How Computers Work

Computers are marvels of engineering, yet very few actually know how they work (this holds true among a lot of software developers). This will help augment [How the Computer Works][andrea-laue] by Andrea Laueu.

Transistors

You may remember in Elementry School building a transistor radio, or if you’re an audiophile, you may use transistors in a special sound system. A transistor is a semiconductor device with magical (well, it can seem that way) properties. They are capable fo amplifiying and switching electronic signals as well as amplifiying electronic power. These properties were discovered in the late 1940s, and today are the basis for every electronic device.

What is a transistor?

Semiconductor devices allow us to amplify and switch electronic signals. Different materials with unique physical properties that make this possible. For example, your car stereo, headsets, TV sets, etc., all use amplification transitors to reproduce sound. With computers, we use transistors that “switch” (on and off) to “do stuff” very quickly. Since a transitor doesn’t have mechanical movement (no magents, springs, etc), you can pack transistors together very tightly.

While the first transistors were quite large, they quickly shrank in size. In fact, the current generation of computer processors contain around 1.4 billion transistors.High-end servers can have up to 5.4 billion transistors per chip. This image of an the chip that runs an iPhone (the A7 ARM chip) contains about 1 billion transistors that are each about 22 nm large.

Processors

The “brain” of a computer is what is called the Central Processing Unit (CPU). This is where all the transistors are contained that can turn on-and-off, and controls your computer.

What Does a Computer Do?

A computer only does four things:

1. Basic mathematical and logical operations
2. Store and read values from its memory
3. Control program execution
4. Control and operate attached devices (your hard drive, video card, network card)

Logical Operations

This is the most basic thing a computer can do:

• AND - if A is true, and B is true, then A and B are true. Otherwise, false;
• OR - if A or B is true, A OR B is true, otherwise, false;
• NOT - the opposite of value
• NOR - shorthandfor NOT OR (opposite value of an OR)
• NAND - shothand for NOT AND
• XOR - eXclusive OR: True if only one of the inputs is true; othewise false

In electrical engineering, these are referred to as logic gates which allow one or more inputs to produce a single logical output (true or false). By connecting a series of transistor states (on/off) through relays, we can represent these logical operations.

Propogation Delay

Logical operations don’t happen instantly.

A value is called ‘stable’ when its the final, correct value (there’s aleays output, but not always stable) Each transistor has a delay where it responds to its inputs, which is relative to the physical characteristics of the transistor (size, materials, voltage, etc.) Overall delay is the delay of the longest-path (the longest chain of transistors) An output is stable after the inputs have been stable for at leat the gate delay.

How computers do math

Computers represent number in binary (0 or 1). Basic math (+, -, *, /) can be done with a series of logical operations on each digit

Numbers can be represented in different formats (called bases). You are probably most familiar with the base 10 where numbers are digits (e.g. 0 - 9). However, you can convert these to other mathematical bases, some of which you may have run across:

1. Base 2 (binary) - digits 0 and 1
2. Base 8 (octal) - digits from 0 - 7
3. Base 16 (octal) - digits from 0 - 9, A-F
</blockquote> Add two binary digits (0 or 1), a carry-in (do I need to carry over), comes out with a sum and a carry out. Can chain for bigger adders * Longest path for 1-bit operations 1-bit 1 nanosecond 2-bit 2 nanoseconds The bigger the number, the longer it takes. #### Peformance * A 4-bit adder takes 4 nanoseconds * 4 nanoseconds = 0.0000004 seconds * In one second, we can do 1/(4x10^-9) which if we simplify is 2.5 x 10⁸ 4-bit additions * This means it's running at 2.5 * 10⁸ per second, or hertz (Hz) * 2.5 x 10⁸ = 250 x 10⁶ = 250 Megahertz If you go faster than this, the result will be invalid because the gates haven't delayed. ## The Clock How do you get different calculations running at different speeds together? Different operations can take different amounts of time (e.g. a 3-bit addition vs a 4-bit addition). You need something to synchronize these calculations that you're working on. Each component of an operation will take the clock as an input, then when the clock changes, starts doing its calculation. For physics sake, the clock can't be faster than the longest path needed to stabelize a value. The clock speed says how fast its going, but not how much work is getting done. Your processor (central processing unit or CPU) is rated at a clock speed, like 2.3Ghz (e.g. 2.3 x 10⁹ times per second). ## Memory Your computer needs to retrieve values they are working on, or store results. Two basic operations: read and write (store) There are a *lot* of different kinds of memory. ### Random Access Memory (RAM) Like a memory in your brain; if you can remember somthing, you can pull it up the same way, no matter how old the memory is, or where it is stored. The way you store things in memory is with an address, a number that represents where the value is. To get the value, you need an address. To write to RAM, you have to tell it what the address and value is you want to store, and it updates only that location with the value. At an electrical engineering level, RAM is made out of flip-flops (sets of transitors, not footware) which can hold a value. A flip-flop holds a value until it is set again with a new value. If power is lost to the flip-flop, so is the value (e.g. you turn off your computer). To give you some idea of the number of transisters in your RAM, 1GB of RAM can be implemented as 2³⁰ one-byte registers (or 1,073,741,824 transistors). ### This seems like a bad idea... Yes, you need something to "persist" data for longer durations. In the early days, tape drives were used to save this data, but this posed another issue; you can only read forward or backward to find the information you want, which takes different amounts of time depending on where your information is located. ### Hard drives Hard drives are magnetic devices that have magnetic platters that read and write what is under it. To get the correct data, you turn the circular platter to the correct position to read the data. Hard drives spin at high speeds (generally 5,400, 7,200, 10,000 rpm) to get data. The faster the disk spins, the faster you get data back. The head moves from the outside to center of the disk. The time it takes to spin the disk to the correct location and move the head to read the data is the seek time. Seek times are in milliseconds, which is much (MUCH) slower than RAM (which is in nanoseconds), but they have far more capacity and persist data from power interuptions. ## Computers run programs A program is a set of instructions that tell a processor what to do Each instruction is a simple operation, like retrieve a value from memory, or perform a simple mathematical operation Instructions are stored in order in memory, where the processor has fast access to them The processor executes each instruction in order, then goes to the next one Some instructions are for control flow, and can tell the processor to go back and forth to perform certain instructions over. These are called branches (or jumps for electrical engineers), and you can have non-conditional branches, or branches that are taken if conditions are met, like a value being equal, less than, or greater than. ## Computers Control Devices A processor has several interfaces (or sets of connections) for interacting with other devices (e.g. monitors, USB drives, etc.) The interface defineds the language of how the devices talk to each other (drivers) By sending instructions to the device, the computer can control and read back data. Some devices, like video cards, read directly from system memory. This is a lot faster and takes less processing time. Some common interfaces: SATA, PCI Express, USB, Thunderbolt # Three types of devices CPU, RAM, I/O Devices Hook them together with a "Mother Board" to hook the different components together and provide power. https://www.youtube.com/watch?v=PN7aO81pktU https://www.youtube.com/watch?v=WIDzNyfVVg0 [andrea-laue]: http://www.digitalhumanities.org/companion/view?docId=blackwell/9781405103213/9781405103213.xml&chunk.id=ss1-3-1&toc.depth=1&toc.id=ss1-3-1&brand=default