Computer data

A bit (short for “binary digit”) is the smallest possible unit of information. Computers represent all information in bits because they are the simplest information to store and manipulate electronically.

Each bit is in one of two states, usually represented as 0 and 1. What these two states represent is up for grabs: maybe they mean on or off, yes or no, true or false, up or down, left or right, etc. Like with all messages, the meaning depends upon pre-established agreement between the sender and recipient. For example, “one if by land, two if by sea” is arbitrary. The only way Paul Revere knew what one or two lights meant depended upon pre-established agreement with the message sender. We can use multiple bits to represent more interesting kinds of information, such as numbers, text, images, and audio.

First, how do we represent numbers as bits? The counting system we use in English is called “decimal”. It uses ten symbols to represent numbers: 0 1 2 3 4 5 6 7 8 9. This choice of ten symbols, though, is arbitrary. We could just as well use seven, fifteen, or thirty-four symbols. The fewest number of symbols we could use, though, is two. The system of counting that uses just two symbols, 0 and 1, is called binary. Because it uses just two symbols, we can naturally represent binary numbers with bits. So to represent numbers in bits, we use binary rather than decimal. For example, the quantity we call “twenty-two” and write “22” is represented in binary as “10110” (read this as “one zero one one zero”). (How to convert between decimal and binary is not something we’ll get into.)

To represent text, we represent each character of text as a number (in binary). The choice of which number represents which character is arbitrary: the important thing is that the sender and receiver agree which number represents which character. So for example, to send the messages “a BANANA”, I can send it as 97 32 66 65 78 65 78 65 as long as the recipient knows I intend 97 to represent “a”, 32 to represent a space, 66 to represent “B”, 65 to represent “A”, etc. (Notice that lowercase “a” and uppercase “A” are different characters.) In practice, it would be horribly messy if every computer program in the world used its own arbitrary mapping of numbers to characters, so instead, most programs use a standardized set of characters called Unicode. Consequently, text data written by most programs can be read correctly by most other programs.

A computer image is made up of a grid of colored dots called “pixels” (short for “picture elements”). If we can represent colors as numbers, we can represent the color of each pixel that makes up an image: each row of pixels can be represented as a series of numbers, one number for each color of pixel. (Most computer images these days look realistic because they are made up of many pixels, but if you go back to the 1970’s and 1980’s, most computer images looked blocky because they were made up of too few pixels to look realistic. Video games, for example, had very block graphics in the 1980’s.) A video is simply one image after another, so if we can represent an image as numbers, we can represent a video as numbers.

Audio can also be reduced to a series of numbers. A sound wave’s height changes over time, so if we record the height of a wave as a number at regular intervals along its length, we get an approximation of the wave. The more height information we record, the more accurate our representation. (The rate at which we record the wave heights is called the “sampling rate”: CD-quality sound has a sampling rate of 44.1 kilohertz, meaning 44,100 samples per second.)

Bits and bytes

For talking about large quantities of bits, we use prefixes:

kilo- (thousand)

mega- (million)

giga- (billion)

tera- (trillion)

So, for example, “six megabits” = 6,000,000 bits.

However, usually when we talk about bits, we talk in terms of bytes. A byte is a group of eight bits. (Why eight? This was a historical decision, but it basically boils down to the fact that eight is a power of two and not too big or too small.)

So, for example, “two kilobytes” = 2,000 bytes = 16,000 bits.

We can abbreviate these terms, and when we do, lowercase ‘b’ means bit while uppercase ‘B’ means bytes, e.g.

40 GB = 40,000,000,000 bytes = 160,000,000,000 bits

40 Gb = 40,000,000,000 bits  = 5,000,000,000 bytes

So be careful when talking in terms of bits and bytes: if you mistake bits or bytes, you can be off by a factor of eight. By convention, we almost always talk in terms of bytes. The only exception is when talking about network transfer speeds. My internet connection, for example, is sold to me as having a download speed of 10 megabits per second. (I wish it were 10 megabytes because then it would be 8 times as fast.)

Files and directories

When we store data on our computer, we do so in units called files. A file is simply a sequence of any number of bytes. A small file might have only a few bytes while a large file might be gigabytes in size. Most files are less than a megabyte. When programs create files, what the bytes of the file actually mean are up to that program. In the end, it’s just a bunch of bits. Whether the bits are intended to represent text, images, audio, or something else is up to the writer of the bits.

Files can be stored on many different kinds of storage devices. The most common types of storage devices today include:

hard drive – a device that stores bits on magnetic surfaces of spinning platters. A PC should have at least one hard drive inside, which is where your operating system and other programs reside. A typical hard drive sold today stores between 300 and 1000 GB.

flash drive – a  portable device the size of a stick of gum or smaller that plugs into the computer via a USB port. The typical flash drives sold today hold between 1 and 32 GB.

DVD drive – a DVD disc stores bits, so they can be used to store any kind of data, not just movies. A single DVD disc holds 4.8 or 9.6 GB.

In Windows, each drive is known by an assigned letter. Your main hard drive is usually assigned to the letter C, while your DVD drive is usually drive D. Any remaining drives will be given the letters E, F, G, and so forth.

A directory (a.k.a. “folder”) is a list of files and other directories. Every drive has a special directory called the “root directory”, which otherwise doesn’t have a name (other directories have names). Effectively, the directories on a drive form a hierarchy, with the root directory at the top. The files on the drive are all located in one of these directories.

A file path is a way of expressing where a file is stored on a computer. It begins with the letter of the drive followed by a colon, then the directories in which the file is contained, and then the name of the file itself, all separated by slashes. For example:

C:\washington\eisenhower\lincoln

This refers to a file named lincoln in a directory named eisenhower, which itself is in a directory named washington, which itself is in the root directory of the drive assigned the letter C.

As a user, you mainly interact with files when saving and opening them in programs. For example, in Microsoft Word, when you save your document, it is saved as a file. You the user choose which directory on which drive to store the file. When you wish to open that saved file later, you must remember which directory on which drive you put it in.

By convention in Windows, file names end with an “extension”, a dot followed by a few letters (usually three). The extension is intended to denote the type of the file. For example, the names of Microsoft Word document files should end with the extension “.doc”.

Files and directories can be browsed and manipulated in the Windows file browser, which can be accessed by selecting a location on the right side of the Start Menu.

To rename a file or directory, right-click on it and select “rename”. To create a directory, select “new folder” in the menu bar. To delete files and directories, select them and hit the delete key on the keyboard. (By default, deleted files and directories are not really deleted but instead moved to a special directory called the “recycle bin”. This is to give you a second chance to decide whether or not you really want to delete the file. To really delete a file, delete it from the recycle bin.)

You may wish to move or copy one or more files or directories to another directory, possibly on another drive. To do this, you simply open the directory, select the file(s) and directories, cut or copy them, then navigate to the destination directory and paste. (Copying or moving a directory will copy or move all of the contents of that directory along with the directory itself.)

(Moving files and directories to a different directory on the same drive shouldn’t take any time because the files don’t have to actually be rewritten: the files are simply delisted from one directory and then listed in another. When moving files to another drive, however, the file contents have to actually be written on the destination drive, and this can take awhile.)

When you start Windows, you must log in to a user account before you can do anything else. The idea of user accounts is that multiple people using the same computer may want to keep their own settings and files separate. If you don’t care about this, everyone who uses your computer can just use the same computer. Otherwise, you should create a separate user account for each person.

Each user account has a “user directory”, a directory for that user’s files and personal settings. These folders are located in the Users directory on the drive on which Windows is installed. So for example, my user account is named brian, so my user directory is:

                C:\Users\brian\

Inside each user directory, you’ll find directories called “My Documents”, “My Pictures”, “My Music”, and “My Videos”, which are where Microsoft intends you to put most of your personal files. While nothing requires you to store personal files in these directories, these are logical places to put them.

Also in each user directory, you’ll find a directory called Desktop. In truth, the Desktop you see behind your windows is really a directory, and the icons you see on the Desktop are file or directories contained within. So if you modify the content of this directory, you’ll modify what you see on your desktop and vice versa. The idea behind the Desktop is it’s just a directory that is especially convenient to access because it’s always open and visible (aside from windows getting in the way).

An archive file is a special kind of file which is like a directory in that it contains other files and directories. Whereas a directory is a just a list of other directories and files stored on the drive, an archive file contains the actual bytes that make up those directories and files. Archive files exist because, in some contexts, packaging things into a single archive file makes them easier to organize and move around.

Most archives use data compression techniques. To compress data means to produce a copy which is smaller than the original; to get back the original data, we must decompress the compressed copy. With compression, it’s often possible to reduce large files into much smaller files, thus making them more practical to store and move around.

A zip archive is a popular format of compressed archive. Zip files end in the extension “.zip”. When you open a zip file in Windows, Windows displays the contents just as if the zip file were a directory. To actually use files in a zip archive with other programs, however, you first have to copy them out of the zip file to a regular directory (you can do this simply using copy and paste).

A shortcut file is a special kind of file which doesn’t have its own content but simply points to another file or directory using a file path. When you open a shortcut file, Windows knows you want to open the file pointed to by the shortcut. For example, if on my Desktop I have a shortcut file that points to “D:\madison\jefferson”, opening the shortcut will actually open “D:\madison\jefferson”. A shortcut to a file is created by right-clicking that file and selecting “create shortcut” from the context menu.

Installing programs

Windows includes with it a number of programs. The most notable include:

·         Internet Explorer (Microsoft’s web browser)

·         Windows Media Player (for playing audio and video files)

·         WordPad (basically a stripped down version of Microsoft Word)

·         Calculator (a simple calculator)

·         Paint (a simple image editor)

None of these are terribly good. Better alternatives can be downloaded from the internet for free. For example, rather than using Internet Explorer for a web browser, you should use either Mozilla Firefox or Google Chrome, both of which are free and easy to download from the Internet.

Notable programs you might wish to purchase include:

·         Microsoft Word (word processor)

·         Microsoft Excel (spreadsheet)

·         Microsoft PowerPoint (slide presentations)

These are all available together in a package called Microsoft Office, which will set you back about $150 for the version called Home and Student (for business use, you legally must buy a different, costlier version). A free alternative to Microsoft Office is a free download called OpenOffice.

The most popular image editing program is from Adobe and called Photoshop. The cheap version for home users is called Photoshop Elements and costs $100 or less. One free alternative is called GIMP.

A program residing on your system is essentially a collection of files. The files which make up a program include executable files (files of actual code to run), dynamic linking library files (more code), “resource” files (containing miscellaneous data used by a program), and configuration files (containing configuration data, such as user preferences).

By convention, the files of installed programs are placed in a directory called “Program Files”. The files for Windows itself, however, are in a directory called “Windows”. As a user, you normally don’t have a reason to look in these directories yourself.

Installing a program on your system may be as simple as copying the program files to your system and opening the program’s executable file when you wish to run it. Most programs, however, require more complicated installation steps, such as modifying configuration settings in Windows itself. Such programs are installed with an installer program, which is just what it sounds like: a program for installing a program. To then remove a program from your system, you run the uninstaller for that program. (The best way to run an uninstaller is through “Programs and Features” in the Control Panel).

When you download a program from the Internet, it usually comes in the form of an installer executable, which should end in “.exe”. (Some other installers end in “.msi”.) Simply run the installer to install the program. Once installation is complete, you can start the program from the Start Menu, and you can delete the installer executable file because it’s no longer needed.

Over time, programs are updated with newer versions. Most programs use a convention of major and minor version numbers separated by a dot. For example, say the current version of our program is “4.13”. If the next version only has minor changes, we’ll call it “4.14”, but if the next release has major changes, we’ll call it “5.0”. (Don’t mistake these for fractions: “3.12” is a smaller quantity than “3.8”, but “3.12” is the later version because 12 comes after 8.)

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Windows itself doesn’t know how to read most kinds of file, so when you open a file, it runs a program which knows how to read that file. For example, Windows doesn’t know how to read a Microsoft Word file, so it opens Microsoft Word to open the file. How does Windows know which program can open a file? It infers from the file extension. Windows associates a program with each known file extension. You can configure these associations yourself, so say, you can configure Windows to open Microsoft Word documents in OpenOffice instead (OpenOffice knows how to read Word files). If you want to simply open a file in a different program on a this-time-only basis, select “open with” from the file’s context menu.

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Terminating programs

Sometimes a running program just goes bad: something unexpected went wrong, it no longer responds to your input, and the program must be terminated and restarted to be useable again. When programs go bad in this way, you often can’t close the program in the normal way. Instead, you must use the Windows Task Manager. The simplest way to get to the Task Manager is by holding ctrl and shift together then hitting the ESC key. In the Task Manager, you can select the errant program and then click the “End Program” button to kill it.

If that doesn’t work, look in the “Processes” tab of the Task Manager. Internally, a running program is known in Windows as a “process”. In this tab, you can select processes and kill them with the “End Process” button. Here you must be a little more careful, however, because the process list contains a number of programs not normally visible to the user and which handle core functionality of Windows. Killing the wrong process might make your computer unusable until you restart it.