little endian and big endian(zz)

简而言之：
Big endian machine: It thinks the first byte it reads is the biggest.
Little endian machine: It thinks the first byte it reads is the littlest.
举个例子，从内存地址0x0000开始有以下数据
 0x0000     0x12
 0x0001     0x34
 0x0002     0xab
 0x0003     0xcd
如果我们去读取一个地址为0x0000的四个字节变量，若字节序为big-endian，则读出
结果为0x1234abcd；若字节序位little-endian，则读出结果为0xcdab3412.
如果我们将0x1234abcd写入到以0x0000开始的内存中，则结果为
                big-endian     little-endian
0x0000     0x12              0xcd
0x0001     0x23              0xab
0x0002     0xab              0x34
0x0003     0xcd              0x12
x86系列CPU都是little-endian的字节序.