List and explain key characteristics of computer memory
1 Answer

The key characteristics of memory devices or memory system are as follows:

  1. Location
  2. Capacity
  3. Unit of Transfer
  4. Access Method
  5. Performance
  6. Physical type
  7. Physical characteristics
  8. Organization

1. Location:

It deals with the location of the memory device in the computer system. There are three possible locations:

  • CPU : This is often in the form of CPU registers and small amount of cache
  • Internal or main: This is the main memory like RAM or ROM. The CPU can directly access the main memory.
  • External or secondary: It comprises of secondary storage devices like hard disks, magnetic tapes. The CPU doesn’t access these devices directly. It uses device controllers to access secondary storage devices.

2. Capacity:

The capacity of any memory device is expressed in terms of: i)word size ii)Number of words

  • Word size: Words are expressed in bytes (8 bits). A word can however mean any number of bytes. Commonly used word sizes are 1 byte (8 bits), 2bytes (16 bits) and 4 bytes (32 bits).

  • Number of words: This specifies the number of words available in the particular memory device. For example, if a memory device is given as 4K x 16.This means the device has a word size of 16 bits and a total of 4096(4K) words in memory.

3. Unit of Transfer:

It is the maximum number of bits that can be read or written into the memory at a time. In case of main memory, it is mostly equal to word size. In case of external memory, unit of transfer is not limited to the word size; it is often larger and is referred to as blocks.

4. Access Methods:

It is a fundamental characteristic of memory devices. It is the sequence or order in which memory can be accessed. There are three types of access methods:

  • Random Access: If storage locations in a particular memory device can be accessed in any order and access time is independent of the memory location being accessed. Such memory devices are said to have a random access mechanism. RAM (Random Access Memory) IC’s use this access method.

  • Serial Access: If memory locations can be accessed only in a certain predetermined sequence, this access method is called serial access. Magnetic Tapes, CD-ROMs employ serial access methods.

  • Semi random Access: Memory devices such as Magnetic Hard disks use this access method. Here each track has a read/write head thus each track can be accessed randomly but access within each track is restricted to a serial access.

5. Performance: The performance of the memory system is determined using three parameters

  • Access Time: In random access memories, it is the time taken by memory to complete the read/write operation from the instant that an address is sent to the memory. For non-random access memories, it is the time taken to position the read write head at the desired location. Access time is widely used to measure performance of memory devices.

  • Memory cycle time: It is defined only for Random Access Memories and is the sum of the access time and the additional time required before the second access can commence.

  • Transfer rate: It is defined as the rate at which data can be transferred into or out of a memory unit.

6. Physical type: Memory devices can be either semiconductor memory (like RAM) or magnetic surface memory (like Hard disks).

7.Physical Characteristics:

  • Volatile/Non- Volatile: If a memory devices continues hold data even if power is turned off. The memory device is non-volatile else it is volatile.

8. Organization:

  • Erasable/Non-erasable: The memories in which data once programmed cannot be erased are called Non-erasable memories. Memory devices in which data in the memory can be erased is called erasable memory.

    E.g. RAM(erasable), ROM(non-erasable).


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