spu0 gcd

gcd:
    ldi r3 0
    beq r0 r3 gcd_zero
gcd_loop:
    beq r1 r3 gcd_end
    bgt r0 r1 gcd_a_greater_than_b
    sub r1 r0
    jmp gcd_loop
gcd_a_greater_than_b:
    sub r0 r1
    jmp gcd_loop
gcd_zero:
    mov r2 r1
    rts
gcd_end:
    mov r2 r0
    ldi r0 0
    ldi r1 0
    rts