5.3 What's inside?
I will now examine a physical desktop PC by dismantling it and looking for the places where the processing happens. This exploration will take me down into the central processing unit, where I will see that at the lowest level of the computer there are simple electronic switches.
So let’s take a look and see what can be found inside a modern desktop computer. You should keep in mind that we can think about the computer system as a symbolic message-passing and manipulation system making use of standard data formats and communication protocols.
12 (Exploratory)
Download this video clip.Video player: Inside the box: section 2
Transcript: Inside the box: section 2
[Graphic title: Opening the case – what’s inside?]
[Music – duration 00:12]
Spencer Kelly
Let us now turn our attention to the components inside the system box. Now Darien, is this a fairly standard PC?
Darien Graham-Smith
Yeah, I mean, it’s not a brand new one obviously, but you’ll find systems like this in plenty of businesses and homes.
Spencer
OK. Parisa, is any of this of interest to a computer scientist?
Parisa Eslambolchilar
I very rarely look inside my computers. I leave it to the hardware people.
Spencer
You have people to do that? She actually has people …
Parisa
(Laughs) I’m only interested in what the machine can do.
Spencer
Right. Now Darien, there’s a fan in here. What’s that about?
Darien
Well, the central processing unit which is inside the computer, it can get very hot in use, it can get to more than 80 degrees Celsius, and that’s actually hot enough for it to destroy itself. So we have a fan inside there, we also have a heat sink with thermal paste on it there, and that keeps it all cool.
Spencer
OK, Parisa.
Parisa
The fan has nothing to do with my generic view of computing. This is only an engineering component.
Spencer
Right. [to Darien] You can keep that, then.
And I recognise this as well. This is the power supply, and these are the power cables that supply electricity to all the components inside. Now Darien, the message I’m getting from you is that this is yet another standard component, it’s just there to make all the other bits work, yeah?
Darien
That’s right. If you’re building a PC you need to make sure that all the components are receiving the right power through the right connectors, and that means using a standard power supply.
Spencer
OK, and Parisa, the message I’m getting from you is anything that helps the components to actually physically operate is not part of your view of computing.
Parisa
Yes, that’s right.
Spencer
OK.
[pause]
More standard components here. That’s the DVD drive, and this is the internal hard disk drive.
Darien
That’s right. These are both storage devices, but they’re a little bit different. This one’s self-contained; this one uses removable disks, obviously. Also they’ve got different connectors. This one uses a SATA (pronounced: satter) or SATA (pronounced: sarter) connector; this one has the older IDE interface.
Parisa
In my view of computing devices, these two are both memory devices and both capable of storing symbolic information and data permanently.
Spencer
Now, I recognise this because I’ve just put a new one in my computer. This is a graphics interface card. Now this is a pretty old one, but the newer ones will actually have a fan on top as well. I guess that’s because they generate quite a lot of heat, yeah?
Darien
That’s right, yes, graphics cards have their own processor – it’ll also have its own memory. And they’re both optimised to produce 2D and 3D images at very high speeds, so yes, it does generate quite a lot of heat.
Parisa
Yes, graphic cards used to be quite simple output devices, but modern graphic processing units have become so specialised they’ve become actually a computing device in their own right. They are capable of running image-processing algorithms and they can do any computational algorithms such as codebreaking very, very efficiently. So I can draw the same generic model for the graphic processing unit. So I can actually have the same big box but this time for the graphics card, with the graphics processor and the memory to store the symbolic data and just outside I have input/output devices. And these three units are communicating and talking to each other in the communication channels or data buses.
Spencer
So just to be clear, you are considering this graphics card as both a component of this computer and a computer in its own right.
Parisa
Yes, exactly. So in my view this is both an example of an input/output device in this generic model and also a computing device in its own right.
And these two symbolic manipulation devices are communicating and exchanging symbolic data continuously and just talking to each other.
Spencer
So is this what you do, then, when you’ve got complex components like this? You consider them as two different computers that exchange information and also one will control the other if necessary.
Parisa
Exactly. Or take the example of the digital camera. This is a very complex computing device. It’s got memory, it’s got a processor, it’s got input and output units and also the communication channels. But when you connect the digital camera to the main computer, the main computer sends symbolic instructions to this device and they start exchanging symbolic data with each other.
Spencer
And the same applies to other devices as well, so it sounds like a pretty powerful generalisation.
Parisa
Exactly.
Spencer
OK. Now, this has got an aerial on it, so I guess that makes this the wireless card.
Darien
That’s right. The wireless card lets the PC connect to a wireless network, and most laptops now have built-in wireless adapters so you can connect to the internet at home or perhaps in a coffee shop, for example.
Parisa
Yes, in my view this is an example of an input/output interface device. It turns symbolic electronic data inside a computer into a symbolic wireless signal outside and vice versa.
Spencer
Now, there’s a few wires left inside here and they’re not actually connected to the power supply, so what are these for?
Darien
Well, these cables simply connect some of the components together and they connect to some of the external ports, and generally these just bring everything together into one complete system.
Spencer
It’s a complete mess in here, I tell you. There you go.
Parisa
Oh, thank you. In my view these are data address and control buses. They allow the communication between the processor, memory, input/output devices to take place. So they allow the models of input/output devices, processor and the memory I have built to be treated as a single interconnected system.
Spencer
Right then. Well, let’s just remove the final piece of the puzzle. Let’s get rid of the case.
Darien
Well, don’t discard the case because this is a component as well. Obviously it needs to be big enough to hold all the various parts of the computer, it needs to have the correct mountings, and also we’ve spoken about fans – it needs to be designed to allow air to flow through to keep everything cool.
If you’re designing a high-end desktop system you could spend quite a long time just thinking about the air flow.
Spencer
OK, fair enough. I stand corrected.
Right, so we’ve been inside the box, and we’ve disconnected the various components attached using this wiring to this central circuit board here. Now, from an engineering viewpoint these are all the bits that make the computer work: the power, the cooling, the wiring and, of course, the standard components connected using standard interfaces. The computer scientist has a general model which ignores the specific detail of the hardware and concentrates instead on what the computer is capable of doing.
Inside the box: section 2
Interactive feature not available in single page view (see it in standard view).
The important point to establish is that the computer operates by its subsystems exchanging symbolic values. The exchange of symbols is governed by protocols, and electronic components ensure that the symbol representations (such as voltage levels, timed signals, etc.) are correct for each subsystem. When a complex sequence of symbols makes up a message, either the computer executes an algorithm (represented as another set of symbols) that implements a communications protocol, or the electronic components involved in the message exchange are constructed in such a way that they follow a series of steps that implement the protocol.
To a computer engineer the power supplies, cooling fans, wiring, etc. are important because they make the computer work, but to a computer scientist or a software engineer they’re simply the stuff that allows the computer science – the implementation of the algorithms – to happen.
Now let’s think about what is happening inside the central processing unit of the computer.