2^0 = 1
2^1 = 2
2^2 = 4
2^3 = 8
2^4 = 16
2^5 = 32
2^6 = 64
2^7 = 128
11111111 是八位 转换呢就是 1+2+4+8+16+32+64+128=255咯.