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Technology
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What do the parts of a computer do? You are pretty familiar with your TV, maybe too familiar! The TV is mostly an output machine or device. This means that the stuff, be it information or junk, flows from the TV out to you. It also has some input controls so that you can give it some input. These are the volume control, the on off switch and the channel changer, which lives under the couch. So output is the flow of information to you and input is the flow from you to the device. Computers have:
What is kind of surprising is how much effort is spent getting input and giving output, and how little is actually spent on doing work! By work I mean heavy duty arithmetic or searching through long lists for a certain item.
The Parts of a Computer and their duty are: Monitor (screen)
- Output The computers main job is to run programs. Programs are instructions in a language that computers and programmers understand. A programmer, or a team of programmers writes a program and saves it on a long term storage device like a floppy drive, a hard disk or a CD-ROM. Then, when you want to run that program, you type its name and the computer goes and gets the program off the hard drive, or wherever it is stored, and brings it into its short term storage, called RAM, and runs the program. I think that people, who were forced to find their way around computers running the MS-DOS operating system, are at an advantage because they know what files are and where they live. Operating environments such as Windows and the MAC try to cover up these details, making it harder to learn what really is going on. Picture yourself sitting at a computer with just the DOS prompt on the screen. The DOS prompt usually looks like this C:> Did you know that there is a program running? Well, yes there is, it's the disk operating system, DOS. It's busy doing what it does best, which is waiting for you to tell it what to do. So you slide up to your trusty input device, the keyboard, and you type a word, say dor and then you press Enter. Before talking about what happens next, let's look at what just happened. Each time you pressed a letter, it actually did go to the computer, because it appeared on the output device, the monitor. If you follow the wires, you see that the only way to get to the monitor from the keyboard is through the computer! So, we learned something... DOS really is running because somehow the action of pressing a certain key on the keyboard input device, resulted in a letter being printed in the proper location on the screen output device! That is actually quite a feat, and computers had been around a long time before a program could do that. The fact that DOS puts the character whose key was just pressed on the screen is called echoing and it's done only as a convenience to you, the user. And they say that DOS is not user friendly! DOS also knows enough not to try to interpret what you have typed until you press the Enter key, indicating you are finished. There is nothing magical about the Enter key, it is just another key, but one that the DOS program, and many other programs, have been programmed to recognize as "OK, I'm finished inputting, you figure out what I input.". Okay, so now DOS has three letters in a row, in computer terms called a string. The DOS program has a list of steps to follow when it receives a string from the keyboard. Check to see if it is a command you recognize. If yes, then do it. If not, then check to see if there is a file in the current directory that ends in .com or .exe or .bat but has the same first part, such as dor.com or dor.exe or dor.bat. If there is, then get a copy of that file from the long term storage device and into the short term storage, the RAM, and run the program. If there is no such file in the current dir, then look in each directory that DOS has in a list of directories called the Path. If you can find such a file, then, again, pull it into RAM and run it (called executing the file). If DOS has taken all of these steps and still can't interpret what you want, then, using the monitor output device, tell the user 'Bad command or file name'. Just be thankful that the computer wasn't given boots as an output device, because a kick in the shins would be more appropriate, after sending the poor thing on such a wild goose chase! So, one thing we've learned is that dor is a pretty meaningless word to both DOS and us! Now try typing dir Hey, that's better. dir is a command that DOS knows, and it means "list all the files and directories that are in the current directory." Now, when you see 'Bad command or file name' after telling DOS to do something, you don't have to just sit there with this confused look on your face. On the logical side of your brain you can figure out why you got that ouptut instead of the one you were expecting. If you had typed the name of a program, say wp and were expecting to start-up Word Perfect, but received that friendly 'Bad command or file name' instead. then you can go over in your head exactly what DOS had tried to do. First, it checked to see if wp was a command it recognized - no it wasn't. Then it looked for wp.com or wp.exe or wp.bat in its current directory, the directory the operating system was left in. If it was there, DOS would have grabbed the file off the hard drive and put a copy in the RAM. But it wasn't there. Then DOS looked in all the directories that are listed in its path variable. You can see what directories it looks in by giving DOS the command path. [ Giving DOS the command path means typing path at the dos prompt C:> and pressing Enter ]. DOS spits out the list of directories it has been told to look in. It didn't find such a file there, so it told you so. Files: An Overview: The idea about files and directories is very important and actually if you can get a firm grasp of files, you are then on the downward side of the learning curve. If you understand files, you'll be able to figure out most things, and a lot of the black magic involved in using a computer, and a lot of the fear, will simply disappear. First I'll give a quick overview of files and then we will make comparisons between files and things you already know. What do I mean by Files? Files are clumps of computer data stored somewhere in your computer. Each file has a name, a location, and a length, and usually a date of when it was last changed. Files are stored in several places: on floppy disks; on hard drives; on CD's; in the computers memory (RAM); in the computers special read only memory (ROM); on tapes of a tape back-up device. Any files that are in the memory of the computer are lost when the power is turned off. That is not as scary as it sounds because what you usually have in memory is only a temporary copy of the file, the original stays on the hard drive or floppy or wherever it is usually kept. Pretty well the only time the original is in memory is when you are creating a new file and haven't saved it yet. What Do Files Do? Files hold and store information that can be read by the computer. That's all they do. What Kinds of Files Are There? All files are basically the same, they all have a name, a location, a date and they all hold information. However, they can hold lots of different kinds of information, so we often think of this as different kinds of files. Actually it is just the type of information that is different. Here are some of the main types of files you will encounter. They often have certain letters at the end of their names so you can tell from their names what sort of information is inside. Some files you can easily look at the information inside and many others you can't, at least not without a program that is specifically designed to look inside that type of file. I will use the following format (to avoid using a text table): name anything.com anything.exe anything.bat readme.txt anything.gif
What you do with a file depends on the type of information that it holds, but some things can be done with all files. Run them - if they hold a program. Look inside them, if they hold graphics information or text. Listen to them, if they hold audio information. Copy them - this is one of the main things you will do. When you run a program, what actually happens is that the file, with the program inside, is copied from the long term storage device into the RAM, where its' list of instructions are executed (run). You also copy files from a floppy to your hard drive or from one place on your hard drive to another. Move them - This is like copying except that the original is NOT left behind. It is often safer to copy a file from one place to another and then delete the original, once you are sure the copy went well. Delete them. When you no longer need a particular file, you can zap it. Create them. When you compose a document in a word processor or any other program such as an editor or spreadsheet, you are creating a new file. Usually saving goes hand in hand with creating. That is enough for an overview. You are now probably sure what I mean when I talk of a file. Next comes two comparisons of files to things that are familiar and then a sort of history of files. A lot of this information is repeated again and again, so if you don't understand what I mean, don't worry about it, I'll be saying it again later! As I said, getting a firm grasp of handling files is essential to becoming a computer guru. It's almost all you need.
How
Files Are Like LP Records One way of understanding something is to compare it to something that you already know. LP Records First of all... let's look at what is a file? One comparison is to records, I mean the record player, 33 RPM vinyl records that have been replaced by CD's. If you are too young to remember these, then just think of audio CD's. If you look at the surface of a record, you can see where each of the 'cuts' or songs is. Each one of these songs represents a file. It is a file of audio information, that can be understood and turned into music by your stereo player. Each song file has a title, a length and a location on the record. Each computer file has a title, a length and a location. We cannot make use of the contents of a computer file without a computer and a program to understand the file. We can't make use of the song file without a stereo that has been programmed to understand that type of record. Although all song files are essentially the same, we classify them as rock, classic and so on, according to what is inside the file. We do the same with computer files. We call them graphics files, text files and programs, depending on what is inside the file. Having songs on a record or CD is not hard to understand, and yet they are very similar to computer files, so files in a computer system can't be too hard to understand either. One of the things you often do with computer files is to copy them, from one place to another. With records, you can copy a song from the record to a tape, but you can't copy it to another record. Computer files can be copied more easily. The reason for this is that a stereo player can only play files; it can read them and turn them into sound and put that sound on the output device, the speakers. This is what is meant by read only files. But a stereo player can not write files, whereas a tapedeck can and a computer can. The computer can
read files and put them on the output device. It can also write files.
It has to be this way or you wouldn't be able to copy files. So while
songs on a record and computer files on a hard disk are very very similar,
the one major advantage that computers have is that they can make the
files. It's like you have your own recording studio for computer files!
It you want DOS to put 'play' the file called wp.exe, you simply type wp . Or, if you want to see what is inside a file, you can use the DOS command type and tell DOS to type readme.txt ... DOS will 'play' the file readme.txt . Using a Windows operating system, you can simply open up the file to read it on your monitor. While the old records had about 12 files, modern hard drives can have many thousands of files. This could be very difficult to organize. That is the reason, and I guess the only reason, that there are directories, so you can organize all your files. Which leads up to the next lesson!
Organizing Files Is Like Organizing the Fridge How Do You Keep Track Of All Those Files? As you know, a file is a collection of information. It has a name, a size and a date. DOS computers allow names that are 8 characters maximum, then a dot, then a 3 character maximum extension. Such as readme.txt or lesson5.htm Computers using Windows 98, Windows ME etc allow longer file names but the extension at the end is always 3 characters in length. After working with a computer for a while, you can end up with a huge quantity of files. So, 'they' invented directories. Directories were invented to help you organize your files, but, they often seem to confuse people. The people at Apple call directories 'Folders' but that doesn't help much either. One of the reasons it doesn't help is that people are not used to putting folders inside folders inside other folders, however that is exactly what you do with Directories. Think of directories as boxes to put things in. Boxes, with things in them, can be put inside other boxes. We do it all the time. Look inside your kitchen, your kitchen is a big box. Inside that box is a refrigerator box and a lot of cupboard boxes. We don't need to label the kitchen box with a sign that says 'Kitchen' because everyone has learned what the kitchen looks like. However with directories on our hard drive, we can't actually see the boxes so we label (name) them so we can see them, and know which box we are in. So let's walk into the 'kitchen' directory-box. In DOS you do that by typing cd \ Enter to make sure you are starting fresh (at the 'root', more later) and then typing cd kitchen. This is all in your head, of course, you probably don't have a directory-box called 'kitchen' on your hard drive, but you could. And for sure you do in your house. So now we are in the kitchen directory-box. Over there is the 'refrigerator' directory-box. Everyone recognizes it but it could have a label and if it was a directory-box on your hard drive, you might call it 'fridge'. It is a directory-box inside the kitchen directory-box. In Windows, you can go to your Program Manager or Explorer and files and directories are listed like a family tree/ Here comes a very important fact that will cause you no end of grief until you really 'get it'. Directories, with files in them, can be inside other directories, AND YOU CAN'T SEE INSIDE UNTIL YOU GO IN. You can't see what's in the fridge until you open the door! Inside the fridge, once you open the door is a bunch of files, I mean food, and a lot of other little directory-boxes. What's inside those other directory-boxes? WHO KNOWS! YOU CAN'T TELL TILL YOU GO IN THEM! Sure some of them are labelled, like 'Milk'. But you don't really know it's milk until you peak inside! It could be gravy from last week, or it could be anything. Same with all those other little Tupperware directory-boxes. Some have labels, but that doesn't mean that the label is right - you have to look inside. So, with directory-boxes
on your hard drive, you can put boxes inside boxes. Please note that you can only see the names of the files and directory-boxes that are inside the same box as you are. You can't see the files and directory-boxes that are in the Kitchen, without going into the Kitchen. Now you decide you want to separate personal documents from business docs so you make a directory-box called 'mystuff' inside the directory-box called Docs. Now in Docs you have both files and a directory-box. That is very common and the way things work. As time goes by and you accumulate lots of files in 'mystuff' you may decide to make another directory-box inside Docs for 'HisStuff'. Now you have files and two directory-box inside the directory-box called Docs. After a while, the directory-box MyStuff gets a lot of stuff in it so you make a directory-box inside MyStuff called OldStuff. And on and on until you have a real rats nest in there. Now you know what hard drives look like. When you are thinking about how files are stored on the hard drive, think of them as in directory-boxes. The whole hard drive then, can be thought of as one big directory-box with many files and directory-boxes in it. This outer box is called the Root directory and is shown on the DOS screen as '\'. The name of the
hard drive, or floppy drive for that matter is C: or A: or B: or D:
etc. Most directory-boxes are named by you, for example 'mystuff', but
that first big 'Mother of all Directory-Boxes' is named by the computer
and is always called \, which is a pretty silly name. So we call it
'root', which is just a silly nickname for the silly name \.
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