1
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Continue with the divide by zero handler. 

2
00:00:03,190 --> 00:00:09,970
As you can see we have added push and pop instructions. What the push instructions do is saving the state of cpu 

3
00:00:09,970 --> 00:00:13,120
when interrupt or exception occurs. 

4
00:00:13,930 --> 00:00:21,060
When that happens, only some of registers are saved, such as rip and rsp registers.  

5
00:00:21,070 --> 00:00:25,570
In the handler, we will use and modify registers to perform specific tasks. 

6
00:00:26,560 --> 00:00:32,140
Therefore when we return from the handler, the value of registers may have changed 

7
00:00:32,140 --> 00:00:36,260
of CPU is not the same as it was before the entropic and the state of cpu is not the same as it was before the interrupt occurs.

8
00:00:36,760 --> 00:00:42,430
So in order to restore the state of cpu after the interrupt handling is done, we store all the general purpose registers 

9
00:00:42,430 --> 00:00:44,080
on the stack. 

10
00:00:44,980 --> 00:00:52,030
In 64-bit mode, we have 16 general purpose registers. Rsp is pushed on the stack 

11
00:00:52,030 --> 00:00:53,650
when cpu calls the handler, 

12
00:00:53,770 --> 00:00:56,080
So here we push 15 registers.

13
00:00:58,290 --> 00:01:04,440
After we handle the interrupt, we pop the original value in those registers and return. 

14
00:01:06,200 --> 00:01:11,980
Stack is like a last-in first-out structure.  So we pop the value on the stack in reverse order. 

15
00:01:13,420 --> 00:01:19,390
Ok now, if the interrupt is fired, the handler will save all the general purpose registers first, 

16
00:01:20,510 --> 00:01:27,230
do its work and then pop the value on the stack back to those registers. 

17
00:01:27,230 --> 00:01:32,500
When we return from the handler, the previous work CPU is doing is resumed just like the interrupt never happened. 

18
00:01:34,430 --> 00:01:40,840
As you see, when divided by zero handler is called, we just stop here. So we don’t test the code in this lecture

19
00:01:40,850 --> 00:01:41,480
.

20
00:01:42,760 --> 00:01:47,790
In next video, we will discuss the timer interrupt and test it in that example.

21
00:01:49,340 --> 00:01:50,750
That's it for this lecture.

22
00:01:50,780 --> 00:01:52,040
See you in the next video.