In the last chapter, we had a discussion on memory devices and their characteristics. Read this chapter to understand the characteristics of the two most important types of memories named, RAM (Random Access Memory) and ROM (Read Only Memory) which are used in digital systems like computer, laptops, smartphones, etc.

What is RAM?

A RAM constitutes the internal memory of the CPU for storing data, program and program result. It is read/write memory. It is called Random Access Memory (RAM).

Since the access time in RAM is independent of the address to the word that is, each storage location inside the memory is as easy to reach as other location & takes the same amount of time. We can reach into the memory at random & extremely fast but can also be quite expensive.

RAM is a volatile memory i.e., data stored in it is lost when we switch off the computer or if there is a power failure. Hence, a backup uninterruptible power system (UPS) is often used with computers. RAM is small, both in terms of its physical size and in the amount of data it can hold.

Types of RAM

RAM or Random Access Memory is classified into the following two types −

• Static RAM (SRAM)
• Dynamic RAM (DRAM)

Let’s discuss about these two types of RAMs in detail.

Static RAM (SRAM)

The word “static” indicates that the memory retains its contents as long as power remains applied. However, data is lost when the power gets down due to volatile nature. SRAM chips use a matrix of 6-transistors and no capacitors. Transistors do not require power to prevent leakage, so SRAM need not have to be refreshed on a regular basis.

Because of the extra space in the matrix, SRAM uses more chips than DRAM for the same amount of storage space, thus making the manufacturing costs higher. Static RAM is mainly used as cache memory needs to be very fast and small.

Characteristics of SRAM

The following are some important characteristics of SRAM −

• Being a type of RAM, the SRAM is also a volatile memory. Thus, it requires a continuous power supply to maintain its stored data. If power supply is removed or switched off, the data stored in the SRAM will delete.
• SRAM is a high-speed random access memory. SRAM does not need to be refreshed to maintain its stored data.
• SRAM is made up of semiconductor components called flip-flops which store data. SRAM has lower storage density. This is mainly because of its complex memory cell structure. This also results in larger physical size.
• SRAM is mostly employed in digital systems in which high-speed data access is important. For example, it is used as CPU cache memory, high-speed buffers, and registers in microprocessors and microcontrollers.
• SRAM is relatively more expensive. This is mainly because of its lower storage density and higher manufacturing cost.

Dynamic RAM (DRAM)

DRAM, unlike SRAM, must be continually refreshed in order for it to maintain the data. This is done by placing the memory on a refresh circuit that rewrites the data several hundred times per second. DRAM is used for most system memory because it is cheap and small.

All DRAMs are made up of memory cells. These cells are composed of one capacitor and one transistor.

Characteristics of DRAM

The important characteristics of DRAM (Dynamic Random Access Memory) are listed below −

• Since DRAM is also a random access memory, hence it is also a volatile memory and thus requires a continuous power supply to retain its stored data. The data stored in DRAM is lost, when power supply to is turned off.
• In DRAM, the memory cells are made up of capacitors and transistors. Where each memory cell can store a 1-bit of data in the form electric charge in a capacitor.
• In DRAM, to prevent losing stored data due to leakage in capacitor, a refresh circuit is required for periodic refresh cycles. This is the primary reason the term “dynamic” is used in DRAM.
• For DRAM, the access time is typically of the order in nanoseconds (ns). DRAM is less expensive than SRAM.

This is all about RAM (Random Access Memory) and its types. Let us now discuss about another type of memory device called ROM.

What is ROM?

ROM stands for Read Only Memory. The memory from which we can only read but cannot write on it. This type of memory is non-volatile. The information is stored permanently in such memories during manufacture.

A ROM, stores such instruction as are required to start computer when electricity is first turned on, this operation is referred to as bootstrap. ROM chip are not only used in the computer but also in other electronic items like washing machine and microwave oven.

Types of ROM

The following are some important types of Read Only Memory (ROM) −

• MROM
• PROM
• EPROM
• EEPROM

Let’s discuss these different types of ROMs in detail along with their important characteristics.

MROM (Masked ROM)

The very first ROMs were hard-wired devices that contained a pre-programmed set of data or instructions. These kinds of ROMs are known as Masked ROMs. It is inexpensive ROM.

Since it is a type of ROM, thus it is also a non-volatile memory. The MROMs are programmed at the time of manufacturing and its data cannot be modified or changed at a later point of time.

Characteristics of MROM

The following are some important characteristics of MROM −

• MROM is a non-volatile memory. Hence, it can retain its data even when power supply is turned off or removed.
• MROM is mainly used for storing permanent software and instructions like firmware, bootloader code, and other system data essential for system operations.
• In MROM, data and programs are written during the manufacturing process. Once it is programmed, the stored data cannot be modified or changed. Thus, it is a one-time programmable memory.
• Another important characteristic of MROM, it is a read-only memory. Thus, it supports read only operations.
• MROM is an inexpensive type of read only memory.
• The major disadvantage of the MROM is its limited flexibility that means once it is programmed, the stored data cannot be changed or deleted.

PROM (Programmable Read Only Memory)

PROM is a read-only memory that can be modified only once by a user. The user buys a blank PROM and enters the desired contents using a PROM programmer.

Inside the PROM chip, there are small fuses which are burnt open during programming. It can be programmed only once and is not erasable.

Characteristics of PROM

Here are the important characteristics of the programmable read only memory −

• PROM also retains its stored data, when the power supply is turned off.
• PROM is a programmable memory, but it can be programmed by the user only once. Then, its stored data cannot be changed, deleted, or rewritten.
• In PROM, the memory cells are made by using either fuse-based technology, in which storing the data involves blowing of tiny fuses.
• PROM is also a read only memory, thus it supports read operations only.
• Like MROM, PROM also offers limited flexibility, as the data cannot be changed or erased, once it is programmed.

EPROM (Erasable and Programmable Read Only Memory)

The EPROM can be erased by exposing it to ultra-violet light for a duration of up to 40 minutes. Usually, an EPROM eraser achieves this function.

During programming, an electrical charge is trapped in an insulated gate region. The charge is retained for more than ten years because the charge has no leakage path. For erasing this charge, ultra-violet light is passed through a quartz crystal window (lid). This exposure to ultra-violet light dissipates the charge. During normal use, the quartz lid is sealed with a sticker.

Characteristics of EPROM

Some of the key characteristics of EPROM are highlighted below −

• EPROM provides a permanent storage for data even in the absence of power supply.
• EPROM is an electrically programmable memory. Hence, it can be programmed by applying specific voltage levels to its write circuit.
• EPROM can be erased multiple times by exposing it to UV rays for around 20 to 30 minutes. Then, it can be reprogrammed again.
• EPROM chips have a quartz window on its top. This is provided to penetrate the UV rays to erase the stored data.
• EPROM provides the high storage density. Hence, it can hold large amounts of data in a relatively small physical space.
• EPROM is slower to write, that can affect overall performance of the system.

EEPROM (Electrically Erasable and Programmable Read Only Memory)

The EEPROM is programmed and erased electrically. It can be erased and reprogrammed about ten thousand times. Both erasing and programming take about 4 to 10 ms (millisecond).

In EEPROM, any location can be selectively erased and programmed. EEPROMs can be erased one byte at a time, rather than erasing the entire chip. Hence, the process of re-programming is flexible but slow.

Characteristics of EEPROM

The important characteristics of electrically erasable programmable read only memory are highlighted below −

• Being a ROM, EEPROM stores data permanently.
• EEPROM can be erased and reprogrammed electrically and does not require any special equipment or UV rays for erasing the stored data.
• EEPROM can be erased and reprogrammed multiple times.
• EEPROM also provides random data access capabilities. This feature allows for efficient and fast data manipulation and management.
• EEPROM consumes very less power. Hence, it is better suited to use in battery-powered devices where energy efficiency is important.
• EEPROM is a cost effective read only memory device designed use in modern digital systems.

Conclusion

In this chapter, we explained the basics RAM and ROM along with their different types and characteristics.