00000000
00000001
00000010
00000011
……
01111111 127 = 2^7-1
10000000负的最大 01111111 10000000 -2^7 = -128
10000001 01111110 01111111 -(2^7-1) = -127
10000010 01111101 01111110 -(2^7-2) = -126
10000011
10000100 10000000 负的最大 +1+1+1+1 负的越来越少
10000101
10000110 01111001 01111010 -(2^7-5) = -122
11111110 00000001 00000010 -2
11111111负的最小 00000000 00000001 -1
-1 = 0-1 = 0000000000000-1 = 111111111111111
-128 = 127+1（越界）= 100000000000000
