Hardware Fundamentals
© Copyright Brian Brown, 1992-2000. All rights reserved.

STORAGE DEVICES
The objective of this section is to

• discuss various storage devices
• explain the characteristics and terminology of selected storage devices

At the end of this section, you should be able to

• compare storage devices using a number of criteria
• describe the operation of storage devices

Disk Storage
Disks are used to store data, applications software and operating systems software. Whereas the primary form of storage in the early days of computing was magnetic tape, this has been replaced by predominantly disk based medium today. The reasons for this trend has been

• decreasing cost per bit
• reliability
• reduced access times
• higher transfer rates (more data per second)
• reduced size and power requirements
• increased capacity

One trend that is appearing is a move to CDROM and optical storage medium. Many software companies offer both operating systems software and application software on CDROM today.

Disk storage systems are essentially based on magnetic properties. This is the same principle as used in cassette tape recorders. A rotating disk is coated with fine magnetic particles. When writing data, a write head magnetises the particles on the disk surface as either north or south poles. When reading data, a read head converts the magnetic polarisation's on the disk surface to a sequence of pulses.

The read and write heads are generally combined into a single head unit. There may be more than one read/write head. Consider the example shown below, where a group of heads is used to write data onto concentric rings on the magnetic drum. In this example, the heads are fixed (non-moveable).

Fig 5.1: Magnetic Drum

The problem with this drum approach is limited capacity. To increase the capacity requires an increase in the circumference size of the drum, or more read/write heads.

A more common approach used today is to use a read/write head attached to a moveable arm, which steps across (by small increments) the surface of the disk. The disk is a platter coated with magnetic particles. This arrangement is shown below.

Fig 5.2: Hard Disk Drive

Early drives were large. IBM developed a smaller rigid disk drive with one fixed and one removable pack. Each pack held about 30 megabytes (MB) of data, so it was dubbed model '3030', and became known as the Winchester drive.

Most Winchester drives have common features

• the disk and read/write heads are enclosed in a sealed airtight unit
• the disk(s) spin at 3600 revolutions per minute
• the read/write heads do not actually touch the disk surface
• the disk is lubricated (heads fly above when the disk is up to speed)
• the disk surface contains a magnetic coating
• the disk surface (platters) is fixed

Data is arranged as a series of concentric rings. Each ring (called a track) is subdivided into a number sectors, each sector holding a specific number of data elements (bytes or characters).

Fig 5.3: A track subdivided into sectors

The smallest unit that can be written to or read from the disk is a sector. Once a read or write request has been received by the disk unit, there is a delay involved until the required sector reaches the read/write head. This is known as rotational latency, and on average is one half of the period of revolution.

The storage capacity of the disk is determined as (number of tracks * number of sectors * bytes per sector * number of read/write heads)

Disk Drive Characteristics
This section discusses the terminology and characteristics of disk drives.

Tracks and Sectors
The disk is divided into concentric rings called tracks. A track is thus one complete rotation of the disk underneath the read/write head. The width of a track is determined by the size of the read/write head, and the distance between tracks determined by the mechanics of the stepper motor which controls the positioning of the arm to which the read/write head is attached.

Each track is subdivided into a number of sectors. Each sector contains a specific number of bytes or characters. Typical sector capacities are 128, 256, 512, 1024 and 4096 bytes.

Increasing the number of tracks is one way to increase the storage capacity of a disk drive. Often the physical size of the disk imposes space restrictions which make this impractical. The most common choice is increasing the number of sectors per track (from 17 to 34), or increasing the number of bytes stored in each sector.

Bad Blocks
The drive maintains an internal table which holds the sectors or tracks which cannot be read or written to because of surface imperfections. This table is called the bad block table, and is created when the disk surface is initially scanned during a low level format.

Partitions
A disk partition is a sub-division of the disk into one or more areas. Each partition can be used to hold a different operating system. The computer system boots from the active partition , and software provided allows the user to select which partition is the active one.

Sector Interleave
This refers to the numbering of the sectors located in a track. A one to one interleave has sectors numbered sequentially, 0, 1, 2, 3, 4, etc. The disk drive rotates at a fixed speed, 3600rpm, which means that there is a fixed time interval between each sector. A slow computer can issue a command to read sector 0, storing it in an internal buffer. Whilst it is doing this, the drive makes available sector 1, but the computer is still busy storing sector 0. Thus the computer will now have to wait one full revolution till sector 1 becomes available again.

Renumbering the sectors like 0, 8, 1, 9, 2, 10, 3, 11 etc gives a 2:1 interleave. This means that sectors are alternated, giving the computer slightly more time to store sectors internally than previously.

Drive Controller
The drive is managed by a special peripheral card called a drive controller. It may handle multiple drives or only a single drive. The controller is often responsible for issuing commands to position the read/write head (especially in MFM and RLL drives).

In SCSI and IDE drives, the controller is simplified, and the intelligence is placed onto the drive itself. These drive offer sophisticated features like caching and hot fix (write errors are redirected to another free sector).

Rotation Speed
This refers to the speed of rotation of the disk. Most hard disks rotate at 3600rpm. To increase data transfer rates, higher rotational speeds are required, or multiple read/write heads arranged in parallel, or disk arrays (multiple disks arranged in parallel).

Low/High level Formatting
Low level formatting is placing track and sector information, plus bad block tables and other timing information on the disk. Sector interleave can also be specified at this time.

High level formatting involves writing directory structures and file allocation tables to the disk. Often this also means transferring the boot file for the operating system onto the hard disk.

Access Time
Access time refers to how soon the drive makes data available once issued with the command to read the data. Once a read command is issued, the drive must position the read/write head at the appropriate track number and wait for the correct sector to arrive.

Latency
This refers to the delay between the read/write request, and the appearance of the required sector under the read/write head.

Timing tracks
In larger drives used on main frame computers, the disk drives often had timing tracks written. These tracks were used for alignment purposes, to ensure that the read/write head was accurately positioned over the track.

The read/write head was moved until the pulses picked up from the timing head were at a maximum. This meant that the read/write head was correctly positioned over the data track.

Newer more accurate mechanisms have tended to make this obsolete.

Hard Drive Construction
This picture shows the physical construction of a hard disk drive.

Fig 5.4: Hard Disk Drive Construction

Disk Drive Types
This section discusses the various drive types available for IBM-PC compatible computers.

• mfm (modified frequency modulation)
  These drive types were the first to be made for the IBM-XT model. Storage capacity was 30MB with access types of about 30ms. Capacity ranges from about 10MB to 80MB. Due to their design, it was costly to make drives any larger. MFM refers to the encoding technique used to store data on the disk. They are all but replaced by IDE.
  
• rll (run length encoding)
  This is similar to MFM drives except that the encoding technique used to store data essentially doubles the capacity of an MFM drive. Few, if any, manufacturers still make MFM or RLL drives today.
  
• esdi (enhanced small disk interface)
  These drive types range in capacity from about 80MB to about 300MB. They are becoming less common. The advantage they have over RLL or MFM drives is higher data transfer rates (about twice as fast).
  
• scsi (small computer systems interface)
  The common drive choice for servers or high end workstations, drive capacities range from 100MB to 4GB. They have fast access times and high data transfer rates, but are expensive. One advantage of these drives is that a single controller can handle seven drives.
  
• ide (integrated disk electronics)
  A common drive used today for workstations with capacities of 40MB to 1000MB. Good access times of about 20ms, but slower data transfer rates than SCSI drives. Drives are reasonably cheap, but the controllers can only handle a maximum of two drives (some will handle only one).

Floppy Drives
The floppy drive uses a thin circular ceramic disk for data storage. The disk is coated with magnetic particles and is flexible (hence the term floppy).

The disk rotates at 360rpm. A read/write head makes physical contact with the disk surface. Data is recorded as a series of tracks subdivided into sectors. Typical values for an IBM-PC compatible are

stepper motor. The stepper motor moves a small amount for each pulse applied to it. A mechanical switch detects when the read/write head is in the outermost position (track 00).

Fig 5.5: 5.25" Floppy Drive

The two main methods of identifying the beginning sector of a track are

• Hard Sectored Floppy
  In hard sectored, the start of each sector is identified by a physical hole. A light is shone through the hole and picked up on the other side by a detector. As the disk rotates, this causes a sequence of pulses to occur as the sector ID holes allow the light to pass through to the detector. The holes are called index holes. This method was used in the Apple II computer.
• Soft Sectored Floppy
  In soft sectored, only one index hole is used, and each sector is preceded with data bytes which identify the sector number. This format is used in IBM-PC compatible computers.
  

Fig 5.6: Floppy Drive Cleaning Kit

The image on the left shows a floppy diskette cleaning system. It consists of a floppy disk containing a cleaning disk and some cleaning fluid. The fluid is placed on the floppy disk and then the disk is inserted. Any foriegn material on the read/write heads are transferred to the disk. The cleaning disk can only be used a limited number of times.

Hard Disks
Hard disk drives use a thicker non-flexible disk. The disk is made from aluminium or ceramic material, and coated in magnetic particles. The disk rotates at a much higher speed (3600rpm). In addition, multiple disks are stacked on top of each other (with a gap in between), with a read/write head allocated for each surface (top and bottom).

Storage capacities range from 10MB (first PC disk used in XT) to several gigabytes.

• Exchangeable Disks
  The disks are provided as removable cartridges which can be removed and inserted into the drive. The drive unit contains the motor drive, electronics and read/write heads. When the cartridge is inserted, the read/write heads are positioned to make contact with the disk platters inside the cartridge pack.

  This allows a flexible method of data storage. As each disk pack becomes full, it can be removed and a new pack inserted to handle the next lot of transactions etc.

• Fixed Disks
  Fixed disks are permanent disks which cannot be removed. Data is archived off the disk onto another storage medium in order to free up space for new data.

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© Copyright Brian Brown, 1992-2000. All rights reserved.