1
00:00:01,010 --> 00:00:05,790
In this lecture, we will look at the basic operating system concepts briefly.

2
00:00:06,460 --> 00:00:09,320
These concepts are used in the entire course.

3
00:00:09,770 --> 00:00:12,680
We will talk about them in detail later in this course.

4
00:00:13,940 --> 00:00:16,520
So the first concept is address space.

5
00:00:17,460 --> 00:00:22,650
Address space is the set of addresses that a process can use to address memory. 

6
00:00:23,160 --> 00:00:25,650
We will see what the process is in a moment.

7
00:00:26,250 --> 00:00:29,370
In this course, we focus on 64-bit addresses, 

8
00:00:30,440 --> 00:00:36,970
each number references a specific address giving us an address space of two to the power of 64 bytes 

9
00:00:37,130 --> 00:00:38,370
which is really large.

10
00:00:39,240 --> 00:00:45,470
Also, note that the system we are talking about is byte addressing which means the smallest addressable unit is byte

11
00:00:45,470 --> 00:00:46,880
.

12
00:00:47,480 --> 00:00:54,730
So each address references a storage location which is a byte long.

13
00:00:54,740 --> 00:01:00,040
Within this address space, we can keep part of it for RAM and part for hard disk and some other devices. 

14
00:01:00,770 --> 00:01:04,310
So, the address space is normally larger than the physical memory. 

15
00:01:06,670 --> 00:01:14,800
The next concept is process. A process is basically a program in execution. Process contains program instructions, 

16
00:01:14,800 --> 00:01:20,890
the data the program needs, heap and stack within the address space.  

17
00:01:21,950 --> 00:01:28,460
Each process has its address space which process can read and write.

18
00:01:28,460 --> 00:01:36,080
Process also contains other resource such as registers, a list of open files, etc.  So a process is like a container 

19
00:01:36,080 --> 00:01:38,950
holding all the information in order to run the program.

20
00:01:39,770 --> 00:01:46,880
Here is an example of memory map when process is running,  as you see, it includes program instructions, 

21
00:01:46,880 --> 00:01:48,050
data and stack, 

22
00:01:49,140 --> 00:01:55,710
typically the address is from 0 to max size which can be defined by the OS. 

23
00:01:55,710 --> 00:02:01,800
The operating system kernel resides in memory above the max size. The kernel code, data, etc. 

24
00:02:03,880 --> 00:02:09,699
When we have multiple processes running on the computer, what happens is that each process has its own address space

25
00:02:09,699 --> 00:02:10,660
,

26
00:02:11,510 --> 00:02:16,000
the program instructions, data, stack are different among processes. 

27
00:02:16,750 --> 00:02:23,840
The memory reference within one process does not affect the address space of other processes.  

28
00:02:23,860 --> 00:02:30,880
However, the kernel which resides in the higher part of address space are the same across all the processes 

29
00:02:30,880 --> 00:02:31,300
.

30
00:02:31,300 --> 00:02:38,320
The operating system kernel includes the essential information about each process, such as process state, etc. 

31
00:02:38,320 --> 00:02:41,790
And user programs are not allowed to access data in kernel space. 

32
00:02:42,190 --> 00:02:44,160
This brings us operation mode. 

33
00:02:44,920 --> 00:02:46,840
There are two modes of operations, 

34
00:02:47,500 --> 00:02:49,170
kernel mode and user mode. 

35
00:02:49,840 --> 00:02:53,270
The operating system runs in kernel mode. 

36
00:02:53,310 --> 00:03:00,250
In this mode, we can execute all the instructions and access all the hardware. The application programs run in user mode 

37
00:03:00,370 --> 00:03:04,300
where they can only execute a subset of the instructions 

38
00:03:04,300 --> 00:03:06,790
and they cannot access the data in the kernel space. 

39
00:03:08,140 --> 00:03:14,950
When a program wants to access data in kernel or hardware, it calls a special set of functions called system calls.

40
00:03:14,950 --> 00:03:21,860
The system call acts as an interface between user programs and the operating system.  

41
00:03:22,480 --> 00:03:27,880
When a program calls a system call, the control is transferred to the operating system. 

42
00:03:28,420 --> 00:03:34,030
The operating system will perform the operation the program needs and return to the user program

43
00:03:34,030 --> 00:03:35,100
once it is done.