1vbits=0x00000000 ctz(0x00000000)=32 2vbits=0x00000000 ctz(0x00000001)=0 3vbits=0x00000040 ctz(0x00000090)=4 4vbits=0x00000a00 ctz(0x00000500)=8 5vbits=0x001e0000 ctz(0x000f0000)=16 6vbits=0xfffffffe ctz(0xffffffff)=0 7