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Storing Your Work: Hard and Removable Disks
In this lesson, you learn how a PC saves information for later use on a hardor floppy disk, and why disks are vital PC components.
When you're using your computer, your data exists only in the computer's RandomAccess Memory (RAM). RAM holds the data and program instructions your computeris working with while the computer is turned on. Turn the computer off, and RAM'scontents are lost.
To work with that data again, you need to save the data in a more permanent way;you put it in storage. Disks are your computer's storage area. They hold yourdata for you until you delete it or move it. When you save information, your computertakes what's in RAM and writes it to a disk. When you want to use stored data again,your computer looks at that data on the hard disk drive and reads a copy of it backinto RAM. If you change your data, you'll need to save it to the hard disk driveagain to preserve your changes, and so on.
New users frequently use the terms "floppy disk" and "hard drive"to refer to the two main storage tools of the computer. A "disk" and a"drive," however, are not the same things.
The disk is the spinning platter of magnetic materi al that holds the storeddata. The hard "drive" within your computer actually holds a stack of thesemagnetic disk platters--that's how it holds so much data. In contrast, the spinning"platter" that you can see within a floppy disk is actually a tough filmcoated with magnetic material.
The actual collection of mechanisms that spins the disk, writes information onthe disk, and reads information from the disk is the disk drive. The harddisk drive in your computer has a sealed case that holds both the drive and the stackof disks. You can insert and remove disks from floppy disk drives and other removabledrives through a slot on the front of the drive (and the front of the system unit).
At its most basic level, your computer can only understand two digits: 0 and 1.The computer combines 0s and 1s to create unique characters, and then those charactersare combined to form words, values, commands, and so on.
Whether hard or floppy, your disks are coated with tiny magnetic particles, eachof which can be magnetized by an electrical charge. Each of these particles is knownas a bit--short for BInary digiT. When a bit has a high charge or ismagnetized (the "on" state), the computer reads it as a 1. When a bit hasa low charge or is not magnetized (the "off" state), the computer readsit as a 0. A combination of eight bits forms a byte, which is one characterof data. There is a unique set of bits for each letter of the alphabet, for everysingle-digit number, and for many special characters (like $).
The capacity of every disk and memory chip can be measured in bytes. A kilobyteis just over 1,000 bytes. A megabyte is just over 1,000,000 bytes, and a gigabyteis over a billion bytes (or a thousand megabytes).
These magnetic bits aren't just floating around on the disk's surface. They'reactually formatted or arranged in a very specific pattern to make it easy for thedisk drive to store and find data. Each disk has concentric rings called tracksdividing it. To further organize the data, the disk has sectors, pie-shapedwedges separated by imaginary lines radiating from the center of the disk, as shownin Figure 3.1.
Formatting a Disk You format a disk--arrange the magnetic bits into tracks and sectors--so your computer can write information to the disk. A PC usually comes with the hard disk already formatted. But, you may need to format a floppy disk before using it. To learn how to do this, see Lesson 14, "Formatting a Disk," in Part 2.
Figure 3.1 Magnetic information is arranged into concentricrings called tracks, which are divided into wedges called sectors.
Tiny magnetic codes on the disk identify the tracks and sectors that store everygroup of bits. Your system manages data in units called clusters or allocationunits. Each cluster might hold one or more sectors. Under older versions of DOS(operating system software) and Windows, the smallest cluster size possible for harddisks over 256M is 8k (8,000 bytes or characters). Because all your files won't necessarilycontain even multiples of 8,000 characters, some space is wasted. For example, afile with 8,001 bytes would occupy two full clusters, even though most of the secondcluster is wasted. Windows 95 OEM Service Release 2 (OSR 2), which has shipped onnew computers since the fall of 1996, uses the more efficient FAT32 system with smaller4K clusters. With 4K clusters, you can pack 10 - 15 percent more information on thesame drive. This is yet another reason to ensure any new computer you buy uses OSR2, not the original release of Windows 95. As a bonus, the new FAT32 system supportsmuch larger hard disks than earlier file systems. You'll learn more about what theFAT does in later lessons.
File This is a named collection of information, stored on a disk. You can think of a file as a single document, such as a letter from a word processor, a budget worksheet from a spreadsheet program, or a picture file created in a graphics program.
When a drive writes to a disk, it's actually magnetizing selectedbits to create bytes, or characters. When the drive reads from a disk, itreads the bits comprising each byte and places a copy of that information in RAM.A mechanical part called the read/write head is connected to an arm that floatsover the disk surface (much like the tone arm on old record turntable), magnetizingbits or reading their magnetic charges, as shown in Figure 3.2.
Figure 3.2 The read/write head magnetizes bits on a disk and readsmagnetic charges.
The drive's read head is smart and fast. It knows exactly where the different partsof a file are stored and finds them all to load them into RAM. (Lesson 8 of Part6 explains why a file migh t become divided into pieces on a drive.) Similarly, it'sadept at finding available space on a disk to copy a file to. A disk drive's speedis called its access time. Smaller access times indicate faster drives. Forexample, you should look for a hard drive with an access time of 12 ms (milliseconds)or less. Otherwise, you'll find yourself tapping your toe while you're waiting foryour drive to perform various operations.
Your computer stores information semipermanently on the hard disk (alsocalled the hard disk drive or hard drive), which consists of severalmetal disks in an airtight case. When you install a program, it's placed on the harddisk so it can be retrieved into RAM for use over and over. Hard disks can storemore information than floppy disks and tend to be more reliable (because they takeless abuse), so you should also store important data files on the hard disk insteadof a floppy. You can later delete programs and data to reuse the space they occupied.
Hard disk capacities are measured in megabytes (1,000M equals a gigabyte,or 1G). A too-small hard drive limits the number of programs you can install;these days the minimum is usually 1.2G. You also want to consider a hard drive'saccess time. A slow hard drive can slow down your whole system, no matterhow fast the processor is. A lower access time indicates a faster drive (10ms isfaster than 12ms). Most drives available today are Enhanced IDE (which are fasterthan the older IDE drives), but some systems come with faster SCSI hard drives.
Install To make a program available on your com puter, you must install it to the hard disk. Each program offers a setup or install file that automates the installation process, copying the program's files to your disk and altering key files on your system to make sure the program will run correctly. Lesson 1 in Part 3 explains how to install and remove programs under Windows 95.
Bigger Is Better Don't underestimate how rapidly you'll fill your hard drive. To have enough space for your current needs and the long term, buy at least a 2.5G drive, with 12ms access time or better.
Like a hard drive, a floppy disk lets you store data and program instructionssemipermanently. The difference: floppy disks store data on portable disksyou can slide in and out of a slot in the system unit. Floppies enable you to movefiles from one computer to another, such as when you want to take a memo home fromwork to edit it on your home system. Older floppy disk drives used flexible 5.25-inchdisks, which are now almost obsolete. Recent floppy drives use 3.5-inch HD (high-density)disks that hold 1.44M of data; these have hard plastic shells that better protectyour data.
You may have noticed that your floppy disks or disk labels have "HD"or something similar on them. These hieroglyphics refer to the disk's densityor capacity, which is as important as the floppy's size. Both affect how muchdata a floppy disk can hold.
Manufacturers indicate this amount in kilobytes (each K is just over 1,000characters) and megabytes (each M is more than o ne million characters). Allfloppy disks sold today are double-sided (DS); they store data on both sides(many early floppies were single-sided) and hence store more information.
The disk's capacity depends on how efficient its magnetic storage material is.The newer high-density (HD) disks hold more data than double-density(DD) disks of the same size. For a comparison of various disk capacities, see Table3.1.
Table 3.1 Floppy Disk Capacities
|3.5-inch||DSDD, double-sided double-density||720K|
|3.5-inch||DSHD, double-sided high-density||1.44M|
|5.25-inch||DSDD, double-sided double-density||360K|
|5.25-inch||DSHD, double-sided high-density||1.2M|
To translate the capacity of any disk into real-world terms, think of each byteas a character. A 3.5-inch HD (high-density) floppy disk holds 1.44 megabytes, whichis roughly 1.4 million characters. So if the average page of data has 35 lines of65 characters each (2,275 characters), a single 1.44-megabyte disk can hold morethan 600 pages of data (2,275 characters times 600 pages equals 1,365,000 characters),which is about as many pages as there are in this book--if the data doesn't havea lot of fancy formatting.
Floppy disks give you almost unlimited (if not completely convenient) capacityfor storing files. When you want to use the files on a floppy, you need to insertit into the floppy disk drive on the system unit. When you're finished working withthe floppy, you then remove it from the disk drive. Follow these steps to insertand remove a floppy disk:
- 1. Turn your 3.5-inch disk so the label side is up. Grip the edge opposite the edge covered by the sliding door. (If you were inserting a 5.25-inch disk, you would need to hold it label up, with the square notch to the right and the side with two smaller notches facing the drive slot.)
2. Insert the leading edge of the disk (it's covered by the metal sliding door on a 3.5-inch floppy) into the drive slot (Figure 3.3). Push it (gently) all the way into the drive. When the disk seats, the drive's eject button pops out. (For a 5.25-inch disk, you may have to press the drive lever down until it clicks in place in front of the slot.)
Figure 3.3 Insert the leading edge of the floppy disk intothe floppy drive slot on the system unit.
- 3. To finish using the disk, save your work on it. After the drive's indicator light goes out, you can remove the disk (which you should do before you use another one or shut down your computer).
4. Press the drive's eject button (or lift the lever if your 5.25-inch drive has one); the disk pops part of the way out.
5. Pull the disk the rest of the way out and store it in a safe place.
Drive Light ever remove a floppy disk from the drive when the drive indicator light is still on. Doing so usually damages your files, and you could lose data permanently.
Some applications today create huge files. A single graphic image file, for example,can be a few megabytes in size. In fact, even a word processing document with fancyformatting can greatly exceed 1M. This created a problem in transferring files betweencomputers. These large files don't fit on the typical 1.44M floppy disk.
In the last few years, hardware manufacturers have come to the rescue, offeringfloppy-like drives that use larger disks or cartridges. The most common removabledisk drives include the 100M Zip drive and 1G Jaz drive created by Iomega), 230MEZFlyer and SyJet 1.5G from SyQuest, and 90M and 150M drives from Bernoulli. Thedrives themselves range in price from $149 for the ZIP to $400 or more for a larger,faster removable drive. The disks or cartridges themselves start at about $15 forthe 100M ZIP disk to more than $90 for a 1G Jaz cartridge.
Figure 3.4 This compact Jaz drive (external model shown) uses 1Gcartridges that st ore more than the typical hard drive available a few years ago.
At this point, you have to add most of these removable disk alternatives after youpurchase a system, but some systems now include a Zip drive for extra removable storage.Some models connect to a parallel (printer) port. You plug the drive in, plug yourprinter into the back of the drive, and install the software that runs the drive.Other models must be used with a SCSI (Small Computer Systems Interface) adapter,a faster type of disk and device connection that isn't standard on most PCs. If youdon't have a SCSI adapter, you'll need to install one to use a SCSI removable drive(see Lesson 4 in Part 5). If you don't have a SCSI adapter and don't want to buyand install one, make sure you buy a parallel port model.
SCSI An acronym for Small Computer Systems Interface, SCSI is a type of disk and device connection. SCSI usually offers the fastest method for devices like disk drives to communicate with the system, so it's a faster type of connection. In addition, one you've installed a SCSI adapter, you can create a "daisy chain" of devices; plug the first device into the adapter, then plug the next device into the first one, and so on. Lesson 4 in Part 5 covers SCSI in more detail.
Note that the parallel models are external--you don't have to install them insidethe system unit, so you can use them with more than one computer as long as you'veinstalled the software for the drive on each. SCSI models are usually internal, orinstalled in the system unit.
Safe Ejection Inserting a removable disk resembles the process for inserting a floppy. Generally, you just gently push the disk in until it clicks into place. You may also need to close a lever, as for some SyQuest drives. While you simply reverse the process to eject the removable disk, be aware that some removables require you to use software to unlock or unmount the drive before you remove it. Failing to unlock the drive could result in lost files.
As you've already noticed, your computer probably has at least two drives. Yourcomputer uses a simple system to tell them apart: the ABCs. You'll need to know yourcomputer's naming system when you want to save, retrieve, or otherwise work withdata files. Here's how to identify your system's drives:
- If your computer has only one floppy disk drive, it's drive A. If your system has two floppy disk drives, the one that's to the left or on top is usually drive A (although this may not be true if the leftmost floppy drive is oriented vertically).
- On a system with two floppy disk drives, the bottom or right one is usually drive B.
- Generally, the hard disk drive inside your computer is drive C. If you have multiple hard disk drives, the first will be C, the second will be D, and so on. Hard disks are assigned drive letters before CD-ROM drives, removable disk drives, and any network drives your system can access.
- Your CD-ROM drive, if you have one, is usually drive D. However, if you have multiple hard disks or removable dr ives attached to your system, the CD-ROM drive will usually be bumped to a later letter. For example, one of my systems has three hard disks, a Syquest removable disk drive, and a CD-ROM drive. The three hard disk drives are C, D, and E; the Syquest is F, and the CD-ROM is G. If your system is attached to a network, the drives on the network normally are assigned a much higher letter, such as I or H.
This lesson introduced you to hard, floppy, and other removable disks, that enablePCs to store information. The next lesson covers devices you use to give the computerinformation, or input.