Up one level

In my last post (well, last excepting Wednesday’s little topical deviation), I talked about the real nuts and bolts of a computer, detailing the function of the transistors that are so vital to the workings of a computer. Today, I’m going to take one step up and study a slightly broader picture, this time concerned with the integrated circuits that utilise such components to do the real grunt work of computing.

An integrated circuit is simply a circuit that is not comprised of multiple, separate, electronic components- in effect, whilst a standard circuit might consist of a few bits of metal and plastic connected to one another by wires, in an IC they are all stuck in the same place and all assembled as one. The main advantage of this is that since all the components don’t have to be manually stuck to one another, but are built in circuit form from the start, there is no worrying about the fiddliness of assembly and they can be mass-produced quickly and cheaply with components on a truly microscopic scale. They generally consist of several layers on top of the silicon itself, simply to allow space for all of the metal connecting tracks and insulating materials to run over one another (this pattern is usually, perhaps ironically, worked out on a computer), and the sheer detail required of their manufacture surely makes it one of the marvels of the engineering world.

But… how do they make a computer work? Well, let’s start by looking at a computer’s memory, which in all modern computers takes the form of semiconductor memory. Memory takes the form of millions upon millions of microscopically small circuits known as memory circuits, each of which consists of one or more transistors. Computers are electronic, meaning to only thing they understand is electricity- for the sake of simplicity and reliability, this takes the form of whether the current flowing in a given memory circuit is ‘on’ or ‘off’. If the switch is on, then the circuit is represented as a 1, or a 0 if it is switched off. These memory circuits are generally grouped together, and so each group will consist of an ordered pattern of ones and zeroes, of which there are many different permutations. This method of counting in ones and zeroes is known as binary arithmetic, and is sometimes thought of as the simplest form of counting. On a hard disk, patches of magnetically charged material represent binary information rather than memory circuits.

Each little memory circuit, with its simple on/off value, represents one bit of information. 8 bits grouped together forms a byte, and there may be billions of bytes in a computer’s memory. The key task of a computer programmer is, therefore, to ensure that all the data that a computer needs to process is written in binary form- but this pattern of 1s and 0s might be needed to represent any information from the content of an email to the colour of one pixel of a video. Clearly, memory on its own is not enough, and the computer needs some way of translating the information stored into the appropriate form.

A computer’s tool for doing this is known as a logic gate, a simple electronic device consisting of (you guessed it) yet more transistor switches. This takes one or two inputs, either ‘on’ or ‘off’ binary ones, and translates them into another value. There are three basic types:  AND gates (if both inputs equal 1, output equals 1- otherwise, output equals 0), OR gates (if either input equals 1, output equals 1- if both inputs equal 0, output equals 0), and NOT gates (if input equals 1, output equals 0, if input equals 0, output equals 1). The NOT gate is the only one of these with a single input, and combinations of these gates can perform other functions too, such as NAND (not-and) or XOR (exclusive OR; if either input equals 1, output equals 1, but if both inputs equal 1 or 0, output equals 0) gates. A computer’s CPU (central processing unit) will contain hundreds of these, connected up in such a way as to link various parts of the computer together appropriately, translate the instructions of the memory into what function a given program should be performing, and thus cause the relevant bit (if you’ll pardon the pun) of information to translate into the correct process for the computer to perform.

For example, if you click on an icon on your desktop, your computer will put the position of your mouse and the input of the clicking action through an AND gate to determine that it should first highlight that icon. To do this, it orders the three different parts of each of the many pixels of that symbol to change their shade by a certain degree, and the the part of the computer responsible for the monitor’s colour sends a message to the Arithmetic Logic Unit (ALU), the computer’s counting department, to ask what the numerical values of the old shades plus the highlighting is, to give it the new shades of colour for the various pictures. Oh, and the CPU should also open the program. To do this, its connections send a signal off to the memory to say that program X should open now. Another bit of the computer then searches through the memory to find program X, giving it the master ‘1’ signal that causes it to open. Now that it is open, this program routes a huge amount of data back through the CPU to tell it to change the pattern of pretty colours on the screen again, requiring another slue of data to go through the ALU, and that areas of the screen A, B and C are now all buttons, so if you click there then we’re going to have to go through this business all over again. Basically the CPU’s logical function consists of ‘IF this AND/OR this happens, which signal do I send off to ask the right part of the memory what to do next?’. And it will do all this in a miniscule fraction of a second. Computers are amazing.

Obviously, nobody in their right mind is going to go through the whole business of telling the computer exactly what to do with each individual piece of binary data manually, because if they did nothing would ever get done. For this purpose, therefore, programmers have invented programming languages to translate their wishes into binary, and for a little more detail about them, tune in to my final post on the subject…

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Getting bored with history lessons

Last post’s investigation into the post-Babbage history of computers took us up to around the end of the Second World War, before the computer age could really be said to have kicked off. However, with the coming of Alan Turing the biggest stumbling block for the intellectual development of computing as a science had been overcome, since it now clearly understood what it was and where it was going. From then on, therefore, the history of computing is basically one long series of hardware improvements and business successes, and the only thing of real scholarly interest was Moore’s law. This law is an unofficial, yet surprisingly accurate, model of the exponential growth in the capabilities of computer hardware, stating that every 18 months computing hardware gets either twice as powerful, half the size, or half the price for the same other specifications. This law was based on a 1965 paper by Gordon E Moore, who noted that the number of transistors on integrated circuits had been doubling every two years since their invention 7 years earlier. The modern day figure of an 18-monthly doubling in performance comes from an Intel executive’s estimate based on both the increasing number of transistors and their getting faster & more efficient… but I’m getting sidetracked. The point I meant to make was that there is no point me continuing with a potted history of the last 70 years of computing, so in this post I wish to get on with the business of exactly how (roughly fundamentally speaking) computers work.

A modern computer is, basically, a huge bundle of switches- literally billions of the things. Normal switches are obviously not up to the job, being both too large and requiring an electromechanical rather than purely electrical interface to function, so computer designers have had to come up with electrically-activated switches instead. In Colossus’ day they used vacuum tubes, but these were large and prone to breaking so, in the late 1940s, the transistor was invented. This is a marvellous semiconductor-based device, but to explain how it works I’m going to have to go on a bit of a tangent.

Semiconductors are materials that do not conduct electricity freely and every which way like a metal, but do not insulate like a wood or plastic either- sometimes they conduct, sometimes they don’t. In modern computing and electronics, silicon is the substance most readily used for this purpose. For use in a transistor, silicon (an element with four electrons in its outer atomic ‘shell’) must be ‘doped’ with other elements, meaning that they are ‘mixed’ into the chemical, crystalline structure of the silicon. Doping with a substance such as boron, with three electrons in its outer shell, creates an area with a ‘missing’ electron, known as a hole. Holes have, effectively, a positive charge compared a ‘normal’ area of silicon (since electrons are negatively charged), so this kind of doping produces what is known as p-type silicon. Similarly, doping with something like phosphorus, with five outer shell electrons, produces an excess of negatively-charged electrons and n-type silicon. Thus electrons, and therefore electricity (made up entirely of the net movement of electrons from one area to another) finds it easy to flow from n- to p-type silicon, but not very well going the other way- it conducts in one direction and insulates in the other, hence a semiconductor. However, it is vital to remember that the p-type silicon is not an insulator and does allow for free passage of electrons, unlike pure, undoped silicon. A transistor generally consists of three layers of silicon sandwiched together, in order NPN or PNP depending on the practicality of the situation, with each layer of the sandwich having a metal contact or ‘leg’ attached to it- the leg in the middle is called the base, and the ones at either side are called the emitter and collector.

Now, when the three layers of silicon are stuck next to one another, some of the free electrons in the n-type layer(s) jump to fill the holes in the adjacent p-type, creating areas of neutral, or zero, charge. These are called ‘depletion zones’ and are good insulators, meaning that there is a high electrical resistance across the transistor and that a current cannot flow between the emitter and collector despite usually having a voltage ‘drop’ between them that is trying to get a current flowing. However, when a voltage is applied across the collector and base a current can flow between these two different types of silicon without a problem, and as such it does. This pulls electrons across the border between layers, and decreases the size of the depletion zones, decreasing the amount of electrical resistance across the transistor and allowing an electrical current to flow between the collector and emitter. In short, one current can be used to ‘turn on’ another.

Transistor radios use this principle to amplify the signal they receive into a loud, clear sound, and if you crack one open you should be able to see some (well, if you know what you’re looking for). However, computer and manufacturing technology has got so advanced over the last 50 years that it is now possible to fit over ten million of these transistor switches onto a silicon chip the size of your thumbnail- and bear in mind that the entire Colossus machine, the machine that cracked the Lorenz cipher, contained only ten thousand or so vacuum tube switches all told. Modern technology is a wonderful thing, and the sheer achievement behind it is worth bearing in mind next time you get shocked over the price of a new computer (unless you’re buying an Apple- that’s just business elitism).

…and dammit, I’ve filled up a whole post again without getting onto what I really wanted to talk about. Ah well, there’s always next time…

(In which I promise to actually get on with talking about computers)