【复习】补充

Overflow rule:

If two numbers are added,and they are both positive or both negative,then overflow occurs if the result has the opposite sign.

两个正数或两个负数相加，结果的符号位相反（两个正数相加符号位为1，两个负数相加符号位为0），溢出发生

本题R1>R2，所以CF等于0

关于CF，做的是减法，被减数如果比这个减数要小，需要借位，那么CF才为1

Carry:Set if an operation resulted in a carry(addition) into or borrow(subtraction) out of a high-order bit.Used for multiword arithmetic operations.

router: connect the different networks

switch: connect the different devices

message 报文

segment 段

datagram 数据报

frame 帧