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					Systems Architecture, Fourth Edition                                                                Chapter 5

Chapter 5
Data Storage Technology
Chapter Outline

Instructor Notes
Storage Device Characteristics

The primary characteristics that distinguish storage devices are:
     Speed- faster is better but cost increases with speed. As a result, the quantity of fast storage
       locations (e.g., registers and L1 cache) is usually much less than of slower devices (e.g., magnetic
       disk and tape).

        Volatility – data stored on a storage device are either volatile or non-volatile. A storage device or
         medium is non-volatile if it holds data without loss over long periods of time. A storage device or
         medium is volatile if it cannot reliably hold data for long periods.

Systems Architecture, Fourth Edition                                                                Chapter 5

       Access Method – the data on the storage devices can be accessed using serial, random or parallel
        access. Serial access retrieves data in sequential order. Random access retrieves data in any order.
        Parallel access simultaneously accesses multiple storage locations.

       Portability – data can be made portable by storing it on a removable storage medium. Portable
        storage devices and devices with removable storage media typically have slower access speed than
        permanently installed devices and those with non-removable media.

       Cost and capacity – an increase in speed, permanence or portability generally comes at increased
        cost if all other factors are held constant.

    Characteristic                     Description                          Cost
    Speed                              Time required to read or write a     Cost increases as speed
                                       bit, byte or larger unit of data     increases
    Volatility                         Ability to hold data indefinitely,   For devices of similar type, cost
                                       particularly in the absence of       decreases as volatility increases
                                       external power
    Access method                      Can be serial, random or             Serial is the least expensive;
                                       parallel; parallel devices also      random is more expensive than
                                       are serial or random access          serial; parallel access is more
                                                                            expensive than non-parallel
    Portability                        Ability to easily remove and         For devices of similar type,
                                       reinstall the storage media from     portability increases cost; if al
                                       the device or the device from        other characteristics are held

Systems Architecture, Fourth Edition                                                                   Chapter 5

                                      the computer                         constant
    Capacity                          Maximum data quantity held by Cost usually increases in direct
                                      the device or storage medium         proportion to capacity
    Table 5-1 – Storage device characteristics and their relationship cost

Primary Storage Devices
There are two basic RAM types – Static RAM and Dynamic RAM. Static RAM is implemented with
transistors. The basic unit of storage is a flip-flop circuit, which is an electrical switch that remembers its
last position. SRAM is volatile unless a continuous supply of power can be guaranteed.

Dynamic RAM uses transistors and capacitors. DRAM chips include circuitry that automatically performs
refresh operations. SRAM is more expensive to fabricate than the DRAM chip.

Read-only Memory is a random access memory device that can store data permanently or

Memory packaging:
            Early RAM and ROM – Dual in-line packages (DIP)
            Late 1980’s – Single in-line memory module (SIMM)
            Newer standard – Dual in-line memory module (DIMM)

CPU Memory Access

Systems Architecture, Fourth Edition                                                                Chapter 5

Main memory of a computer system can be regarded as a sequence of contiguous memory locations. The
addressable memory of the CPU is the highest numbered storage byte that can be represented. Memory is
allocated to system software, application software and data. Memory addresses can either be allocated
using absolute or relative addressing. Absolute addressing refers to actual memory addresses. If the
program is moved to a different location in memory, the program must be rewritten. Relative addressing
uses an offset register to hold the offset value. If the program is moved to a different location in memory,
the value in the offset register is added to the memory location.

Magnetic Storage

Magnetic storage devices use magnetism and electricity to store and retrieve data from the storage medium.
A write operation places a data on the storage medium. A read operation is the inverse of the write
operation. Magnetic storage devices have the following undesirable characteristics:
     Magnetic decay
     Magnetic leakage
     Minimum threshold current for read operations
     Storage medium coercivity
     Long-term storage medium integrity

Systems Architecture, Fourth Edition                                                                 Chapter 5

Magnetic Tape

The most common magnetic storage devices are a disk and tape. Magnetic tapes are used primarily to
backup data stored on a hard disk. Linear recording and helical scanning are used to store data on the
magnetic tape. Linear recording places bits along parallel tracks that run along the entire length of the tape.
Helical scanning reads and writes data to or from the tape by rotating the read/write head at an angle to the
tape and moving from tape edge to tape edge. Magnetic tape is susceptible to the problems of magnetic
storage devices and the problems that are unique to tape including:
      leakage problems by winding the tape on itself
      leakage occurring from adjacent bit positions n the same area of tape as well as from the layer of
         tape wound above or below on the reel
      problems from stretching, friction and temperature variations.

Systems Architecture, Fourth Edition                                                             Chapter 5

Linear recording is inherently faster and helical scanning has inherently greater storage density. The
shortcomings of either technology can be mitigated through expensive means, which is one reason why
“industrial strength” tape drives cost thousands of dollars and require expensive media.

Technology Focus – Magnetic Tape Formats and Standards
The Quarter Inch Committee (QIC) helped to create open standards for magnetic tape.

Format            Year             Cartridge Size       Capacity (GB)   Tracks            Recording
                                   (Inches)                                               Density (bpi)
QIC-80            1988             4x6                  .08             28                14,700
QIC-120           1991             4x6                  0.125           15                10,000

Systems Architecture, Fourth Edition                                                               Chapter 5

QIC-525         1992             4x6                      0.525           26                20,000
QIC-2100        1993             4x6                      2.1             30                50,800
QIC-3095        1995             3.25 x 2.5               4               72                67,733
QIC-3220        1997             3.25 x 2.5               10              108               106,400
Table 5-4: Sample QIC tape format specifications.

In the 1980s and 1990s, the use of small servers and minicomputers led to the development of new

        Digital Data Storage (DDS) standards were developed by Hewlett-Packard and Sony.
        Digital Linear Tape (DLT) standards were developed by Quantum.
        Advanced Intelligent Tape (AIT) standards were developed by Sony.
        Linear Tape Open (LTO) standards were developed by Hewlett-Packard.

Magnetic Disk

Magnetic disks store data on flat, circular platters. Data is stored on both sides of the platters. Data is
placed on the tracks that are placed on the disk. The tracks are divided into sectors. Each sector holds 512
bytes. The data is accessed using read/write heads. The data is either stored on a hard disk or a floppy
disk. Typically hard disks are stored inside the computer system and store from 20 to 200 GB. Floppy
disks are removable disks that store from under 1MB to as much as 20 GB. Data is accessed faster from
the hard disk than the floppy disk. The hard disk rotates at speeds of up to 15,000 RPM. The speed at
which data can be read or written depends on several factors including:
      Time required to switch among read/write heads
      Time required to position the read/write heads
      Rotational delay (spin rate)

Average access time is a combination of the head-to-head switching time, track-to-track seek time and
rotational delay. Head-to-head switching time is the time required to switch to the appropriate read/write
head before a sector can be accessed. Track-to-track seek time is the time required to move the read/write
head from one track to another. Rotational delay time is the time the disk controller must wait for the
proper sector to rotate beneath he heads.

Systems Architecture, Fourth Edition                                                            Chapter 5

Technology Focus – Whither the Floppy Disk?
Data transport from one computer system to another using a floppy disk is called SneakerNet and was
commonly employed before networks became commonplace. The storage capacity of the floppy disk has
increased 20 times since 1981.

Year             Capacity (KB)   Platter Size           Number of       Tracks per        Sectors per
                                 (Inches)               Sides           Side              Track
1981            160              5.25                   1               40                8
1982            180              5.25                   1               40                9
1982            320              5.25                   2               40                8
1983            360              5.25                   2               40                9
1985            1200             5.25                   2               80                15
1985            720              3.5                    2               80                9
1987            1440             3.5                    2               80                18
1991            2880             3.5                    2               80                36
Table 5-6: IBM-compatible PC floppy disk formats

The standards in the table have ceased since 1991 because IBM is no longer developing standards for the
diskette. Microsoft and Intel have been defining the standards since the late 1980s. Iomega Corporation
has been a leader in removable magnetic media technology since the mid-1980s.

Optical Mass Storage Devices

Systems Architecture, Fourth Edition                                                                       Chapter 5

Technology/Format                        Writable?                               Description
CD-ROM                                   No                                      Adaptation of musical compact
                                                                                 disc technology, 650 MB
CD-R                                     One time only                           CD-ROM format with a dye
                                                                                 reflective layer that can be altered
                                                                                 by a low-power laser
CD-RW                                    Yes                                     CD-ROM format with a phase
                                                                                 change reflective layer, can be
                                                                                 written up to 1000 times
Magneto-optical                          Yes                                     Combination of optical and
                                                                                 magnetic technology, expensive
                                                                                 and outdated, rapidly giving way
                                                                                 to CDs and DVDs
DVD-ROM                                  No                                      Adaptation of DVD video
                                                                                 technology, similar to CD-RM
                                                                                 but more advanced, 4.7 GB
DVD-R                                    One time only                           DVD-ROM format, same basic
                                                                                 technology as CD-R with
                                                                                 performance and capacity
DVD-RAM                                  Yes                                     Format similar but not
                                                                                 completely compatible with
                                                                                 DVD-ROM updated CD-RW
                                                                                 phase change technology with
                                                                                 performance and capacity
DVD-RW                                   Yes                                     A competitor to DVD-RAM
DVD+RW                                   Yes                                     A competitor to DVD-RAM

Business Focus – A Modern SneakerNet

Further Readings or Resources
See for an up-to-date list of reference materials.

Key Terms
absolute addressing, 172
Describes memory address operands that refer to actual physical memory locations.

access arm, 183
Device within a disk drive containing one or more read/write heads mounted at one end. The other end is
attached to a motor that allows the read/write head(s) to be positioned over a single track of the disk platter.

access time, 161
Elapsed time between the receipt and completion of a read or write command by a storage device.

addressable memory, 171
Maximum amount of memory that can be addressed physically by the CPU.

Systems Architecture, Fourth Edition                                                                       Chapter 5

Advanced Intelligent Tape (AIT), 182
Magnetic tape standard developed by Sony based on Digital Audio Tape technology.

average access time, 161
Statistical average (or mean) elapsed time required by a storage device to respond to a read or write command.

big endian, 171
Describes a CPU architecture that stores a multibyte data item with the most significant byte in the storage
location with the lowest-numbered address.

block, 161
(1) Series of logical records grouped on a storage device for efficient processing, storage, or transport. (2) Unit of
data transfer to and from a storage device. (3) Portion of a program that is always executed as a unit.

CD-DA (compact disc digital audio), 193
Acronym of compact disc–digital audio.

CD-R, 194
Acronym of compact disc–recordable.

CD-ROM (compact disc read-only memory), 193
Acronym of compact disc read-only memory.

CD-RW, 195
Acronym of compact disc–read/write.

coercivity, 177
Ability of an element or compound to accept and hold a magnetic charge.

core memory, 166
Antiquated form of primary storage implemented as a lattice of wires with iron rings wrapped around each wire
junction point.

cylinder, 182
In a disk storage device, the set of all tracks at an equivalent distance from the edge or spindle on all recording

data transfer rate, 161
Rate at which data is transmitted through a medium or communication channel, as measured in data units per

device controller, 158
Processor that controls the physical actions of one or more storage devices.

Digital Audio Tape (DAT), 181
Early magnetic tape technology on which Digital Data Storage standards are based.

Digital Data Storage (DDS), 181
Family of magnetic tape standards developed by Hewlett-Packard and Sony, and based on Digital Audio Tape.

Digital Linear Tape (DLT), 182
Magnetic tape standard developed by Quantum.

direct access, 163
Synonym of random access.

Systems Architecture, Fourth Edition                                                                        Chapter 5

disk defragmentation, 187
Reorganization of data on a disk drive so that all portions of an individual file are stored in sequential sectors.

diskette, 184
Synonym of floppy disk.

double in-line memory module (DIMM), 170
Small standard printed circuit board containing one or more random access memory chips with electrical contacts
on both sides.

drive array, 184
Set of disk drives managed and accessed as if they were a single storage device.

dual in-line package (DIP), 169
Early form of packaging for processor or memory circuits with two rows of electrical contact pins.

DVD, 195
Acronym of both digital video disc and digital versatile disc.

DVD-RAM, 195
Acronym of digital video disc random access memory.

DVD-ROM, 195
Acronym of digital video disc read-only memory.

dynamic RAM (DRAM), 167
Type of electronic memory circuit that implements bit storage with transistors and capacitors.

electronically erasable programmable read-only memory (EEPROM), 169
Read-only memory device that can be erased and written by sending appropriate control signals.

enhanced DRAM (EDRAM), 168
Type of electronic memory circuit primarily composed of DRAM but supplemented with a small SRAM cache.

ferroelectric RAM, 169
Type of nonvolatile random access memory that stores bit values within metallic crystals.

firmware, 169
Software that has been permanently stored in read-only memory devices.

flash memory, 169
Type of electronically erasable read-only memory that requires relatively little time to update memory contents.

flash RAM, 169
Synonym of flash memory.

floppy disk, 184
Small, removable magnetic disk storage medium encased in a protective cover.

hard disk, 183
(1) Storage medium consisting of a rigid platter coated with a metallic oxide on which data are recorded as
patterns of magnetic charge. (2) Synonym of hard disk drive.

head-to-head switching time, 185
Elapsed time required to switch shared read/write circuitry between two adjacent read/write heads.

Systems Architecture, Fourth Edition                                                                      Chapter 5

helical scanning, 179
Tape recording method in which data is read and written by rotating the read/write head at an angle, moving
from tape
edge to tape edge.

indirect addressing, 173
Any addressing method where a program’s memory address operands do not necessarily correspond to physical
storage locations.

least significant byte, 171
Byte within a multiple byte data item that contains the digits of least, or smallest, magnitude.

linear recording, 179
Recording of data onto a tape in which bits are placed along parallel tracks that run along the entire length of the

Linear Tape Open (LTO), 182
Proprietary magnetic tape standard developed by Hewlett-Packard, IBM, and Seagate.

little endian, 171
Describes a CPU architecture that stores a multibyte data item with the least significant byte in the storage
location with the lowest-numbered address.

magnetic decay, 176
Loss in strength of a stored magnetic charge over time.

magnetic leakage, 176
Reduction in strength of a stored magnetic charge because of interference from one or more adjacent magnetic
charges of opposite polarity.

magnetic tape, 178
Polymer ribbon coated with a metallic compound used to store data.

magneto-optical, 194
Secondary storage device that reads and writes data bits using a combination of magnetic and optical methods.

Mammoth, 182
Magnetic tape standard developed by Exabyte based on the Digital Audio Tape standard.

memory allocation
Allocation of primary storage resources to active processes.

most significant byte, 171
Byte within a multiple byte data item that contains the digits of most, or largest, magnitude.

non-volatile, 162
Term describing storage devices that retain their contents over long periods of time.

offset register, 173
Register that holds a memory address offset, which is added to each explicit memory reference.

parallel access, 163
Simultaneous access to multiple portions of a data item through multiple communication channels or
transmission media.

Systems Architecture, Fourth Edition                                                                     Chapter 5

physical memory, 171
Physical primary storage capacity of a computer system, in contrast to virtual memory capacity and addressable

platter, 182
One disk within a disk storage device, the surface or surfaces of which store data.

Quarter Inch Committee (QIC), 181
Committee that promulgates standards for cartridge magnetic tapes.

random access, 163
Ability of a storage device to access storage locations directly, or in any desired order.

random access memory (RAM), 166
(1) Generic description of semiconductor devices used to implement primary storage. (2) Device used to
implement primary storage that provides direct access to stored data.

read-only memory (ROM), 169
Primary storage device that can be read but not written.

read/write head, 174
Mechanism within a storage device that reads and writes data to/from the storage medium.

read/write mechanism, 158
Synonym of read/write head.

recording density, 177
Closeness or spacing of bit positions on a storage medium, typically as measured in bits, bytes, or tracks per inch.

refresh cycle, 167
(1) Period during a dynamic random access memory refresh operation when the storage device is unable to
respond to a read or write request. (2) Transfer of one full screen of data to a video monitor.

relative addressing, 173
Synonym of indirect addressing.

rotational delay, 185
Waiting time for the desired sector of a disk to rotate beneath a read/write head.

sector, 161
(1) Smallest accessible unit of a disk drive. (2) For most disk drives, 512 bytes.

segment register, 173
Register that holds the base address of a memory segment, as used in a central processing unit that uses a
segmented memory model.

sequential access time, 185
Time required to access the second of a stored sequential pair of data items.

serial access, 162
Access technique where data items are read and written in an order corresponding to their position within
allocated storage locations.

single in-line memory module (SIMM), 170
Small printed circuit board with memory chips on one or both sides and electrical contacts on one edge.

static RAM (SRAM), 167

Systems Architecture, Fourth Edition                                                                   Chapter 5

Type of random access memory that implements bit storage with a flip-flop circuit.

storage medium, 158
Device or substance within a storage device that physically stores data.

sustained data transfer rate, 185
Maximum data transfer rate that can be sustained by a device or a communication channel during lengthy data

synchronous DRAM (SDRAM), 168
Read-ahead random access memory that breaks read and write operations into a series of simple steps that can be
completed in one bus clock cycle.

tape drive, 178
Device that contains motors that wind and unwind tapes and read/write heads to access their content.

track, 182
Set of sectors on one side of a disk platter that form a concentric circle.

track-to-track seek time, 185
Time required to move a disk read/write head between two adjacent tracks.

volatile, 162
Storage devices that cannot retain their contents indefinitely.

wait state, 160
Idle processor cycle consumed while waiting for a response from another device.

write once read many (WORM), 194
Type of optical storage media that is manufactured blank and can be written to once.


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