Chapter 5

Data Storage Technology

Chapter goals Describe the distinguishing characteristics of

primary and secondary storage Describe the devices used to implement

primary storage Describe the memory allocation schemes Compare and contrast secondary storage

technology alternatives

Goals cont. Describe factors that determine storage device

performance Choose appropriate secondary storage technologies

and devices Explore storage devices and their technologies Outlines characteristics common to all storage

devices Explains the technology strengths and weaknesses of

primary storage and secondary storage

Storage types Primary storage – memory or RAM

Holds instructions and data for currently executing programs

Volatile – requires electricity to maintain data

Secondary storage – electromagnetic or optical devices Non-volatile storage devices with

large capacities

Storage device components Storage devices are comprised of

Storage medium Read/write mechanism Device controller – interface between

the storage device and the system bus (discussed in chapter 6)

Storage device characteristics

Speed Speed of primary storage (RAM) directly

impacts performance of entire system RAM extends the limited capacity of CPU

registers The CPU continually moves data and

instructions between registers and RAM If a read/write to RAM takes more than one

CPU cycle, then CPU must wait for information

RAM is faster than secondary storage by a factor of 105 or more

Speed cont. Speed is also an issue for secondary

storage Called “access time” or “seek time” Access time is defined as time to

complete one read or write operation Access time for disk or tape storage can

vary depending on location of information, therefore access time is expressed as an average

Access times Primary storage – expressed in

nanoseconds (billionths of a second)

Secondary storage – expressed in milliseconds (thousandths of a second)

Data transfer rate Complete measure of data access

speed consists of access time and the unit of data transfer to/from the storage device Access time plus how much data is

transferred Data transfer unit for primary storage

is based on word size (usually 32 bit)

Data transfer unit Data transfer unit (amount of data

moved at a time) for secondary storage varies depending on the device

Unit is called a “block” Block size is stated in bytes “Sector” is data transfer unit for

magnetic and optical devices Common sector/block size is 512 bytes

Data transfer rate Expressed in terms of

bytes/second Access time combined with data

transfer unit Data transfer rate describes how

much data can be transferred between devices over a period of time

Volatility Volatile – storage device is volatile if it

cannot reliable hold data for long periods of time

Non–volatile – storage device is non-volatile if it can reliably store data for long periods of time

Computer systems need a combination of volatile and non-volatile storage devices

Access method Physical structure of storage

device’s read/write mechanism determines the way(s) data can be accessed Serial access Random access Parallel access

Serial access Stores and retrieves data items in

a linear or sequential order Slowest access method Tape typically used for backup

purposes

Random access Also called a direct access device Can directly access data stored on

the device All primary storage and disk storage

devices are direct access Parallel access – with multiple

read/write heads, can simultaneously access more than one storage location

Portability Data can be made portable by storing it

on a removable storage medium or device.

Portable devices 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.

Storage Device Characteristics

Primary storage devices Random access memory (RAM) is a

generic term for storage device that Microchip implementation using

semiconductors Ability to read and write with equal

speed Random access to stored bytes, words,

or larger data units

SRAM vs. DRAM Static RAM – implemented with

transistors Requires continuous supply of electricity to

preserve data Dynamic RAM – uses transistors and

capacitors Require a fresh infusion of power thousands

of times per second. Each refresh operation is called a refresh

cycle

Read only memory ROM – random access memory

device that can store data permanently or semi-permanently

Typically used to store BIOS (basic input output services)

Instructions stored in ROM is called firmware

Memory packaging

CPU Memory Access Management of RAM is critical to

performance of computer Organization, access, and

management or RAM is done by the operating system

How memory is accessed is large factor in performance of RAM

Physical memory organization Main memory of any computer is a

sequence of contiguous memory cells Addressable memory – highest number

storage byte that can be represented Determined by the number of bits used to

represent an address If 32 bits used to represent and address,

highest address is 232 = 4,294,967,296, or 4 GB Physical memory – actual memory

installed, usually less than addressable memory

Memory addressing & allocation Memory allocation is the assignment or

reservation of memory segments for system software, application programs, and data

Memory allocation is the responsibility of the operating system

Common scheme is to place OS in low memory and applications in high memory This can be demonstrated with C++ program

Memory allocation

Absolute vs. relative addressing Some programming languages (C,

C++) allow instructions that reference explicit memory locations BRANCH to location # STO to location #

Absolute addressing describes memory address operands that refer to actual physical memory locations

Problems with absolute addressing If a program refers to a physical

memory address in the code, then OS loses ability to re-arrange application locations in memory

Instead, programs use relative addressing

Relative addressing Instructions that refer to memory use a

combination of registers to compute addresses

When OS loads application into memory, OS loads starting point of application into one register

Instruction in application that refers to memory location is using an offset (i.e. distance from beginning of application)

OS adds offset to starting point to calculate physical memory location

Segmented memory

Each application has unique starting address

Magnetic storage Uses magnetism to store binary

information onto a storage medium that can store magnetic information

Least expensive medium for secondary storage

Can be portable Retains data without electricity Over longer periods of time will

eventually lose information

Read/write in magnetic device

Magnetic decay and leakage Primary disadvantage is loss of

data over time Magnetic Decay – the tendency of

magnetically charges particles to lose their charge over time

Magnetic Leakage – a decrease in the strength of individual bit charges

Magnetic storage

Organization of tracks and sectors

Optical mass storage devices Advantages:

Higher recording density Longer data life Retain data for decades Not subject to problems of magnetic decay and

leakage

Optical storage Optical storage devices store bit values as

variations in light reflection.

Storage medium is a surface of highly reflective material.

The read mechanism consists of a low-power laser and a photoelectric cell.

Storing binary information

Examples of optical devices

Chapter summary A typical computer system has primary and

secondary storage devices

The critical performance characteristics of primary storage devices are their access speed and the number of bits that can be accessed in a single read or write operation

Summary cont. Programs generally are created as through

they occupied contiguous primary storage locations starting at the first location

Magnetic storage storage devices store data bits as magnetic charges

Optical discs store data bits as variations in light reflection