Memory and Storage of Computer

Storage :

Provides capacity for all the files  and information you need (e.g. capacity of the computer’s hard  disk)

Memory :

Provides your working space (e.g. the amount of RAM installed  in the computer)

Overview 

Primary

-storage of intermediate data

-necessary to run the computer

-RAM, Cache

Secondary

-long-term storage of data

-HDD

Tertiary

-CD, DVD, memory card…

Off-line

-disconnected storage

-unplugged USB flash drive

-external HDD

Main Memory (Primary Memory)

RAM, ROM, Cache, Register

In computing memory refers to the physical  devices used to store programs or data on the  temporary or permanent basis for use in a  computer or other digital electronic device.

Main memory is divided into two parts :

-Random Access memory ( RAM ),  Read Write Memory

-Read Only Memory (ROM)

Random Access Memory (RAM)

Structure :

A set of memory chips, each of them  is an integrated circuit (IC) made of  millions of transistors and capacitors

• Volatile – data is lost or reset when power is removed from the system.

Usage :

Holds data/application programs  from input devices or storages, Also called Main memory or Primary memory perform read and write operation

  Types of RAM

Static Random Access Memory  (SRAM)

• Semi conductor memory
• Use flip-flop to store each bit of memory so does not need to be periodically refreshed
• Faster and consumes low power
• Expensive and have complex structure (6 transistors) so not use in high capacity applications

Dynamic Random Access  Memory (DRAM)

• Store each bit of memory in capacitor in an integrated circuit
• Real capacitors leak charge so capacitors need to be refreshed periodically
• Simple structure ( 1 transistor and 1 capacitor per bit) so it has very high density

Read only memory (ROM)

• ROM is “built-in” computer memory containing data that normally can only be read, not written to.
• Has contents which are fixed when the chip is manufactured.
• Holds the bootstrap loader part of the operating system.
• Retains its data when the computer is switched off.

Types of ROM

Programmable ROM (PROM)

Empty of data when the chip is  manufactured, can be  programmed by the user. Once  programmed the data cannot be  erased.

Erasable PROM (EPROM)

Like PROM only the chip can be  removed from the computer and  the program erased and another  stored in its place using  ultraviolet light.

Electrically EPROM (EEPROM)

Like EPROM but electricity is  used to erase and reprogram

Cache memory

• Faster and expensive than RAM
• It improves the computer’s performance
• Processor can use it to store frequently accessed data and program instructions
• It is two types

–Primary cache   (inside the  processor)

Secondary cache  (in the  motherboard or near the  microprocessor)

Register

A processor register (CPU register) is one of a small set of data holding places that are part of the computer processor. A register may hold an instruction a storage address, or any kind of data.

Secondary storage

Secondary Storage, Magnetic Tape, Magnetic Disk, CD-ROM, Hard Disk, Floppy Disk

Storage – any devices that are capable of holding information, even when the power is off (non-volatile).

Ex: Hard disk drive, USB flash disk, Floppy disk drive,…

Offline storage – type of storage that can be removed and can be accessed from any computers when

 connected.

-USB flash drive

-external HDD

Also called secondary memory

Types of storage device

• Optical storage devices
• Magnetic storage devices
• Flash memory devices

 

 

Optical storage 

Advantages:

Small and portable

Cheap to produce

Large market of music, movies and games

Most desktop computers or laptops are equipped with  CD/DVD/Blu-ray drive

Disadvantages:

Fragile, easy to get scratched

Slower data transferring rate

Required specified drives to write data to the discs  x  Limited number of writes

Magnetic storage devices

Floppy disk

First available in 1971

Capacity: 1.44 MB (3 1⁄2-inch floppy disk)

Discontinued.

Since 2010, no motherboard is still manufactured with the support of floppy disk drive.

Hard disk drive (HDD)

First introduced in 1956 Became dominant in the 1960s

Capacity keeps increasing

Max. capacity: 16 TB

(as of 2017)

Continuously improved.

Maintained an important position of modern servers and PCs.

Advantages (HDD):

Large and increasing capacity

High transferring speed

No data loss when power is off (non-volatile)

Cheap price compared to other storage media (cost per  MB)

Disadvantages (HDD):

Possible damages and data loss due to ‘head’ crash

Because of working on mechanism basis, it will be  eventually broken

In case of internal hard drive, it is not easy to be  transferred to another computer

High power consumption

Flash memory devices

USB Flash drive

Commercially available in 2000

Max. capacity: 1 TB (as of 2017)

Small size

Ideal solution for storing small data

Advantages:

Large and increasing capacity

High transferring speed

Small size, portability

Low power consumption

Work more quietly than physical hard drive

Disadvantages:

High price compared to hard disks (cost per MB)

Limited number of reads/writes (write endurance rating)  x  Easy to lose because of small size