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

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.

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.

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.

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.

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.

Physical type:

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

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.

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).