Memory devices

 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 is required to save data and instructions. Memory is divided into cells, and they are stored in the storage space present in the computer. Every cell has its unique location/address. Memory is very essential for a computer as this is the way it becomes somewhat more similar to a human brain. In this article, we are going to discuss memory and memory units in detail.

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.

Main memory operates at a high speed compared to mass storage which is slower but less expensive per bit and higher in capacity. Besides storing opened programs and data being actively processed, computer memory serves as a mass storage cache and write buffer to improve both reading and writing performance. Operating systems borrow RAM capacity for caching so long as it is not needed by running software. If needed, contents of the computer memory can be transferred to storage; a common way of doing this is through a memory management technique called virtual memory.

Semiconductor memories

Semiconductor memories are the modern data storage and retrieval devices in digital systems and are an essential part of contemporary electronics. Made of semiconductor materials, mostly silicon,  as the base material, supplemented by advanced materials like high-κ dielectrics, phase-change alloys, ferroelectric compounds, and magnetic materials to enhance performance, scalability, and efficiency and works by employing integrated circuits to store binary data as electrical charges or states. This way of processing and storing information has completely transformed new age technology, which makes it possible to quickly access and manipulate the data that powers modern computing.

These memories 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 37 kB memory size, then this memory unit has 37 × 1024 = 37888 memory location or cells. Hence, the address of these locations ranges from 0 to 37887.

Modern computer memory is implemented as semiconductor memory, where data is stored within memory cells built from MOS transistors(open for more) and other components on an integrated circuit. There are two main kinds of semiconductor memory: volatile and non-volatile. Examples of non-volatile memory are flash memory and ROM, PROM, EPROM, and EEPROM memory. Examples of volatile memory are dynamic random-access memory (DRAM) used for primary storage and static random-access memory (SRAM) used mainly for CPU cache.

Fundamental units of memory : Registers

Register(open for more) is an important component of digital devices that stores data and instructions for quick processing. It serves as a temporary storage area where information can be accessed and manipulated quickly in order to carry out complex tasks. Registers are the most basic type of memory in computers and they play a critical role in helping machines process data efficiently. In this blog, we’ll explore what registers are, how they work, and why they are so important for modern-day computing.

A register is composed of multiple flip-flops, which are electronic circuits capable of storing a single bit of information, which is represented through binary data – either a 0 or a 1. By combining multiple flip-flops, registers can store larger binary values, such as bytes or words.

Data and Address registers

Registers also contain control logic circuitry, which allows it to coordinate the flow of data and instructions within the CPU. This can include operations such as decoding control signals, performing data manipulation like data loading, storing, or arithmetic operations, and using mulitplexers to route data to a specific location within the register.

Together, flip-flops and control logic work in partnership within registers. Flip-flops provide the storage capacity, while control logic facilitates the coordination of data transfer, manipulation, and synchronization with other components of the CPU. This enables registers to store and process data efficiently during the execution of instructions.

The data in a memory are stored and retrieved by the process called writing and reading respectively. 

A word is a group of bits where a memory unit stores binary information. A word with a group of 8 bits is called a byte. A memory unit consists of data lines, address selection lines, and control lines that specify the direction of transfer.

Data lines provide the information to be stored in memory. The control inputs specify the direct transfer. The k-address lines specify the word chosen. When there are k address lines, 2^k memory words can be accessed. 

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:

Types of Computer Memory

• Cache Memory: This temporary storage area, known as a cache, is more readily available to the processor than the computer’s main memory source. It is also called CPU memory because it is typically integrated directly into the CPU chip or placed on a separate chip with a bus interconnect with the CPU.
• RAM: It is one of the parts of the Main memory, also famously known as Read Write Memory. Random Access memory is present on the motherboard and the computer’s data is temporarily stored in RAM. As the name says, RAM can help in both Read and write.
• D RAM (Dynamic RAM): D RAM uses capacitors and transistors and stores the data as a charge on the capacitors. They contain thousands of memory cells. It needs refreshing of charge on capacitor after a few milliseconds. This memory is slower than S RAM.
• S RAM (Static RAM): S RAM uses transistors and the circuits of this memory are capable of retaining their state as long as the power is applied. This memory consists of the number of flip flops with each flip flop storing 1 bit. It has less access time and hence, it is faster.
• ROM: ROM full form is Read Only Memory. ROM is a non volatile memory and it is used to store important information which is used to operate the system. We can only read the programs and data stored on it and can not modify of delete it.
• MROM(Masked ROM): Hard-wired devices with a pre-programmed collection of data or instructions were the first ROMs. Masked ROMs are a type of low-cost ROM that works in this way.
• PROM (Programmable Read Only Memory): This read-only memory is modifiable once by the user. The user purchases a blank PROM and uses a PROM program to put the required contents into the PROM. Its content can’t be erased once written.
• EPROM (Erasable Programmable Read Only Memory): EPROM is an extension to PROM where you can erase the content of ROM by exposing it to Ultraviolet rays for nearly 40 minutes.
• EEPROM (Electrically Erasable Programmable Read Only Memory): Here the written contents can be erased electrically. You can delete and reprogram EEPROM up to 10,000 times. Erasing and programming take very little time, i.e., nearly  4 -10ms(milliseconds). Any area in an EEPROM can be wiped and programmed selectively.
• Virtual memory: A memory management technique where secondary memory can be used as if it were a part of the main memory. Virtual memory uses hardware and software to enable a computer to compensate for physical memory shortages by temporarily transferring data from RAM to disk storage.

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.