Help Desk.How It Works.Page Four

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Hard Disks and Storage

It may be hard to believe, but the first Macs didn't have a hard disk drive inside. All the software they needed fit on a single floppy disk! My, how times have changed. Today it takes dozens of disk drives, CD-ROMs, and removable disks to store the instructions our computers need, and all the data files we create. How do these devices work? That's the subject of this page.

The Basics: In many ways, the technology used by computer storage devices is quite similar to your VCR at home: A motorized mechanism moves a storage "media" past one or more "read/write heads," which interpret that data and forward it to the screen.

The big difference is that video tape (the "media") stores data in a "linear" stream from beginning to end. This means you need to move forward or backward through ALL of the stored data to reach the specific information you want. That was fine years ago, but modern computers require more speed. So, almost all modern storage devices have replaced fixed-position heads and linear media with spinning disk "media" and "floating" (moving) heads which provide faster "Random Access" to data stored anywhere on the disk surface

Magnetic Media Devices: Hard disk drives, floppy disks and most removable disks (ZIP, JAZ, Syquest) are magnetic media devices, in which the spinning disk "media" is coated with tiny magnetic particles (metal oxides). As each section of the disk spins past the heads, the heads read the positive or negative charge of each magnetic particle to determine if it represents a "One" or "Zero" digital data bit.

The overall advantages of magnetic storage devices are low cost and fast speeds. But, one major disadvantage is that magnetic media doesn't last forever. Outside influences can damage the magnetic signals, or the signals can just "wear out" (lose their charge) over time.

The simplest of all magnetic media devices is the Floppy Disk. Inside that rigid 3.5" square of protective plastic is a flexible disk coated with magnetic particles. (You can see the disk if you slide the floppy's metal shutter to one side.) When you load the diskette into your computer, the drive mechanism pushes the shutter aside, spins the disk at a constant speed (about 300 rpm), and positions the read/write heads just above the surface of the disk to read the charge of each magnetic particle.

Original floppy disks could store only 400 Kilobytes (K) of data on one side of the flexible disk. Eventually, these were replaced by "double-sided" drives that used TWO read/write heads, one above and one below the spinning disk, and stored a total of 800 K of data. (400 K on each side.) Today's disks are "Double-Sided, High-Density," which means they store even more data on each side, for a total of 1.4 MB per disk.

The only real difference between a floppy disk and all other types of magnetic media are the spinning disks themselves. That's the "hard" part of a hard disk drive. Instead of a flexible disk, hard disk drives and removables use solid metal disks (usually aluminum) that spin much faster and are coated with even smaller magnetic particles, so they can store more data. In addition, modern hard drives may contain a stack of individual disks inside, to increase the total storage capacity even more.

Optical Media Devices: Although CD-ROM and Magneto Optical (MO) drives both use a spinning disk and floating read/write heads, these two "optical media" devices don't use magnetic media. However, since MO drives are becoming rare, due to the growing popularity of recordable CDs, we'll only look at how a CD-ROM drive works.

CDs (Compact Disks) consist of a thin aluminum disk covered with a plastic coating. Each single-sided disk (the label is on the other side) can hold about 640 MB of data. During the process of creating a CD, a laser beam melts tiny holes or "divots" into the aluminum disk (That's why it's called "burning a CD.") When you insert the CD into the drive in your machine, the drive's laser beam can "see" these holes and knows that each represents a "One" data bit. If there is no hole, the drive knows that represents a "Zero" bit. (NOTE: You can't see the holes with your naked eye. They are only a few microns - millionths of a meter - deep.)

The major advantage of CDs (and also MO drives) is security. Once you write a CD, your data should be safe for 10,000 years or more. They "never" wear out. The drawback is that CDs transfer data slower than magnetic media drives.

Of course, CD-ROM speeds are getting faster every day. That's why you'll always see a CD-ROM drive labeled as 4x, 8x, 24x and so on. This doesn't represent the speed of disk rotation. It measures the amount of data the drive can transfer compared to the very first CD-ROM drives (1x speed - the same as audio CDs in your home stereo). Today's fastest drives are 32x, meaning they can transfer data 32 times faster than the original audio CDs. But, that's still slower than the data transfer speed of a hard disk drive!

How Disks Store Data: Except for the "optical" differences noted above, all random access storage devices work the same way. For the sake of understanding, let's look at the typical hard disk drive.

A hard disk drive consists of five major components: The recording media (disks), the read/write heads and armature mechanism, the motor, the controller, and the protective case.

Inside the sealed protective hard drive case are a stack of disks mounted on a central spindle (like a phonograph with a stack of albums). Each disk is coated on both sides with magnetic particles, and there is a small space between the individual disks to allow the heads to fit in between them. Depending on the device, the motor mechanism turns the spindle at a consistent speed, ranging from 3600 to 7200 rpm. Faster speeds allow faster data access. (NOTE: Removable storage devices have only ONE metal disk inside a plastic casing called a "cartridge." When the cartridge is inserted into the drive, a spindle is pushed up through a hole in the case to turn the disk, and a metal shutter is pushed aside so the heads can access the disk surfaces. Rotation speeds for removable disks are 1200 - 2400 rpms.)

The read/write heads are attached to an "armature" assembly that moves all the heads in towards the center of the disk(s) and back out to the edges again in a straight line. The combination of "in and out" head movement and constant disk rotation provides quick access to any position on the surface of the disk. (NOTE: There is one head assembly for each data surface (top and bottom) on each disk (one or a stack). All heads are moved together as a single unit by the armature assembly.)

The heads "read" the magnetic charge of the particles as they pass by on the spinning disk(s), and send this information to the drive's "controller" (a printed circuit board mounted inside the drive). The controller analyzes the magnetic fluctuations, interprets these as a string of digital "Ones" and "Zeros," and forwards the data to the CPU. When data is received back from the CPU, the controller tells the heads to assign a new charge to each magnetic particle passing by on the spinning disk, and the data is "written" to the disk.

Important things to remember about magnetic storage devices:

The read/write heads never touch the disks. Instead, the rotation of the disk creates a cushion of air that keeps the heads floating just a few microns above the disk surface. That's why a sudden power loss can cause severe damage. If the armature doesn't pull the heads completely away from the disk surfaces (as it does during shutdown) a power loss can cause the heads to drop onto the disk surfaces - where they gouge their way through the magnetic particle coating.

Disk drives hate dust. When you consider that the heads are only a few microns from the disk surfaces, even a dust speck we can barely see is an obstacle as big as a mountain for the drive. That's why all hard disk drive mechanisms are sealed inside a protective, dust-free case. You should NEVER open this case. Removables and CD-ROM's are slightly more tolerant of dust, but you should still make every effort to keep dust away from the disk cartridges and drive mechanisms.

Drives are the only "mechanical" devices inside your computer, and the moving parts can suffer from mechanical failures. For example, rough handling can damage the loading mechanisms or misalign the read/write heads. Motors can also fail from old age, and dust buildup can cause spindle or armature mechanisms to stick or "freeze" in place.

How Data is Organized On the Disk

Before you can use a storage device, you have to "format" it to access and record information in a way your computer will understand. The process of formatting first erases the entire drive by resetting ALL magnetic particles to the "Zero" value, then defines small areas on the surface(s) of each disk as "sectors," and further breaks each sector into "blocks" of data storage space. These steps together are referred to as "physical formatting."

When the physical formatting is completed, the "logical formatting" (initializing) of each volume begins. The Initialization process performs the following actions, in the order listed below. (NOTE: When you run a disk-repair utility, you will see it check each of these areas of the disk.)

Creates "Boot Blocks" containing the data which defines the space as a Macintosh volume, and stores the device's "driver" software.

Creates the "Volume Information Block" which keeps track of the volume name and the total number of files stored on the disk.

Creates the "Volume Bitmap," which keeps track of which blocks are used or unused on the disk.

Creates the "Extents Directory" which remembers which blocks on the disk are "contiguous" (next to each other), so your computer knows where to find a large enough storage space for each file.

Creates the "Catalog Tree" which stores the physical location of each file on the disk

Defines the remainder of the disk space as potential storage space for files and folders, and creates the Desktop Directory and Desktop Database. (NOTE: These information files are loaded into RAM when the volume is "mounted" to tell the CPU where to find files and empty spaces on the disk.)

About HFS+: Beginning with MacOS 8, Apple introduced a new type of formatting called HFS+, which changes how a disk's data blocks are "addressed." Apple's original HFS (Hierarchal File System) defined each block address using only 16 bits of data. This allowed for no more than 65,000 different addresses on any one volume.

As hard disk capacity grew, this limitation required the individual blocks to be larger, too. (The total volume size must be divided into no more than 65,000 equal-sized blocks.) Instead of the optimum block size of 512 bytes, larger drives had to set the block size at 1024 Bytes (1K) or more. Since the computer only cares if a block is "Used" or "Unused," any space inside a block that is not completely filled with data is wasted. For example: If a file measures 3.1 K in size, it is big enough to fill three and one tenth blocks. The remaining space in the fourth block is wasted.

To use space on large drives more efficiently, Apple introduced HFS+ addressing which defines each block address using 32 bits (4 Bytes) of information. This can potentially define billions of block addresses, so each block can be smaller and wasted space is dramatically reduced. The Windows 98 operating system has introduced the same change for PC drives using a new addressing scheme called "Fat32."

About Partitions: "Partitioning" breaks the total space available on a device into two or more independent "volumes" (partitions), each with their own boot blocks, directories and data file addresses. The Finder mounts each partition independently, so you "see" two drives on your desktop, even though they both exist on one "device."

Before the introduction of HFS+, power users often partitioned larger drives into smaller, independent volumes. This allowed each volume to use space more efficiently within the 65,000 address limit.

Even after the introduction of HFS+, however, many users continue to partition their larger drives for organizational purposes, and to reduce wear and tear on the drive mechanisms. (The head armature moves less often on a partitioned drive.) In addition, removable drives can't use HFS+ formatting, and older software titles had problems understanding data stored on HFS+ volumes. As a result, partitioning drives using standard HFS addressing is still widely used on many Mac systems.

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Help: How it Works: 4 of 9
Hard Disks and Storage It may be hard to believe, but the first Macs didn't have a hard disk drive inside. All the software they needed fit on a single floppy disk! My, how times have changed. Today it takes dozens of disk drives, CD-ROMs, and removable disks to store the instructions our computers need, and all the data files we create. How do these devices work? That's the subject of this page. The Basics: In many ways, the technology used by computer storage devices is quite similar to your VCR at home: A motorized mechanism moves a storage "media" past one or more "read/write heads," which interpret that data and forward it to the screen. The big difference is that video tape (the "media") stores data in a "linear" stream from beginning to end. This means you need to move forward or backward through ALL of the stored data to reach the specific information you want. That was fine years ago, but modern computers require more speed. So, almost all modern storage devices have replaced fixed-position heads and linear media with spinning disk "media" and "floating" (moving) heads which provide faster "Random Access" to data stored anywhere on the disk surface Magnetic Media Devices: Hard disk drives, floppy disks and most removable disks (ZIP, JAZ, Syquest) are magnetic media devices, in which the spinning disk "media" is coated with tiny magnetic particles (metal oxides). As each section of the disk spins past the heads, the heads read the positive or negative charge of each magnetic particle to determine if it represents a "One" or "Zero" digital data bit. The overall advantages of magnetic storage devices are low cost and fast speeds. But, one major disadvantage is that magnetic media doesn't last forever. Outside influences can damage the magnetic signals, or the signals can just "wear out" (lose their charge) over time. The simplest of all magnetic media devices is the Floppy Disk. Inside that rigid 3.5" square of protective plastic is a flexible disk coated with magnetic particles. (You can see the disk if you slide the floppy's metal shutter to one side.) When you load the diskette into your computer, the drive mechanism pushes the shutter aside, spins the disk at a constant speed (about 300 rpm), and positions the read/write heads just above the surface of the disk to read the charge of each magnetic particle. Original floppy disks could store only 400 Kilobytes (K) of data on one side of the flexible disk. Eventually, these were replaced by "double-sided" drives that used TWO read/write heads, one above and one below the spinning disk, and stored a total of 800 K of data. (400 K on each side.) Today's disks are "Double-Sided, High-Density," which means they store even more data on each side, for a total of 1.4 MB per disk. The only real difference between a floppy disk and all other types of magnetic media are the spinning disks themselves. That's the "hard" part of a hard disk drive. Instead of a flexible disk, hard disk drives and removables use solid metal disks (usually aluminum) that spin much faster and are coated with even smaller magnetic particles, so they can store more data. In addition, modern hard drives may contain a stack of individual disks inside, to increase the total storage capacity even more. Optical Media Devices: Although CD-ROM and Magneto Optical (MO) drives both use a spinning disk and floating read/write heads, these two "optical media" devices don't use magnetic media. However, since MO drives are becoming rare, due to the growing popularity of recordable CDs, we'll only look at how a CD-ROM drive works. CDs (Compact Disks) consist of a thin aluminum disk covered with a plastic coating. Each single-sided disk (the label is on the other side) can hold about 640 MB of data. During the process of creating a CD, a laser beam melts tiny holes or "divots" into the aluminum disk (That's why it's called "burning a CD.") When you insert the CD into the drive in your machine, the drive's laser beam can "see" these holes and knows that each represents a "One" data bit. If there is no hole, the drive knows that represents a "Zero" bit. (NOTE: You can't see the holes with your naked eye. They are only a few microns - millionths of a meter - deep.) The major advantage of CDs (and also MO drives) is security. Once you write a CD, your data should be safe for 10,000 years or more. They "never" wear out. The drawback is that CDs transfer data slower than magnetic media drives. Of course, CD-ROM speeds are getting faster every day. That's why you'll always see a CD-ROM drive labeled as 4x, 8x, 24x and so on. This doesn't represent the speed of disk rotation. It measures the amount of data the drive can transfer compared to the very first CD-ROM drives (1x speed - the same as audio CDs in your home stereo). Today's fastest drives are 32x, meaning they can transfer data 32 times faster than the original audio CDs. But, that's still slower than the data transfer speed of a hard disk drive! How Disks Store Data: Except for the "optical" differences noted above, all random access storage devices work the same way. For the sake of understanding, let's look at the typical hard disk drive. A hard disk drive consists of five major components: The recording media (disks), the read/write heads and armature mechanism, the motor, the controller, and the protective case. Inside the sealed protective hard drive case are a stack of disks mounted on a central spindle (like a phonograph with a stack of albums). Each disk is coated on both sides with magnetic particles, and there is a small space between the individual disks to allow the heads to fit in between them. Depending on the device, the motor mechanism turns the spindle at a consistent speed, ranging from 3600 to 7200 rpm. Faster speeds allow faster data access. (NOTE: Removable storage devices have only ONE metal disk inside a plastic casing called a "cartridge." When the cartridge is inserted into the drive, a spindle is pushed up through a hole in the case to turn the disk, and a metal shutter is pushed aside so the heads can access the disk surfaces. Rotation speeds for removable disks are 1200 - 2400 rpms.) The read/write heads are attached to an "armature" assembly that moves all the heads in towards the center of the disk(s) and back out to the edges again in a straight line. The combination of "in and out" head movement and constant disk rotation provides quick access to any position on the surface of the disk. (NOTE: There is one head assembly for each data surface (top and bottom) on each disk (one or a stack). All heads are moved together as a single unit by the armature assembly.) The heads "read" the magnetic charge of the particles as they pass by on the spinning disk(s), and send this information to the drive's "controller" (a printed circuit board mounted inside the drive). The controller analyzes the magnetic fluctuations, interprets these as a string of digital "Ones" and "Zeros," and forwards the data to the CPU. When data is received back from the CPU, the controller tells the heads to assign a new charge to each magnetic particle passing by on the spinning disk, and the data is "written" to the disk. Important things to remember about magnetic storage devices: The read/write heads never touch the disks. Instead, the rotation of the disk creates a cushion of air that keeps the heads floating just a few microns above the disk surface. That's why a sudden power loss can cause severe damage. If the armature doesn't pull the heads completely away from the disk surfaces (as it does during shutdown) a power loss can cause the heads to drop onto the disk surfaces - where they gouge their way through the magnetic particle coating. Disk drives hate dust. When you consider that the heads are only a few microns from the disk surfaces, even a dust speck we can barely see is an obstacle as big as a mountain for the drive. That's why all hard disk drive mechanisms are sealed inside a protective, dust-free case. You should NEVER open this case. Removables and CD-ROM's are slightly more tolerant of dust, but you should still make every effort to keep dust away from the disk cartridges and drive mechanisms. Drives are the only "mechanical" devices inside your computer, and the moving parts can suffer from mechanical failures. For example, rough handling can damage the loading mechanisms or misalign the read/write heads. Motors can also fail from old age, and dust buildup can cause spindle or armature mechanisms to stick or "freeze" in place. How Data is Organized On the Disk Before you can use a storage device, you have to "format" it to access and record information in a way your computer will understand. The process of formatting first erases the entire drive by resetting ALL magnetic particles to the "Zero" value, then defines small areas on the surface(s) of each disk as "sectors," and further breaks each sector into "blocks" of data storage space. These steps together are referred to as "physical formatting." When the physical formatting is completed, the "logical formatting" (initializing) of each volume begins. The Initialization process performs the following actions, in the order listed below. (NOTE: When you run a disk-repair utility, you will see it check each of these areas of the disk.) Creates "Boot Blocks" containing the data which defines the space as a Macintosh volume, and stores the device's "driver" software. Creates the "Volume Information Block" which keeps track of the volume name and the total number of files stored on the disk. Creates the "Volume Bitmap," which keeps track of which blocks are used or unused on the disk. Creates the "Extents Directory" which remembers which blocks on the disk are "contiguous" (next to each other), so your computer knows where to find a large enough storage space for each file. Creates the "Catalog Tree" which stores the physical location of each file on the disk Defines the remainder of the disk space as potential storage space for files and folders, and creates the Desktop Directory and Desktop Database. (NOTE: These information files are loaded into RAM when the volume is "mounted" to tell the CPU where to find files and empty spaces on the disk.) About HFS+: Beginning with MacOS 8, Apple introduced a new type of formatting called HFS+, which changes how a disk's data blocks are "addressed." Apple's original HFS (Hierarchal File System) defined each block address using only 16 bits of data. This allowed for no more than 65,000 different addresses on any one volume. As hard disk capacity grew, this limitation required the individual blocks to be larger, too. (The total volume size must be divided into no more than 65,000 equal-sized blocks.) Instead of the optimum block size of 512 bytes, larger drives had to set the block size at 1024 Bytes (1K) or more. Since the computer only cares if a block is "Used" or "Unused," any space inside a block that is not completely filled with data is wasted. For example: If a file measures 3.1 K in size, it is big enough to fill three and one tenth blocks. The remaining space in the fourth block is wasted. To use space on large drives more efficiently, Apple introduced HFS+ addressing which defines each block address using 32 bits (4 Bytes) of information. This can potentially define billions of block addresses, so each block can be smaller and wasted space is dramatically reduced. The Windows 98 operating system has introduced the same change for PC drives using a new addressing scheme called "Fat32." About Partitions: "Partitioning" breaks the total space available on a device into two or more independent "volumes" (partitions), each with their own boot blocks, directories and data file addresses. The Finder mounts each partition independently, so you "see" two drives on your desktop, even though they both exist on one "device." Before the introduction of HFS+, power users often partitioned larger drives into smaller, independent volumes. This allowed each volume to use space more efficiently within the 65,000 address limit. Even after the introduction of HFS+, however, many users continue to partition their larger drives for organizational purposes, and to reduce wear and tear on the drive mechanisms. (The head armature moves less often on a partitioned drive.) In addition, removable drives can't use HFS+ formatting, and older software titles had problems understanding data stored on HFS+ volumes. As a result, partitioning drives using standard HFS addressing is still widely used on many Mac systems.
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