/m A general-purpose register or memory location matching the operation’s size, for example rax or qword [rbp-8]. The byte registers ah, bh, ch and dh cannot be used with a REX prefix, including when the memory address uses r8–r15. 
bsf finds the least significant set bit; bsr finds the most significant. The operands have matching 16-, 32- or 64-bit widths, with indices 0–15, 0–31 or 0–63 respectively.
If the source is zero, ZF is set and the destination should be treated as undefined: current processors leave it as it was, but older ones made no such promise, and some clear the upper half of a 64-bit register even then. Otherwise ZF is cleared, and a 32-bit destination write clears the upper half of its 64-bit register.
 global _start

section .text
_start:
        mov rax, 12
        mov rbx, 5
        mov rcx, 3
        mov rsi, 9

        bsr rax, rbx

        mov rax, 60         ; syscall 60: exit
        xor rdi, rdi
        syscall