Memory

Memory refers to electronic holding place for instructions and data where the processor can reach quickly.

Classification of memory:

1. Primary memory

2. Secondary memory

Units to measure memory:

• Bit : It can hold only one of two values 0 or 1. It is represented by b.

• Byte: A unit of 8 bits is known as a byte.It is represented as B.

• Kilobyte: A kilobyte is equal to 1024 bytes. It is represented as KB.

• Megabyte: It comprises of 1024KB. Represented as MB.• Gigabyte: A gigabyte consists of 1024 MB. Abbreviated

as GB.• Terabyte: It refers to 1024 gigabytes. Representred

asTB.

Memory Hierarchy

Registers, Cache memory

Internal Processor Memory

RAM, ROM Primary memory

Hard disk,floppy disk

Secondary memory

CostStorage capacity

Access Speed

Primary memory:

Random Access Memory(RAM): It is the place where the operating system,application programs,and data in current use are kept so that it can be accessed quickly by computer processor.

RAM is much faster to read from and write to than other kind of storage like hard disk,floppy disk.

Types of RAM:

1.Static RAM: The memory retains its contents as long as power remains applied.Data is lost when power goes down.

2.Dynamic RAM:DRAM must be continually refreshed in order to maintain the data.It is used because it is cheap.

Read only memory:

It is a non\-volatile memory. It performs the read operation and restrict themselves to be altered and therefore making it secure than RAM.Designers program ROM chips at the time of manufacturing circuits.

Types of ROM

1.Masked ROM: These are hardwired devices that contain a pre-programmed set of data or instruction.

2. Programmable ROM: Blank ROM chips can be brought and coded by anyone with a special tool such as PROM-programmers.Once a PROM has been programmed, its contents can never be changed.

3. Erasable Programmable ROM: An EPROM can be erased and reprogrammed repeatedly.It can be erased by simply exposing the device to a strong source of ultraviolet light for a certain amount of time.

4.Elecrtically Erasable Programmable ROM:It can be erased by an electrical charge and then written to by using slightly higher than normal voltage.

Types of secondary storage devices:

There are two methods of accessing data from the secondary storage devices:

1.Sequential: Sequential access means the computer system must search the storage device from beginning until it finds the desired information.Ex: magnetic tape

2.Direct:Direct also called random access means that the computer can go directly to information that the user wants.Ex: magnetic and optical disks.

Secondary Storage Devices

Sequential Access devices

Direct Access Devices

Magnetic Disk

Magnetic Tapes

Optical Disk

Magneto Optical Disk

Magnetic Tape:

It is a plastic tape with magnetic coating. The data is stored in form of magnetised and demagnetised portion on the surface of the material. Magnetised portion of surface refers to bit value 1 whereas demagnetised portion refers to bit value 0.Magnetic tape are cheap and reliable storage medium.

Magnetic Disk:

A magnetic disk consists of plastic circular plate which is coated with magnetic oxide layer.Data is represented as magnetised spots on the disk.

Types of magnetic disk:

1.Floppy disk

2.Hard disk

Optical disk

The optical disk storage system consists of a rotating disk coated with a thin layer of metal that acts as a reflective surface and a laser beam which is used as a read/write head for recording data onto the disk.

Ex:

CD-ROM,DVD-ROM