Computer Memory





The computer memory is a data storing devices. There are many categories of memories: primary and secondary memory. RAM, ROM, registers, accumulator, floppy, hard disk, CD-ROM, magnetic tapes etc are example of computer memory. The processing capacity of computer not only depends on processor, but also on capacity of primary memory.

 

The evolution of computer from one generation to next generation show variation in the processing speed, storing capacity, reliability, power consumption, and size. A memory system can be logically divided into three categories: internal memory, main memory, and secondary memory.

Internal memory refers to the set of registers confined to the CPU. It holds temporary results when a computation is in progress. Because these registers and the CPU are fabricated with the same technology, there is no speed disparity between them but the cost factor limits the number of registers incorporated in the CPU.

A memory system may be centralized or distributed. In a centralized system, all processors that exist in a system can be share one common memory unit, which may be referred to as a tightly coupled system. The salient features of this system are as follows:

  • Inexpensive
  • Easy to control
  • High use of the available space.

In a distributed or loosely coupled system, each processor has a private or local memory as well as a common memory. The advantage of it is:

(i) Several memory units can be kept busy at the same time

(ii) Since each processor has its own private memory, they can be simultaneously engaged to perform local processing.

 

Primary Memory

The fast and integrated memory of less capacity is termed as primary memory or main memory. The RAM, ROM, Flash memory, cache memory are types of primary memory. These memories have great effect on processing speed of computer. Main memory has register called MAR (Memory Address Register) to hold the address of the location to be accessed. The memory also has an input-output port through which the data transfer takes place to and from the addressed location. This port is called Memory Buffer Register (MBR) or Memory Data Register (MDR).

The main function of main memory is READ, WRITE and CLEAR.

  • READ: READ signal when applied, READ contents of memory location specified by the MAR into the MBR.
  • WRITE: WRITE signal when applied causes the contents of MBR to be written into the memory location.
  • CLEAR: CLEAR signal has same function as that of the READ signal except the data READ is not loaded with MBR.

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Primary Memory

Secondary Memory

1.

It is a semiconductor memory.

It is a magnetic and optical memory.
2. In memory hierarchy, it is placed in highest position due to high speed, high cost and small size. In memory hierarchy, it is placed in lower position due to low speed, low cost and large size.
3. Its storing capacity is small. Its storing capacity is very large.
4. Processor directly accesses primary memory. Processor does not directly access secondary memory. It is accessed through I/O interface for I/O processor.
5. Example: RAM, ROM Example: Floppy, ZIP-Floopy, Hard Disk, CD-ROM, VCD, WROM

 

RAM and ROM: The RAM and ROM built main or primary memory.

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RAM

ROM

1. It stands for Random Access Memory. It is also called read/write or volatile memory, because its content can be emptied when refresh command is executed or computer shut down.

It stands for Read Only Memory. It is called dead store, field store. Permanent stores and non-Volatile memory.

2. Its content lost if power is off. It retains contents if power is off.
3. RAM is divided into two categories: (A) Static RAM and (B) Dynamic RAM ROM is also divided into (A) PROM (B) EPROM (C) EEPROM and (D) Flash Memory
4. Costly Cheap

 

Static RAM and Dynamic RAM:

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Static RAM (SRAM)

Dynamic RAM (DRAM)

1. Static RAM hold information in a flip-flop circuit consisting six transistors in needed in each memory cell. It is a costly RAM. It requires less number of transistors per memory cell because information is stored in stary capacitors. Only one transistor is needed to form a memory cell of the dynamic RAM. It is a less costly than SRAM.
2. Its speed is high. Its speed is lower than SRAM.
3. It occupies large space. Small space is occupied by DRAM.
4. It consumes less power than DRAM.

More power is consumed.

5. Refresh circuit is not needed. Refresh circuit is needed.

 

The concept of PROM, EPROM and EEPROM:

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PROM EPROM EEPROM
1. It stand for programmable ROM. It stands for Erasable Programmable ROM.

It stands for Electrically Erasable Programmable ROM. It is also called EAPROM (Electrically Alterable PROM).

2. It is a programmable ROM. Its contents cannot be erased. Information stored can be erased in Ultra violet exposure. Its contents can be erased by electricity and again it can be programmed like as EPROM.

 

Video RAM (VRAM):

The CRT screen is a display unit of computer. The operating system organizes the display in the form of grid of X and Y co-ordinates. Each little box or cell is termed as pixel (picture element). To control color of each pixel, operating system provides small amount of memory to each pixels. In aggregate, memory offered by Operating system to pixels is referred as Video RAM of VRAM.

 

Flash Memory:

  • It is an upgraded version of EEPROM. It is a transistor memory with very fast reprogramming capability.
  • It is suitable for storing control code for BIOS (Basic Input Output System).
  • Its entire contents can be quickly erased but in EIEPROM, it is erased and rewritten at byte levels.
  • It is used in digital cellular phones, digital camera, LAN switches etc.

 

Cache memory:

Cache memory is a small, quick and expensive memory placed between Main memory and Processor (CPU). It contains copy of main memory’s data and when Processor request for data, it send data to processor for processing. Its capability is measured in hit ratio. Generally, the value of hit ratio is 0.9 and above. The cache comes from the French, cacher which meaning to hide. Programmers do not generally “see” the cache. Cache should only affect the performance of a program.

  • A buffer memory present between Processor (CPU) and Main memory is called Cache memory.
  •  It is very small in size but expensive. It contains static RAM to store data.
  • It is a 5 to 10 times faster than main memory.
  • It increases storing capacity of main memory and increase speed of Processor.
  • If cache is present in processor is called internal cache and when it is outside of Processor, then it is called external cache.
  • It contains copy of data fetched from main memory. The access of cached copies is more efficient than original data contained in main memory.
  • Cache Hit: At first processor, send address of desired data to cache, if data is available then event is called Cache hit.
  • Cache Miss: When processor sent address is not present in cache, event is called Cache miss. After cache miss, data of main memory is copied to cache for next cache hit.
  • Cache Ratio: The performance of cache is measured in cache ratio. Its value is 0.9 and more.

 

Secondary Memory:

Floppy Disk, Hard disk, CD-ROM, DVD, Zip Disk, Jaz Drive, WORM, magnetic tapes etc are secondary memories used to store data. These memories are also called backup memory because these memories are used to store data for future application. Now a days, operating system, and other software, business data and information are stored in CD-ROM.



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