The Raspberry Pi is a small, affordable single-board computer that you will use to design and develop fun and practical IoT devices while learning programming and computer hardware. In addition, you will learn how to set up up the Raspberry Pi environment, get a Linux operating system running, and write and execute some basic Python code on the Raspberry Pi. You will also learn how to use Python-based IDE (integrated development environments) for the Raspberry Pi and how to trace and debug Python code on the device. Please note that this course does not include discussion forums.
Lecture 2.2 - Processes
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Из курса от партнера University of California, Irvine
The Raspberry Pi Platform and Python Programming for the Raspberry Pi
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University of California, Irvine
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Курс 4 из 6 — Specialization An Introduction to Programming the Internet of Things (IOT)
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Module 1
This module describes the basic functionality the Raspberry Pi B+ board. I'll describe how to set up the board, configure it, and use it. An important point differentiating Raspberry Pi from the Arduino platform which we have talked about previously is that Raspberry Pi uses an operating system. I'll describe some of the implications of an operating system on the behavior of the Raspberry Pi as an IoT device.
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Ian HarrisProfessor
Department of Computer Science
[MUSIC]
So some of the advantages of using an operating system on a Raspberry Pi like
you couldn't under Arduino.
We've been talking about those, and one of the important advantages is the existence,
the fact that you can run multiple processes at the same time.
What does that mean?
Say you had an Arduino and you write a piece of code and you're running it
on there, that program is the only program running on your Arduino at a given time.
Okay.
But with a Linux machine, and
a Rasberry Pi is essentially a Linux machine, so running Linux.
The operating system allows you to run multiple programs at the same time.
Now first, let me say that when I say they're running at the same time,
they're not really running at exactly the same time.
Basically, you've got one processor.
Let's assume one processor, single core, okay?
You've got one processor and that processor, say there are ten processes,
ten different programs running at the same time.
They're not actually running at the same time, they're swapping quickly, so
the operating system swaps them in an out.
But to the human, it swaps them so
fast that to the human it looks like they're running at the same time.
Okay, so I can do 10, 20 processes running at the same time, looking like they're
running at the same time, even though they're swapping in and out fast.
And this can be very convenient to be able to run multiple
processes at the same time.
What we see on the screen here, you see a terminal window, so
this is a Linux, actually in my desktop, so this isn't an actually, I didn't take
this screenshot of a Rasberry Pi, but it looks actually, exactly the same.
And what I'm doing there is, I type the command ps, which gives me a listing of
all the processes running on my machine at the time when I did this.
And every row is a process, and it has all it's information,
we'll go into more detail about this later.
But the idea is that every row is a particular process.
The right hand column where it says command,
that's the name of the job, the name of the task that's running.
Which we can't interpret those,
those are all processes that are part of the operating system.
The idea though, is that many processes can run at the same time.
So, why is it beneficial?
On one hand, you may say, look, a human can only do one thing at a time.
So why do I need multiple things running at the same time?
Well, it's nice to have multiple things running at the same time because you can
have background processes that are handling things that in the background for
you that are helpful.
So while you, the user are interacting, maybe playing a video game or something
like that, in the background there are processes doing other useful tasks.
Which we'll talk more about this later when we get to
in more detail about Linux in the next module.
But that's a helpful thing.
Now another thing that operating systems give you
is the ability to use different hardware devices easily.
Okay? Now you don't need an operating system to
use different hardware devices.
Take an Arduino,
you can use different hardware devices by buying an Arduino shield, stick it on top.
And then, you call in library functions, right?
But in order to use a hardware device in an Arduino, you have to write code.
You as a programmer have to write a program that invokes certain library
functions and uses the hardware device, okay?
So, you can do it, but you have to write code to do it.
Where with an operating system, you're allowed to use these
different hardware devices to plug in, and you don't have to write code to do it.
That's really the difference.
You don't have to write code.
Now if you're comfortable writing code, as I am, then I can write some code and
I don't need that.
But most users don't want to write code for every little thing they want to do.
They want to just be able to plug in a device, and use that device.
And so operating systems allow that, so
this picture here is trying to give you a bit of an idea
about how an operating system, let's say Linux, accomplishes that.
Specifically Linux, but other operating system's are similar.
So, at the top you got a user application, right up there.
So that user application, just some program you wrote, or
some program you bought.
Maybe you bought some application and you're running that, right?
So, that's the program.
Now, say that program wants to use a hardware device.
And by hardware device, maybe I'm talking about a webcam I plugged into USB, right?
So, that program wants to use that webcam.
So, what does it do?
What it does is it accesses a file.
Now that's the next level down.
See it says slash dev slash XXX, that slash notation,
that's the directory notation.
Path notation, right?
So slash dev.
Dev is short for device.
If you look in a Linux box under slash dev, you see a bunch of sub-directories,
a bunch of files and XXX is one them.
It's not one of them, but xx is just a generic term for a file.
And you got a file in there for every device that's connected to your machine.
So, if you've got a web cam plugged into USB,
there will be some file in that slash dev directory just devoted to that web cam.
Okay.
So that file is associated with the hardware device that you plugged in.
Then, from the user point of view, from the user application or user point of
view, all you need to, say you wanna read data from the webcam, you read the file.
You read the file context.
You read data at the file content.
And that data would be the image data, for instance, from the video And
if was another type of device, say it was a disc drive or something like that or
whatever it was, say you want to write to the disc drive.
Then, if you want to write to it, you write to the file and
that will go into the drive, okay?
So what Linux and a lot of operating systems do is they say look let's,
they give you this uniform interface to all these devices.
They say look, all these devices, how you access a disc drive, and a web cam, and
a microphone is all different But a user doesn't want to see that,
that's way too complicated.
So they say look we're just going to give you a set of files.
You write to the file and read from the file and
that allows you to access every device in the same way.
I want to read from a microphone I want to see what the microphone is hearing.
I read from the appropriate file.
If I want to write to a drive I write to the appropriate file.
So it gives you this nice easy interface.
To access all your hardware devices.
So, there's a file and underneath that, or behind that,
inside the operating system, there's a device driver which I've highlighted here.
And that's a bunch of code.
And what the device driver does is it translates these file accesses into actual
hardware device accesses So that's where the real rubber hits the road.
If you, say it's a disk drive.
You write to the disk drive by writing to a file, so then the device driver has to
say, look, they wanna put this data into this disk drive, I'm gonna have to spin
this disk drive and move the head this way and write the data in this fashion.
The details, the physical details Of how you actually
access the hardware components, the microphone, disk drive,
whatever the hardware component is, that is understood by the device driver.
The device driver handles all those details for you.
So that's why if you ever take let's say a windows machine and
you plug in new device, sometimes in the lower right a little pop up will say
downloading device drivers, searching for device driver and so on.
It's looking for the appropriate device driver.
For that component, to know how to access that particular component.
And once it finds that then it can use that and it makes it very easy for
a user to access any hardware device
without having to write code explicitly for that purpose.
So that's what a device driver is and
there are a set of them, libraries of them.
And then underneath that is the hardware device,
whatever the hardware device is, that is controlled by the device driver code.
So this is another advantage of having an operating system,
is you can have these device drives which are part of the operating system, and
they deal with all the hardware interface details for you.
All you have to do is access files and it makes it a much easier,
more pleasant interface.
Thank you.
[MUSIC]
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