vbits=0x00000000 ctz(0x00000000)=32
vbits=0x00000000 ctz(0x00000001)=0
vbits=0x00000040 ctz(0x00000090)=4
vbits=0x00000a00 ctz(0x00000500)=8
vbits=0x001e0000 ctz(0x000f0000)=16
vbits=0xfffffffe ctz(0xffffffff)=0