500 MB Nyheder Direkte fra Internet 2

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Text File | 1994-09-21 | 21KB | 366 lines

BACKGROUND To begin operating a computer, you find its power switch, turn it on, and then what? What do you type? What do you do? How will the computer respond? The answers to those questions depend on which operating system your computer uses. Most IBM clones use an operating system called MS-DOS, supplemented by Windows (which lets you more easily use a mouse). Mac computers use a different operating system instead, called the Mac System. This book explains how to use all three: MS-DOS, Windows, and the Mac System. Other kinds of computers use different operating systems instead. Five issues To judge which operating system is best for you, consider five issues. . . . Hardware requirements Which hardware must you buy before you run the operating system? Does the operating system run on a cheap microcomputer, or does it require a computer that's more expensive? If it requires an expensive computer, can you justify the high cost? Which CPU chip is required? How much RAM? How much space on the hard disk? Does it require a mouse? Speed Does the operating system run quickly, or must you wait a long time for it to obey your commands? After you've given a command, do you spend a long time twiddling your thumbs, waiting for the operating system to catch up to you? If the operating system runs too slowly, maybe you'd get your work done faster if you'd use an old-fashioned typewriter or calculator instead of a computer! Multitasking To what extent can the operating system handle many people, tasks, and devices simultaneously? Can the operating system begin working on a second task before the first task has finished, so the operating system is working on both tasks simultaneously? For example, while the operating system is making the printer print and waits for the printer to finish printing, can the operating system let you simultaneously use the keyboard, so you can feed the computer further commands ___ and can the operating system start obeying those commands, even though the printer hasn't finished obeying your previous command yet? User interface How do you feed commands to the operating system and tell it what to do? Do you type the commands on the keyboard, or are you supposed to use a mouse instead? Are the commands or mouse-methods easy for you to remember, or must you keep referring back to the instruction manual to remember how to use the damned computer? Program availability Think about your goal (why you bought the computer and what kind of programs you want the computer to run), and ask whether those kinds of programs have been written for this operating system yet. For example, have programs been written yet to make this operating system handle word processing, databases, spreadsheets, graphics, desktop publishing, speech, music, multimedia, telecommunication, networks, and accounting ___ and do those programs work excellently, meet all your needs, and thrill you? Three kinds of user interface How do you give commands to the computer? The answer depends on what kind of user interface the operating system uses. Three kinds of user interface have been invented. Command-driven In a command-driven interface, you give commands to the computer by typing the commands on the keyboard. For example, MS-DOS uses a command-driven interface. To command MS-DOS to copy a file, you sit at the keyboard, type the word ``copy'', then type the details about which file you want to copy and which disk you want to copy it to. To command MS-DOS to erase a file so the file is deleted, you type the word ``erase'' or ``del'', then type the name of the file you want to delete. Menu-driven In a menu-driven interface, you act as if you were in a restaurant and ordering food from a menu: you give orders to the computer by choosing your order from a menu that appears on the screen. For example, Pro DOS (an operating system used on some Apple 2 computers) has a menu-driven interface. When you start using Pro DOS, the screen shows a menu that begins like this: 1. Copy files 2. Delete files If you want to copy a file, press the ``1'' key on the keyboard. If you want to delete a file instead, press the ``2'' key. Afterwards, the computer lets you choose which file to copy or delete. Icon-driven In an icon-driven interface, the screen shows lots of cute little pictures; each little picture is called an icon. To give orders to the computer, you point at one of the icons by using a mouse, then use the mouse to make the icon move or disappear or turn black or otherwise change appearance. For example, the Mac's operating system (which is called the Mac System) has an icon-driven interface. When you turn the Mac on, the screen gets filled with lots of little icons. If you want to copy a file from the Mac's hard disk to a floppy disk, just use the mouse! Point at the icon (picture) that represents the file, then drag the file's icon to the floppy disk's icon. Dragging the file's icon to the floppy's icon makes the computer drag the file itself to the floppy itself. One of the icons on the screen is a picture of a trash can. To delete a file, drag the file's icon to the trash-can icon. When you finish, the trash can will bulge, which means the file's been deleted, thrown away. Multitasking methods Our country is being run by monsters! Big monster computers are running our government, our banks, our insurance companies, our utility companies, our airlines, our railroads, and other big businesses. How do those big computers manage to handle so many people and tasks simutaneously? Here are the methods used by those maxicomputers ___ and by many minicomputers and microcomputers, too. . . . Scheduling the CPU Suppose your organization buys a multi-million-dollar maxicomputer. How can the employees all share it? The answer depends on what kind of operating system you buy. Why batch-processing was invented In the 1950's, the only kind of operating system was single-user: it handled just one person at a time. If two people wanted to use the computer, the second person had to stand in line behind the first person until the first finished. The first improvement over single-user operating systems was batch processing. In a batch-processing system, the second person didn't have to stand in line to use the computer. Instead, he fed his program onto the computer's disk (or other kind of memory) and walked away. The computer ran it automatically when the first person's program finished. That procedure was called batch processing because the computer could store a whole batch of programs on the disk and run them in order. Why multiprogramming was invented While running your program, the CPU often waits for computer devices to catch up. For example, if your program makes the printer print, the CPU waits for the printer to finish. When the printer finishes, the CPU can progress to your program's next instruction. While the CPU is waiting for the printer (or another slow device), why not let the CPU temporarily work on the next guy's program? That's called multiprogramming, because the CPU switches its attention among several programs. In a simple multiprogramming system, the CPU follows this strategy: it begins working on the first guy's program; but when that program makes the CPU wait for a slow device, the CPU starts working on the second program. When the second program makes the CPU wait also, the CPU switches its attention to the third program, etc. But the first program always has top priority: as soon as that first program can continue (because the printer finished), the CPU resumes work on that program and puts all other programs on hold. Why round-robin time-slicing was invented Suppose one guy's program requires an hour of computer time, but another guy's program requires just one minute. If the guy with the hour-long program is kind, he'll let the other guy go first. But if he's mean and shoves his way to the computer first, the other guy must wait over an hour to run the one-minute program. An improved operating system can ``psyche out'' the situation and help the second guy without waiting for the first guy to finish. Here's how the operating system works. . . . A jiffy is a sixtieth of a second. During the first jiffy, the CPU works on the first guy's program. During the next jiffy, the CPU works on the second guy's program. During the third jiffy, the CPU works on a third guy's program, and so on, until each program has received a jiffy. Then, like a card dealer, the CPU ``deals'' a second jiffy to each program. Then it deals a third jiffy, etc. If one of the programs requires little CPU time, it will finish after being dealt just a few jiffies and ``drop out'' of the game, without waiting for all the other players to finish. In that scheme, each jiffy is called a time slice. Since the computer deals time slices as if dealing to a circle of card players, the technique's called round-robin time-slicing. To make that technique practical, the computer must be attached to many terminals so each guy has his own terminal. The CPU goes round and round, switching its attention from terminal to terminal every jiffy. If you sit at a terminal, a few jiffies later the CPU gets to your terminal, gives you its full attention for a jiffy, then ignores you for several jiffies while it handles the other users, then comes back to you again. Since jiffies are so quick, you don't notice that the CPU ignores you for several jiffies. That technique's an example of timesharing, which is defined as ``an operating system creating the illusion that the CPU gives you its full attention continuously''. In that system, you might not get a full jiffy on your turn. For example, if your program needs to use the printer, the CPU sends some data out to the printer but then immediately moves on to the next person, without waiting for the printer to catch up. When the CPU has given all the other people their jiffies and returns to you again, the CPU checks whether the printer has finished the job yet. Fewer switches While the CPU works on a particular guy, the state of that guy's program is stored in the CPU and RAM. When that guy's jiffy ends, the CPU typically copies that guy's state onto the disk, then copies the next guy's state from disk to the CPU and RAM. So every time the CPU switches from one guy to the next, the CPU must typically do lots of disk I/O (unless the CPU's RAM is large enough to hold both guy's programs simultaneously). Such disk I/O is ``bureaucratic overhead''. It consumes lots of time. The only way to reduce that overhead is to switch guys less often. Let's develop a way to make the CPU switch guys less often but still switch fast enough to maintain each guy's illusion of getting continuous attention. Suppose a guy's a ``CPU hog'': he's running a program that won't finish for several hours. Instead of giving him many short time slices, the CPU could act more efficiently by totally ignoring him for several hours (which will make everybody else in the computer room cheer!) and then giving him a solid block of time toward the end of those hours. He'll never know the difference: his job will finish at the same time as it would otherwise. And the CPU will waste less time in bureaucratic overhead, since it won't have to switch attention to and from him so often. To determine who's the hog, the CPU counts how many jiffies and how much RAM each guy's been using. If a guy's count is high, he's been acting hoggish will probably continue to be a hog, so the CPU ignores him until later, when he's given a solid block of time. If that block is too long, the other guys will be ignored too long and think the CPU broke; so that solid block should be just a few seconds. If he doesn't finish within a few seconds, give him another block later. The Decsystem-20 and other great timesharing systems use that strategy. Distributed processing Now you know how to make many people share a single CPU efficiently. But since the CPU chip in an IBM PC costs just $3, why bother sharing it? Why not simply give each guy his own CPU? Today many companies are abandoning maxicomputers that have fancy timesharing operating systems and are replacing them by a collection of IBM PC clones, each of which handles just one person at a time. That's called distributed processing: tying together many little CPU's instead of forcing everybody to share one big CPU. Scheduling the memory Suppose your program is too big to fit in the RAM. What should you do? The obvious answer: chop the program into little pieces, and run one piece at a time. Virtual memory If you buy a computer system that has virtual memory, the operating system automatically chops your program into little pieces, puts as many pieces as possible into the RAM, and runs them. The remaining pieces are put on a disk instead. When the CPU finishes processing the pieces in the RAM and needs to get to the pieces on disk, the operating system automatically fetches those pieces from the disk and copies them into the RAM ___ after copying the no-longer-needed pieces from the RAM to the disk. Each piece of the program is called a page. On most computers, each page is 4K. Copying a page from the disk to the RAM (or vice versa) is called paging or swapping. Multi-user RAM If many people try to share the RAM, the operating system tries to squeeze all their programs into the RAM simultaneously. If they won't all fit, the operating system temporarily puts some of their programs onto disk until the other programs are done (or have used up a time-slice). If lots of users try to share a tiny RAM, the operating system spends most of its time shuttling their programs from disk to RAM and back to disk. That bad situation ___ in which the operating system spends more time copying programs than running them ___ is called thrashing, because the operating system is as helpless as a fish out of water. If the operating system is big and thrashing, it's called a beached whale. The only solution is to buy more RAM or tell some of the users to go home! Scheduling the I/O After the CPU sends a command to a slow device (such as a printer or terminal), the CPU must wait for the device to obey. While the CPU waits, it ought to do something useful ___ such as processing somebody else's program, or processing a different part of the same program. Buffers Here's how the CPU handles the problem of waiting. When the CPU wants to send a list of commands to a slow device, it puts that list of commands into a buffer, which is a special part of the RAM. The device peeks at the commands in the buffer. While the device reads and obeys those commands, the CPU switches attention to other programs or tasks. The buffer's in RAM. But which RAM? In a traditional computer system, the buffer's in the main RAM. In a more modern system, each I/O device contains an auxiliary RAM, just big enough to hold the device's buffer. Buffers are used not just for printers and terminals but also for disk drives and tape drives. Each slow device needs its own buffer. Spooling Suppose you share a maxicomputer with other users. What if your program says to write answers on the printer, but somebody else is still using the printer? In that situation, the CPU pretends to obey your command: it pretends to send your answers to the printer. But actually it sends your answers to a disk instead. The CPU keeps watching the printer: as soon as the other person finishes using the printer, the CPU automatically copies your answers from the disk to the printer and erases them from the disk. That technique of putting your answers temporarily on disk is called spooling; it's handled by a part of the operating system called the spooler. Spooling is used mainly for the printer but can also be used for other output devices, such as the plotter. The word ``spool'' is an abbreviation for ``Simultaneous Processing Of On-Line devices''; spooling handles people who try simultaneously to use the printer and other on-line devices. If several people try to use the printer simultaneously, the spooler stores all their answers on disk temporarily and then prints those answers one a time. The answers waiting on the disk are called the printer's waiting-line or the printer's queue or (even more briefly) the print queue. If the printer is slow and many people try to use it simultaneously, the print queue can get quite long. Suppose you want to use the printer, but somebody else is using it to print paychecks. If your operating system lacks a spooler, it says ``please wait for the other person to finish'' or else makes a mess ___ by printing your answers in the middle of the other person's paycheck! Moral: get a spooler! Personality The operating system can give the computer a personality, so the computer acts as a conversationalist, a drudge, or a boss. Conversationalist A ``conversationalist'' is a computer you chat with by using a keyboard and screen. The conversation might be about a computer game you're playing, an airplane seat you're reserving, or your personality (if the computer is trying to play therapist). Throughout the conversation, what you say to the computer is brief, and so are the computer's replies. Typically, you're asking the computer for more output, or the computer's asking you for more input. Instead of calling such a computer a ``conversationalist'', computists call it an interactive computer. If it handles many programs simultaneously, with each program on a different terminal, it's called a timesharing computer. If its memory contains lots of data and it answers questions about the data, it's called a data-retrieval system or customer-inquiry system or commercial real-time system. Drudge A ``drudge'' is a computer that takes in piles of data and spits out piles of answers. Traditionally, the data comes on cards (or forms that are scanned), and the answers are printed on paper. While the computer works, it asks you no questions; you don't need to chat with it. It just does its job faithfully, blue-collar style. If the drudge lets you start feeding it a second program before the first program has finished, it's a batch processor. If the batch processor starts working on the second program before the first program has finished, so that it's working on both programs simultaneously, it's called a multiprogramming batch processor. Boss A ``boss'' is a computer that monitors another machine (such as a burglar-alarm system, a microwave oven's timer, a series of synchronized traffic lights, an electronic organ, a life-support system giving periodic intravenous injections to an unconscious patient, or a computerized asssembly line). The computer makes sure everything's running smoothly. Typically, you don't see the computer: it hides inside the machine it monitors. Instead of calling such a computer a ``boss'', computer ads call it a controller or a scientific real-time system.