The key characteristics of memory devices or memory system are as follows:
- Unit of Transfer
- Access Method
- Physical type
- Physical characteristics
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.
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).
- 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.
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).