Memory Hierarchy in Computer

The processor is the “brain” of the computer where all the essential computing takes place. But unlike a human brain, a computer processor has very limited memory. Thus, it has to rely on other kinds of memories to hold data and instructions and to store results. The memory In a computer system is of three fundamental types which are given below-

(1) Internal Processor Memory :

This memory is placed within the CPU processor or is attached to a special fast bus. Internal memory usually cache memory and special registers, both of which can be directly accessed by the processor. This memory is used for temporary storage of data and instructions on which the CPU is currently working. Processor memory is the fastest among all the memories but is the most expensive also. Therefore a very diminutive part of internal processor memory is used in the computer system. It is generally used to compensate for the speed gap between the primary memory and the processor.

(2) Primary Memory :

Random Access Memory (RAM) and Read Only Memory (ROM) fall under the category of the primary memory, also known as main memory. Every computer comes with a small amount of ROM, which contains the boot firmware (called BIOS). This holds enough information to enable the computer to check its hardware and load its operating system into RAM at the time of system booting. RAM is the place where the computer temporarily stores its operating system, application programs and current data so that the computer’s processor can reach them quickly and easily. It is volatile in nature, that is, when the power is switched off, the data in this memory in lost. Unlike RAM, ROM is non-volatile. Even when the computer is switched off the contents of the ROM remain available.

(3) Secondary Memory

This memory is also known as auxiliary memory, secondary memory provides backup storage for instructions (computer programs) and data. The most commonly used secondary storage devices are magnetic tapes and magnetic disc. These are the least expensive as well as have much larger storage capacity than primary memory. The instructions and data stored on secondary storage devices are permanent in nature. They can only be removed if the user wants it so or the device is destroyed. Secondary memory can also be used as overflow memory, when the capacity of main memory is surpassed.

Unlike processor memory and main memory, secondary memory is not directly accessible to the processor. Firstly, the data and instructions from secondary memory have to be shifted to main memory and then it to the processor.

Primary memory is of two types which are given below-

(I) Random Access Memory (RAM)

Random access memory is like the computer’s scratch pad. It allows the computer to store data for immediate manipulation and to keep track of what is currently being processed. It is the place in a computer where the operating system, application programs and data in current use are kept so that they can be accessed quickly by the computers processor. RAM is much faster to read from and write to than the other kinds of storage in a computer like the hard disk or floppy disk. However, the data in RAM stays there only as long as the computer is running. When the computer is turned off, RAM loses all its contents. When the computer is turned on again the operating system and other files are once again loaded into RAM. When an application program is started, the computer loads it into RAM and does all the processing there. This allows the computer to run the application faster. Any new information that is created is kept in RAM an since RAM is volatile in nature, one needs to continuously save the new information to the hard disk.

(II) Read Only Memory (ROM)

Just as a human being needs instructions from the brain to perform actions in certain event, a computer also needs special instructions every time it is started. This is required because during the startup operation, the main memory of the computer is empty due to its volatile property so there has to be some instructions (special boot programs) stored in a special chip that could enable the computer system to perform startup operations and transfer the control to the operating system. This special chip, where the startup instructions are stored, is called ROM. It is non-volatile in nature, that is, its contents are not lost when the computer is switched off. The data and instructions stored in ROM can only be read and used but cannot be altered thereby making ROM much safer and secure than RAM. ROM chips are used not only in the computer but also in other electronic items like washing machine and microwave oven.

What are the difference between RAM and ROM ?

Some important differences in RAM and ROM are given below-