Memory organization

Computer Memory:

     Memory is storage part in computer. It is store the data, information, programs during processing in computer. It stores data either temporarily or permanent basis. Memory used to important role in saving and retrieving data.

Primary Memory:

     Primary memory holds only those data and instructions on which the computer is currently working. It has a limited capacity and data is lost when power is switched off. It is generally made up of semiconductor device. These memories are not as fast as registers. The data and instruction required to be processed resides in the main memory. It is divided into two subcategories RAM and ROM.

 

Characteristics of Main Memory:

• These are semiconductor memories.
• It is known as the main memory.
• Usually volatile memory.
• Data is lost in case power is switched off.
• It is the working memory of the computer.
• Faster than secondary memories.
• A computer cannot run without the primary memory.

Types of Primary Memory:

      Primary memory is generally of two types.

·        RAM

·        ROM

RAM (Random Access Memory):

     It stands for Random Access Memory. RAM is known as read /writes memory. It generally refereed as main memory of the computer system. It is a temporary memory. The information stored in this memory is lost as the power supply to the computer is switched off. That’s why RAM is also called “Volatile Memory”. There are basically two kinds of RAMs.

Static RAM:

     

      Static RAM also known as SRAM, retain stored information as long as the power supply is ON. SRAM are of higher coast and consume more power. They have higher speed than Dynamic RAM.

Dynamic RAM:

      Dynamic RAM also known as DRAM, its stored information in a very short time (a few milliseconds) even though the power supply is ON. The Dynamic RAM are cheaper and moderate speed and also they consume less power.

ROM (Read Only Memory):

       It stands for Read Only Memory. ROM is a Permanent Type memory. Its content are not lost when power supply is switched off. Content of ROM is decided by the computer manufacturer and permanently stored at the time of manufacturing. ROM cannot be overwritten by the computer. It is also called “Non-Volatile Memory”. ROMs are basically is three types, names are following.

PROM (Programmable Read Only Memory):

    PROM chip is programmable ROM.it is PROM chips to write data once and read many.once chip has been programmed ,the recorded information cannot be changed. PROM is also nonvolatile memory.

EPROM (Erasable Programmable Read Only Memory):

     EPROM chip can be programmed time and again by erasing the information stored earlier in it. Information stored in EPROM exposing the chip for some time ultraviolet light .

EEPROM (Electrically Erasable Programmable Read Only Memory):

     The EEPROM is programmed and erased by special electrical waves in millisecond. A single byte of a data or the entire contents of device can be erased.

Difference between RAM & ROM:

	RAM	ROM
Data	The data is not permanent and it can be altered any number of times.	The data is permanent. It can be altered but only a limited number of times that too at slow speed.
Speed	It is a high-speed memory.	It is much slower than the RAM.
CPU Interaction	The CPU can access the data stored on it.	The CPU can not access the data stored on it. In order to do so, the data is first copied to the RAM.
Size and Capacity	Large size with higher capacity.	Small size with less capacity.
Usage	Primary memory (DRAM DIMM modules), CPU Cache (SRAM).	Firmware like BIOS or UEFI. RFID tags, micro controllers, medical devices, and at places where a small and permanent memory solution is required.
Cost	It doesn’t come cheap.	Way cheaper than RAM.

Cache Memory:

     

     Cache memory is a very high speed semiconductor memory which can speed up the CPU. It acts as a buffer between the CPU and the main memory. It is used to hold those parts of data and program which are most frequently used by the CPU. The parts of data and programs are transferred from the disk to cache memory by the operating system, from where the CPU can access them.

 

Cache hit:- when cpu ask about data to cache memory is called hit.

cache miss:- When cache dosnt have data which asked by cpu the its is called miss.

Advantages:

The advantages of cache memory are as follows −

• Cache memory is faster than main memory.
• It consumes less access time as compared to main memory.
• It stores the program that can be executed within a short period of time.
• It stores data for temporary use.

Disadvantages:

The disadvantages of cache memory are as follows −

• Cache memory has limited capacity.
• It is very expensive.

Virtual Memory:

      In the most of the computer system, the physical main memory is not as large as address space of the processor. Suppose user tries to run a program. If the program run by the user does not completely fit into the main memory, then the parts of its currently being executed are stored in main memory and remaining portion is stored in secondary storage device such as HDD.

      When a new part of program is to be brought into main memory for execution and if the memory is full, it must replace another part which is already is in main memory. As this secondary memory is not actually part of system memory, so for CPU, secondary memory is considered as Virtual Memory. Virtual memory is a memory management technique that is implemented using both hardware and software.

Advantages:

• Large programs can be written, as virtual space available is huge compared to physical memory.
• Less I/O required, leads to faster and easy swapping of processes.
• More physical memory available, as programs are stored on virtual memory, so they occupy very less space on actual physical memory.

Disadvantages:

• Applications run slower if the system is using virtual memory.
• It takes more time to switch between applications.
• Less hard drive space for your use.
• It reduces system stability.

 Secondary Memory:

    This type of memory is also known as external memory or non-volatile. It is slower than the main memory. These are used for storing data/information permanently. CPU directly does not access these memories, instead they are accessed via input-output routines. The contents of secondary memories are first transferred to the main memory, and then the CPU can access it. For example, disk, CD-ROM, DVD, etc.

 

Characteristics of Secondary Memory:

 

• These are magnetic and optical memories.
• It is known as the backup memory.
• It is a non-volatile memory.
• Data is permanently stored even if power is switched off.
• It is used for storage of data in a computer.
• Computer may run without the secondary memory.
• Slower than primary memories.

Hard Disk Drive:

    Hard disk drive is made up of a series of circular disks called platters arranged one over the other almost ½ inches apart around a spindle. Disks are made of non-magnetic material like aluminum alloy and coated with 10-20 nm of magnetic material.

     Standard diameter of these disks is 14 inches and they rotate with speeds varying from 4200 rpm (rotations per minute) for personal computers to 15000 rpm for servers. Data is stored by magnetizing or demagnetizing the magnetic coating. A magnetic reader arm is used to read data from and write data to the disks. A typical modern HDD has capacity in terabytes (TB).

 

CD Drive:

      CD stands for Compact Disk. CDs are circular disks that use optical rays, usually lasers, to read and write data. They are very cheap as you can get 700 MB of storage space for less than a dollar. CDs are inserted in CD drives built into CPU cabinet. They are portable as you can eject the drive, remove the CD and carry it with you. There are three types of CDs –

CD-ROM (Compact Disk – Read Only Memory) − The data on these CDs are recorded by the manufacturer. Proprietary Software, audio or video are released on CD-ROMs.

CD-R (Compact Disk – Recordable) − Data can be written by the user once on the CD-R. It cannot be deleted or modified later.

CD-RW (Compact Disk – Rewritable) − Data can be written and deleted on these optical disks again and again.

 

DVD Drive:

     DVD stands for Digital Video Display. DVD are optical devices that can store 15 times the data held by CDs. They are usually used to store rich multimedia files that need high storage capacity. DVDs also come in three varieties – read only, recordable and rewritable.

 

Pen Drive:

   Pen drive is a portable memory device that uses solid state memory rather than magnetic fields or lasers to record data. It uses a technology similar to RAM, except that it is nonvolatile. It is also called USB drive, key drive or flash memory.

Differences between Primary and Secondary Memory

S.N.	Primary memory	Secondary memory
1	Primary memory is temporary	Secondary memory is permanent
2	Primary memory is directly accessible by Processor/CPU	Secondary memory is not directly accessible by CPU
3	Nature of Parts of Primary memory varies. RAM- volatile in nature. ROM- Non-volatiler	It’s always Non-volatile in nature
4	Primary memory devices are more expensive than secondary storage devices	Secondary memory devices are less expensive when compare to primary memory devices
5	The memory devices used for primary memory are semiconductor memories	The secondary memory devices are magnetic and optical memories
6	Primary memory is also known as Main memory or Internal memory	Secondary memory is also known as External memory or Auxiliary memory
7	Examples: RAM, ROM, Cache memory, PROM, EPROM, Registers etc	Examples: Hard Disk, Floppy Disk, Magnetic Tapes etc