Operating Systems


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.
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.
Multiuser systems
Our country is run by monsters! Big monster computers run our government, banks, insurance
companies, utility companies, airlines, and railroads. To handle so many people and tasks
simultaneously, those computers use advanced operating systems. Here's how they arose:
Back 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.
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. While the CPU
waits for the printer (or another slow device), you should 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.
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 mean and insists on going first,
the other guy must wait 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 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, attach the computer 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 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, 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, and without giving you a full jiffy of attention. After the CPU's given the other people their
jiffies, the computer returns to you again and checks whether the printer has finished your job yet.
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"
consuming lots of time. To reduce that overhead, switch guys less often. Here's how 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 should act more efficiently by totally ignoring him
for several hours (which will make everybody else in the computer room cheer!) and then give
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 and will probably continue to 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 have used that strategy.
Now you know how to make many people share a single CPU efficiently. But since the CPU chip
in an IBM PC costs just a few dollars, 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.
Prepare to operate
Here's how to start using an IBM-compatible computer. (If you're using a Mac instead, skip ahead
to page 150.)
If you ever have difficulty following my instructions, phone me anytime for free help at
617-666-2666.
Unpack the computer When you buy a computer system, it typically comes in three cardboard
boxes. Open them, and put the contents on your desk.
One box contains the monitor.
One box contains the printer.
One box contains the computer's main part (system unit), keyboard, mouse, speakers, and disks.
Each box also contains power cords, cables, and instruction manuals.
Here are exceptions:
If you bought a portable computer (notebook or laptop), there is no monitor.
If you didn't buy a printer, the printer box is missing.
If you bought a tower computer, put it on the floor instead of on your desk.
Your computer might have no mouse or no speakers.
Into the back of the system unit, plug the cables that come from the monitor, printer, keyboard,
mouse and speakers. Into your wall's electrical socket (or power strip), plug the power cords that
come from the monitor, printer, speakers, and system unit. (On some computers, the cabling is
different.)
Empty the floppy drives At the front of the system unit, you'll see one or two slots. (In most
computers, the slots are horizontal.) You can put floppy disks into those slots. Those slots are
called the floppy drives.
Exception: if your computer is a notebook or laptop, the floppy drives might be in the computer's
right side instead of in the front.
Remove any disks from the floppy drives, so that the floppy drives are empty and you can start
fresh.
Turn on the computer Flick the computer's power switch to the ON position.
Can't find the power switch? Here are some hints:
The power switch is on or near the system unit's right side. If you don't find the switch on the
right side, check the right part of the front side or the right part of the back side. On traditional
computers, the power switch is red. It might say "1" instead of "ON" and "0" instead of "OFF".
On some computers (such as Quantex's), the power "switch" is actually a pushbutton on the front,
near the right.
Turn on the screen Turn on the computer's screen (monitor).
After a few seconds, the screen will display some messages. (If you don't see the messages clearly,
make sure the cable from the screen to the system unit is plugged in tightly, and adjust the screen's
contrast and brightness knobs.)
Examine the keyboard Test your powers of observation by staring at the keyboard. Try to find the
following keys (but don't press them yet).... 
Find the ENTER key. That's the big key on the right side of the keyboard's main section. It has a
bent arrow on it. It's also called the RETURN key. Pressing it makes the computer read what you
typed and proceed.
Find the BACKSPACE key. It's above the ENTER key and to the right of the + key. It has a
left-arrow on it. You press it when you want to erase a mistake.
Find the key that has the letter A on it. When you press the A key, you'll be typing a small "a".
Near the keyboard's bottom left corner, find the SHIFT key. It has an up-arrow on it. Under the
ENTER key, you'll see another SHIFT key. Press either SHIFT key when you want to capitalize a
letter. For example, when you want to type a capital A, hold down a SHIFT key; and while you
keep holding down the SHIFT key, tap the A key.
Find the key that looks like this:
U
3!  3
31  3
AU
It's near the keyboard's top left corner. That's the 1 key. You press it when you want to type the
number 1. Press the keys to its right when you want to type the numbers 2, 3, 4, 5, 6, 7, 8, 9, and
0. If you press the 1 key while holding down a SHIFT key, you'll be typing an exclamation point
(!). Here's the rule: if a key shows two symbols (such as ! and 1), and you want to type the top
symbol (!), you must typically hold down a SHIFT key.
Find the key that has the letter U on it. To the right of that key, you'll see the letters I and O.
Don't confuse the letter I with the number 1; don't confuse the letter O with the number 0.
In the keyboard's bottom row, find the wide key that has nothing written on it. That's the SPACE
bar. Press it whenever you want to leave a blank space.
Which operating system? Figure out which operating system you're using, by looking at your
screen.
If the screen says "Starting Windows 95" or "Microsoft Windows 95" or "Welcome to Windows
Setup" or the screen's bottom left corner says "Start", you're probably using Windows 95, so turn
to page 95. If the screen says "Program Manager", you're probably using Windows 3.1 or 3.11, so
turn to page 108. If the screen says "C:\>", you're probably using just MS-DOS, so turn to page
114. If the screen says something else, ask your dealer which operating system you got and then
turn to the appropriate page.
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Operating systems

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