Memory Devices

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Memory is one of the important parts in a computer or any other digital system. It is used to hold data and programs required for processing and performing tasks.

Memory also affects the performance, efficiency, and speed of the digital system. These days, semiconductor memories are popular, as they provide a very high-speed operation, large storage capacity, and compact size.

Here, we will explain the basic to advanced concepts related to semiconductor memory devices.

What is Memory?

In the field of digital electronics, the memory is a device that is used to store data and instruction in the digital systems like computers and other microprocessor-based systems. In modern digital systems, the memory is made up of semiconductor materials and known as semiconductor memory.

The memory is the device that provides the storage space in computer or any other digital system where data is to be processed and instructions required for processing are stored.

The memory is divided into a large number of small parts. Each part is called a memory cell. Each memory cell or location has a unique address assigned to it which varies from zero to total memory size minus one.

For example, if a computer has 64 kB memory size, then this memory unit has 64 × 1024 = 65536 memory location or cells. Hence, the address of these locations ranges from 0 to 65535.

Classification of Memory

Memory is primarily classified into two types, they are: Internal Memory and External Memory.

Internal Memory

Internal memory is also known as primary memory, as it is directed connected to the hardware architecture of the digital system. It is typically installed in the system’s motherboard in form of ICs.

Examples of internal memory include cache memory, RAM (Random Access Memory), ROM (Read Only Memory), etc.

External Memory

External memory is also known as secondary memory. This memory is not directly connected to the hardware architecture of the systems, instead it is connected through cables as a peripheral device.

The external memory is primarily used to provide additional storage space to store data and instructions permanently. Examples of external memory are CD, DVD, HDD, SSD, USB drives, etc.

Memory Hierarchy

Memory hierarchy is defined as an arrangement of different types of memory devices used in a digital system depending on their characteristics, primarily speed and capacity. The memory hierarchy helps us to select an appropriate memory to use in our system at a specific level.

A typical memory hierarchy of memory for different memory devices is shown in the following figure −

Some of key characteristics of this memory hierarchy, when we go from top to bottom −

• Capacity in terms of storage increases.
• Cost per bit of storage decreases.
• Frequency of access of the memory by the CPU decreases.
• Access time by the CPU increases.

Functional Block Diagram of Memory

A memory is basically a group of multiple storage cells having a support circuit to perform data read/write operations. The following figure depicts the functional block diagram of a typical memory device −

It consists of the following main parts −

Address Lines

These lines are used to load the address of a specific memory location or cell.

Data Lines

These lines are used to read and write the data from/to the memory cell.

Read and Write Signal (R/W’)

This signal is used to read and write the data from and to the memory cells. When the R signal is high, the data of the selected cell gets loaded on the data line. When the W’ line goes low, the data on the data line is loaded into the selected memory cell.

Chip Select Signal (CS’)

This signal is used to enable or disable the memory chip. It is an active low signal, which means when this signal goes low, the memory chip is enabled and allow the read and write operations to execute. Otherwise, the memory chip will be disabled.

Important Terms Related to Memory Operation

The following are some important terms and definitions which are related to the read and write operations of memory −

• Write Cycle Time − The write cycle time is defined as the minimum amount of time for which a valid cell address is available for data writing operation in the cell. Typically, it is of the order of 200 ns.
• Write Pulse Time − The minimum duration of a write pulse is termed as write pulse time and it is of the order of 120 ns.
• Write Release Time − The minimum time for which the memory address is valid before the write pulse is known as write release time.
• Data Setup Time − The minimum amount of time for which the data remains valid before the write pulse ends is known as data setup time. Typically, it is around 120 ns.
• Data Hold Time − The minimum amount of time for which the data remains valid after the write pulse ends is known as data hold time.
• Read Cycle Time − The minimum amount of time for which a valid memory address remains available for reading the data from a memory cell is known as read cycle time. It is typically of the order of 200 ns.
• Access Time − The amount of time required to access data from a memory cell is referred to as access time of the memory. It is also of the order of 200 ns.
• Read to Output Active Time − The minimum time that required for enabling the output buffer after starting of the read pulse is called the read to output active time. Typically, this time is of the order of 20 ns.
• Read to Output Valid Time − The maximum delay time between the beginning of the read pulse and the availability of the valid data at the data output line is known as "read to output valid time".

These are some key terms whose knowledge is required to understand the read and write operations of a memory device.

Characteristics of Memory Devices

In this section, we will focus on studying some key characteristics of memory devices and their definitions and importance −

Storage Capacity

This parameter denotes the total memory of the device. It is generally expressed in terms of number of Bytes that it can store. For example, a memory of 1k × 8 bits can store 1024 × 8 = 8192 Bytes of digital data.

Unit of Data Transfer

The number of bits that can be read or written in a single read or write cycle is called the unit of data transfer. In general, the unit of data transfer is equal to the word length or size of the data bus of the processor.

Modes of Access

It refers to the way in which the data can be read or write to the memory. There are following three modes used in digital memory devices −

Sequential Access

In this mode, the data is read from or write to the memory in a predefined sequential manner. In other words, to access the second file, we first access the first file, to access the third file, firstly access the first and second files, and so on.

Random Access

In this mode, we can directly access any memory location in any order.

Direct Access

This mode is a combination of sequential and random access modes. It is also termed as semi-random access mode.

Data Transfer Rate

It is defined as the amount of data that is read or write in one second. It is generally measured in bits per second. Data transfer rate is referred to as bandwidth of memory.

Types of Memory Devices

Some of the important classifications of memory devices used in computers and digital systems are listed and explained here.

Classification of memory on the basis of nature of data storage −

• Volatile Memory
• Non-Volatile Memory

Classification of memory on the basis of access modes −

• Sequential Access Memory
• Random Access Memory

Now, let us discus all these types of memories in detail along with their subtypes and characteristics.

Volatile Memory

A type of memory that requires continuous power supply to maintain the stored data is called a volatile memory. If the power supply to the memory is turned off, the data stored in it will be lost. Therefore, it is also termed as temporary memory.

Properties of Volatile Memory

The volatile memory loses its stored data when power supply to it is turned off. Volatile memory has fast operational speed; thus, it can read and write data in a short span of time.

Volatile memory is used to store data required to be accessed and perform operations. RAM (Random Access Memory) is an example of volatile memory.

Non-Volatile Memory

A type of memory which can retain stored data even when no power supply is present is known as non-volatile memory. It is also known as permanent memory and is used for long-term storage of digital data.

Properties of Non-Volatile Memory

Non-volatile memory stores data permanently. It can retain the stored data even when the power supply is switched off.

Non-volatile memory is slower than volatile memory. Hence, this memory has longer read and write cycles.

Examples of non-volatile memory includes ROM (Read Only Memory), magnetic tapes, optical discs, magnetic discs, USB drives, etc.

Sequential Access Memory

A type of memory in which the stored data and information is accessed in a predefined sequential manner is known as a sequential access memory.

Sometimes, it is also known as serial access memory, as the stored data is retrieved in a serial order.

In a sequential access memory, the system must search the storage device from the beginning of the memory address until it finds the required piece of data. In other words, to retrieve the desired data, the system must access all memory addresses until it reaches the desired data.

Properties of Sequential Access Memory

In a sequential access memory, data retrieval process executes in a sequential manner. Where, the system starts from beginning of the memory and go sequential through all memory addresses until the desired data is obtained.

Sequential access memory has slower access speed and longer read/write time. Magnetic tapes are the examples of sequential access memory.

Random Access Memory

Random access memory, also called as direct access memory, is a type of memory in which the desired data can be accessed directly, without going through the preceding data. Hence, this memory allows to access any data in any order.

In other words, the direct access memory or random access memory has the ability to read from or write to data in any memory location in the same time. Hence, the access time for all the memory cells is the same and it does not depend on the physical location of the cell within the memory array.

Properties of Random Access Memory

Random access memory allows data to be accessed in any random order. It provides high speed data access i.e., fast read and write operations.

All memory locations of the random access memory are directly accessible to the processing element of the digital system.

Examples of random access memory include RAM, ROM, hard disk, optical disks, and other semiconductor memories, etc.

Conclusion

In conclusion, a memory device is an important component in a digital system like computer used to store data and information.

Different types of memory devices are used for different purposes. For example, a volatile memory like RAM is used to hold temporary data which are required till the process is complete.

On the other hand, a non-volatile memory is used to hold data permanently for a longer period of time. For example, a hard disc is used to store user’s data in the computer system.