Hard Disk or secondary storage

Till now we have understood two hardware components – CPU and RAM. CPU does the processing of data and instructions and RAM is the place from where it reads which instructions and data to process. The next question to ask is from where data comes into RAM.

This is where the hard disk comes into picture. As discussed in the previous tutorial RAM is a temporary storage and remember its contents only till the computer is on. But we need a way to store the data permanently so that when we turn off our computer the data is not lost. For this a hard disk is added to the computer. It is a permanent storage device which is the main source of all the data in the computer.

This is how a hard disk looks in real

Image 

Differences between RAM and hard disk

Following are the important differences between RAM and hard disk

RAM Hard Disk
It is an electronic device It is an electronic + mechanical device. There is a magnetic disk which is rotated by a motor.
Data is stored in the form of charge Data is stored in the form of magnetic signals
It is a temporary storage device. All data is lost once the power to the computer is turned off It is a permanent storage device. Data is remembered even when the power to the computer is turned off
The data read and write speed of the RAM is measured in MHz The data read and write speed of hard disk is measured in RPM’s (Rotations Per Minute)
It is a random read write device. It is a sequential read write device
RAM is logically divided into memory blocks of size 1 byte and each memory block is given a unique numeric address Hard disk is logically divided into cylinders and sectors and each sector is identified by its address
It is also called the primary memory It is also called the secondary memory


Relation between CPU, RAM and Hard disk

Having understood the differences between RAM and hard disk let’s try to understand how CPU, RAM and hard disk interact with each other in order to make the computer function.

CPU’s job is to process the data and instructions

CPU reads its data and instructions from RAM

RAM gets its data from the hard disk

Diagram

From the above statements and diagram you can very well understand how the data travels from the hard disk -> RAM -> CPU. It is very interesting to know why this happens. The major reason behind this is the difference in the operating speeds of CPU, RAM and hard disk. To give you a rough estimate if the data read write speed of the hard disk is X then the processing speed of CPU is 2400000000X. You can well imagine how difficult it will be for the CPU to talk directly with the hard disk in order to read write data. On the other hand if the data read write speed of a RAM is X then the processing speed of CPU as compared to RAM is around 4X. So the CPU is happier interacting with the RAM instead of interacting with the hard disk.

What is stored on the hard disk?

The important data which is stored on the hard disk includes

  1. The operating system files
  2. Your personal files and folders
  3. You movies and songs
  4. Your office documents like word files, excel files and presentations

If due to some reason your hard disk gets destroyed all your data will be lost.

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