MEMORY

MEMORY

Memory Chart

RAM [DDR-2 RAM]

ROM (BIOS)

Hard Disk Drive (HDD)

Floppy Disk

Magnetic Tape

Tape Drive

Primary Storage or Main Memory:

Main memory is the principle internal memory system of the computer. It is a temporary storage area where the program and data being operated on by processor must reside. Main memory is considered temporary because that data program will remain there only as long as the computer has electrical power or is not reset.

Random Access memory (RAM)

• RAM is the "read and write" memory of computer.
• RAM is the form of volatile memory used to hold temporary instructions and data for manipulation while the system is running.
• The term "random" is applied because the CPU can access or place data to and from any of the locations of this memory on the system.
• If power to the system is lost, all information on the RAM is lost as well hence RAM is called volatile.
• Ram is classified into two types:
  • Static Random Access Memory (SRAM)
  • Dynamic Random Access Memory (DRAM)

Dynamic Random Access Memory (DRAM):

• Stores data as charge on the capacitors
• Cell memory gets lost due to leakage in the capacitor. To overcome this problem capacitor is refreshed periodically.
• It more popular because it less expensive and can pack a lot of bytes into a very small chip.

Static Random Access Memory (SRAM):

• It is much faster than DRAM
• Transistors are used instead of capacitor. Transistor doesn't lose their charges so refreshing is not required like in DRAM.
• Its chips are both physically large and store very less bytes overall.
• It is also expensive then DRAM.

Read Only Memory or ROM:

• ROM (read-only memory) is nonvolatile memory that retains data when the machine is turned off.
• This is necessary so that the system can access the data it needs to start up. When stored in ROM, information that is required to start and run the computer cannot be lost or changed. The BIOS, software in the form of programs stored on ROM chips, is used during the startup routine to check out the system and prepare to run the hardware. The system BIOS is a ROM chip on the motherboard used by the computer during the boot process to check out the system and prepare to run the hardware. The BIOS is stored on a ROM chip because ROM retains information even when no power is being supplied to the computer.
• Since ROM chip can only be read, it does not accept any input data or instructions from the users. The actual contents of ROM are set by the manufacturers and they are unchangeable and permanent.
• ROM is mainly of three types:

• PROM (Programmable ROM),
• EPROM (Erasable PROM),
• EEPROM (Electrically Erasable PROM)

PROM (Programmable ROM):

• Once the operations are written into a PROM then cannot be changed.
• PROM programmer or ROM burner is used to write micro code program into PROM

EPROM (Erasable Programmable ROM):

• Program in a memory can be erased and reused
• It is erased by the exposure to intense ultra violet light
• Used in product development and experimental project

EEPROM (Electrically Erasable Programmable ROM):

• EEPROM can be erased with electrical signals instead of ultra violet light
• By using EEPROM it is possible to erase and reprogram the ROM in a computer without removing the chip from the system.

Difference between RAM and ROM:

RAM	ROM
RAM is read and writes memory of the computer.	ROM is read only memory.
It stores data to be and being processed, instructions to be and being executed and intermediate results.	It stores the instructions that co-ordinates and controls the operating system.
It is  volatile memory	It is non-volatile memory.
Data and instructions are stored into it during its operation.	Instructions are stored into it during its manufacturing.

Secondary memory or Auxiliary storage:

In some data processing, all of the instructions and data are entered in primary storage, the computer completes its processing, results are presented, and the application ends. The computer may have sufficient primary storage to hold all the instructions and data that are needed during processing. So, because primary storage may not be large enough to hold all the required instructions and data, and because RAM is volatile and doesn't provide long-term memory, supplemental storage is necessary. Device that supplement to main memory is called Secondary memory. A secondary-storage medium is usually used for the storage of large amounts of data or for permanent or long-term storage of data or programs. Secondary storage is also used for storing backups, or copies of data and programs so that they are not permanently lost if primary-storage power is interrupted.
Some of the commonly used secondary storage devices are floppy disks, hard disks, magnetic tapes, paper tapes, compact disks, etc

Hard disk

 A hard disk drive contain rigid disk shaped platter, usually constructed of aluminum or glass unlike floppy disk the platter cannot be bent (twisted, turned or curved)hence the termed as hard disk. In most hard disk drive, the platters cannot be removed so they are sometime called fixed disk drives.
A hard disk is set of stacked disk called platter made up of aluminum material that are coated with iron oxide on both sides which allows data to be magnetically recorded, the major component inside the hard disk are the platter Stack, Read/write head assembly, head actuator and spindle motor, that spine the platter and move the arms. Each platter is connected to a common central spindle magnetic head. it is used to read and write data from and to the disk. The head actuators control the movement of the head.
The hard disk spins the disk upto72000 or even more RPM (Rotation Per minute).

Floppy Disk

Floppy disk are made up of Mylar (Plastic material) coated with magnetic material (Iron Oxide). The disk is cased in a protective plastic cover the disk jacket it is also called floppy (Flexible) disk because it can be bent unlike hard disk. Floppy disk is also called diskette. They are removable unlike hard disk.
It can be used as an input as well as output medium because program and data can be stored and retrieved forms it

Magnetic Tape

 Magnetic Tape is a serial access type storage device. It is suitable for backup storage. Floppy disk can also be used for backup storage but large no of disk are required so floppies are used for backup in small single user computer. Specially magnetic tapes are used for backup in servers, work stations and large Computers. A high speed magnetic tape is employed for hard disk backup. Only a few minutes are required to dump or load the entire contents of the hard disk to or from a magnetic tape. A magnetic tape is a low cost device and it has large storage Capacity.
A Magnetic tape is made of Mylar coated with magnetic material only one side of the tape, Magnetic Tape are available in half inches, ¼ inches, 8 mm and 3 mm size. These tapes runs form 50-2400 ft.
Magnetic tape is used for backup purpose because of the following reasons

• Suddenly fluctuation in the electricity
• Overwrite the file by mistake
• Formatting the hard disk
• Failure of hard disk
• Lost of valuable data due to a computer related virus
• Catastrophic damages of the Computer by storm, Fire, thief, Lighting etc.