CPU, Computer Memory and Mass Storage Devices

Central Processing Unit (CPU) consists of the following features

  • CPU is considered as the brain of the computer.
  • CPU performs all types of data processing operations.
  • It stores data, intermediate results, and instructions (program).
  • It controls the operation of all parts of the computer.

CPU itself has following three components.

  • Memory or Storage Unit
  • Control Unit
  • ALU(Arithmetic Logic Unit)


Memory or Storage Unit

This unit can store instructions, data, and intermediate results. This unit supplies information to other units of the computer when needed. It is also known as internal storage unit or the main memory or the primary storage or Random Access Memory (RAM).

Its size affects speed, power, and capability. Primary memory and secondary memory are two types of memories in the computer. Functions of the memory unit are −

  • It stores all the data and the instructions required for processing.
  • It stores intermediate results of processing.
  • It stores the final results of processing before these results are released to an output device.
  • All inputs and outputs are transmitted through the main memory.

Control Unit

This unit controls the operations of all parts of the computer but does not carry out any actual data processing operations.

Functions of this unit are

  • It is responsible for controlling the transfer of data and instructions among other units of a computer.
  • It manages and coordinates all the units of the computer.
  • It obtains the instructions from the memory, interprets them, and directs the operation of the computer.
  • It communicates with Input/Output devices for transfer of data or results from storage.
  • It does not process or store data.

ALU (Arithmetic Logic Unit)

This unit consists of two subsections namely,

  • Arithmetic Section
  • Logic Section

Arithmetic Section

Function of arithmetic section is to perform arithmetic operations like addition, subtraction, multiplication, and division. All complex operations are done by making repetitive use of the above operations.

Logic Section

Function of logic section is to perform logic operations such as comparing, selecting, matching, and merging of data.

Computer Memory

RAM (Random Access Memory) is the internal memory of the CPU for storing data, program, and program result. It is a read/write memory which stores data until the machine is working. As soon as the machine is switched off, data is erased.

Access time in RAM is independent of the address, that is, each storage location inside the memory is as easy to reach as other locations and takes the same amount of time. Data in the RAM can be accessed randomly but it is very expensive.

RAM is volatile, 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.

RAM is of two types

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

Static RAM (SRAM)

The word static indicates that the memory retains its contents as long as power is being supplied. 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 be refreshed on a regular basis.

There is extra space in the matrix, hence SRAM uses more chips than DRAM for the same amount of storage space, making the manufacturing costs higher. SRAM is thus used as cache memory and has very fast access.

Characteristic of Static RAM

  • Long life
  • No need to refresh
  • Faster
  • Used as cache memory
  • Large size
  • Expensive
  • High power consumption

Dynamic RAM (DRAM)

DRAM, unlike SRAM, must be continually refreshed in order 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 as it is cheap and small. All DRAMs are made up of memory cells, which are composed of one capacitor and one transistor.

Characteristics of Dynamic RAM

  • Short data lifetime
  • Needs to be refreshed continuously
  • Slower as compared to SRAM
  • Used as RAM
  • Smaller in size
  • Less expensive
  • Less power consumption


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 instructions that are required to start a computer. This operation is referred to as bootstrap. ROM chips are not only used in the computer but also in other electronic items like washing machine and microwave oven.

Let us now discuss the various types of ROMs and their characteristics.

MROM (Masked ROM)

The very first ROMs were hard-wired devices that contained a pre-programmed set of data or instructions. These kind of ROMs are known as masked ROMs, which are inexpensive.

PROM (Programmable Read Only Memory)

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

EPROM (Erasable and Programmable Read Only Memory)

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 10 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.

EEPROM (Electrically Erasable and Programmable Read Only Memory)

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 reprogramming is flexible but slow.

Advantages of ROM

The advantages of ROM are as follows −

  • Non-volatile in nature
  • Cannot be accidentally changed
  • Cheaper than RAMs
  • Easy to test
  • More reliable than RAMs
  • Static and do not require refreshing
  • Contents are always known and can be verified

Mass Storage Devices

The first invention that revolutionized storage for the personal computer industry was the introduction of floppy disks. Similar to tape, these magnetic storage devices were flat discs that rotated within a sleeve. The original 5-inch size soon had an 8-inch larger version to store more data, and both of these were eventually replaced by a smaller 3½-inch size that actually stored more data–up to 1.44 megabytes. These were great improvements, but speed and reliability were still a problem. Today, floppy disks have been almost entirely replaced by other storage devices, such as CDs and DVDs, that are faster, more reliable, and have huge storage capacities.

Hard Disk Drives

Modern external hard drive

The addition of hard disk drives finally gave personal computers more reliable storage, with faster loading and saving of programs and data. Similar to the concept of floppy disks, the magnetic medium was placed on a hard metal platter that could spin much faster. Hard drives soon became standard internal devices on PCs, although storage capacities of 20, 32 and 40 megabytes were filled rather rapidly. External hard drives, which are still in use, became desirable add-ons. Today, however, external hard drives are small, fast, inexpensive and available in storage capacities of 500 gigabytes and even 1 terabyte (1,000 gigabytes).

Flash drive

Solid state memory devices called “flash drives” have become a welcomed storage medium for personal computer users. These small devices are only about 2 inches long and conveniently plug into a computer’s USB port. This makes them portable, enabling an easy transfer of files between desktop and laptop computers, and even between PCs and Apple computers. To the computer user, flash drives appear as another hard drive to which data can be written and read at high speeds. Because there are no moving parts, flash drives are very reliable and can sustain “bumps and bruises.” Storage capacities of up to 32 megabytes are common. Flash drives are also referred to as “travel drives” and “removable disks.”

CDs and DVDs

CDs and DVDs are optical storage media that are well suited for external storage, with DVDs storing up to 4.7 gigabytes. These discs are easy to read and write, and are very reliable, though scratches caused by mishandling can cause problems.

The Future Is Here

Now, even DVDs are being improved upon, with Blu-Ray versions increasing storage capacity and “light scribe” technology enabling users to utilize the laser in their DVD drives to etch labels on their discs.

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