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[MUSIC]. 

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Hi, there. 
We're now starting a new section. 

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We're going to be talking about 
processes, which is an abstraction 

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provided by the operating system together 
with the hardware. 

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Just so we can run programs in 
interesting ways, okay? 

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So then, we're going to see, we're going 
to start seeing what a process is. 

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Then, we're going to see how we create 
processes using operating system 

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services. 
And then, we'll see how we can do 

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interesting things with processes. 
So, what is a process? 

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And, but first of all, why are we 
learning about processes? 

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Well, because they're an abstraction of 
the computer system that provides this 

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interface between the program, and the 
underlying CPU plus memory. 

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Okay, and this is provided by the 
operating system. 

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Okay, so what the process have to do with 
exceptional control flow? 

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Well, the operating system's going to 
have to use tricks with exceptional 

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control flow to give the illusion that 
multiple processes are running 

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simultaneously in the system, okay? 
So what is a program? 

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A program is just a list of instructions, 
a collection of instructions and data 

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that form your code, that form your 
software, okay? 

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A processor, obviously, is the piece of 
hardware that runs these structures in a 

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program. 
In a process, well a process is an 

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instance of a running program. 
When you start running a program, you 

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create a process to run the program, 
therefore the process is an instance of a 

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running program, okay? 
So, that's a very important idea in 

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Computer Science, and it's not the same 
as a program or a processor, okay? 

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A processor executes instructions, and a 
program is a collection of instructions 

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and data. 
A process is an instance of a program 

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running in a processor, so and they 
provide two key abstractions. 

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The first one is logical control flow. 
So, that means that each process has the 

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illusion that it has full control of the 
CPU as exclusive use of the CPU. 

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Which might actually be in the, in 
reality the case, but processes give this 

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abstraction to programs that so to 
processes such that it seems like they 

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have full use of the CPU, okay? 
And now, the other one is private virtual 

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address space. 
Means that each, this gives the illusion 

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that each process. 
Has control over the entire memory. 

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Nobody else is touching memory. 
Okay, so this is very important because 

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it makes it much easier to isolate 
programs to manage memory layout and so 

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on, okay? 
So why are they important? 

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Well, we simplify writing programs a lot 
because you give the illusion that they 

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have the program, the program has entire 
memory. 

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And also gives it illusion that it has 
complete control of the process, or 

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complete use of the process. 
And how are these illusions maintained? 

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Well, first of all process, process 
executions are interleaved. 

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That means from the process point of 
view, which means it's a switch between 

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one process and the other. 
And the address space is managed by 

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neutral memory by, by the visionary 
system which is going to be a topic of 

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our next section. 
So processes can be concurrent. 

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And it means if they run concurrently, if 
the instruction flows overlap in time. 

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Otherwise, they are sequential. 
So, let me give you an example. 

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Suppose that I have Process A here, 
Process A is running, having control in 

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the CPU, then now it's paused for a 
second. 

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And then it continues running and it ends 
here, okay? 

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Process B starts here and ends here. 
Process C starts here, runs, pauses for a 

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bit and ends right there. 
So, it means that processes that are 

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concurrent are A and B are concurrent, 
okay? 

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And also A and C, because you see, they 
are actually running parallel. 

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However, B and C are not concurrent 
because B ends here right before C 

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starts. 
now, the user view of concurrent 

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processes as if they are actually running 
at the same time. 

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Okay, that's the illusion that they have 
full control of the CPU. 

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When in reality, what's going on is that 
one, a processor might be paused, but 

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still conceptually executing, but not 
really. 

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It's not actually making use of the 
processor. 

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so we can really think of concurrent 
processes as executing in parallel. 

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Now, how, how does that work? 
Let's say they have a single process or 

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you have multiple processes running at 
the same time, how does that work? 

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We need some mechanism that bounces 
between processes that are running at the 

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same time. 
So, this is what we call context 

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switching, okay? 
So, the process are managed by a shared 

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chunk of OS code called the Kernel, okay? 
And it's important to note that the 

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kernel is not a seperate process, but 
it's just a piece of code. 

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A special piece of code that runs as part 
of a user process. 

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But the kernel code has special 
privileges. 

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So, here's how the context switch 
happens. 

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Say that Process A is running, and then 
suddenly it's time-based on a timer. 

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There's a timer interrupt that says okay, 
the time, processor time for Process A, 

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CPU time for Process A is up. 
So now, there's an interrupt here, time 

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interrupt, that switches to kernel code. 
And the kernel code is going to save the 

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state. 
You save A here, save the state of A. 

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And then you restore B, and then B 
executes then times up again. 

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The, the kernel code executes through the 
context switch, and the context switch 

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saves B here and restores the state of A 
and continue. 

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So that's, that's a process called 
Context Switch. 

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Now what's actually saved here? 
What will we have to save here? 

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We have to save the instruction pointer 
because the we have to know where Process 

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A stopped. 
So, we need to restore it again and 

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continue allocating from that point. 
It also needs to save register states, 

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okay? 
Because we need to restore those to 

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continue executing the program. 
And the, there are a few other things 

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that also happen there, but that's that's 
subject for a another time.