OS Memory Management Introduction

What is Memory?

Computer memory can be defined as a collection of some data represented in the binary format. On the basis of various functions, memory can be classified into various categories. We will discuss each one of them later in detail.

A computer device that is capable to store any information or data temporally or permanently, is called storage device.

How Data is being stored in a computer system?

In order to understand memory management, we have to make everything clear about how data is being stored in a computer system.

Machine understands only binary language that is 0 or 1. Computer converts every data into binary language first and then stores it into the memory.

That means if we have a program line written as int α = 10 then the computer converts it into the binary language and then store it into the memory blocks.

The representation of inti = 10 is shown below.

The binary representation of 10 is 1010. Here, we are considering 32 bit system therefore, the size of int is 2 bytes i.e. 16 bit. 1 memory block stores 1 bit. If we are using signed integer then the most significant bit in the memory array is always a signed bit.

Signed bit value 0 represents positive integer while 1 represents negative integer. Here, the range of values that can be stored using the memory array is -32768 to +32767.

well, we can enlarge this range by using unsigned int. in that case, the bit which is now storing the sign will also store the bit value and therefore the range will be 0 to 65,535.

Need for Multi programming

However, The CPU can directly access the main memory, Registers and cache of the system. The program always executes in main memory. The size of main memory affects degree of Multi programming to most of the extant. If the size of the main memory is larger than CPU can load more processes in the main memory at the same time and therefore will increase degree of Multi programming as well as CPU utilization.

Let's consider, 
Process Size = 4 MB
Main memory size = 4 MB 
The process can only reside in the main memory at any time. 
If the time for which the process does IO is P, 

Then, 

CPU utilization = (1-P) 
let's say, 
P = 70% 
CPU utilization = 30 % 
Now, increase the memory size, Let's say it is 8 MB. 
Process Size = 4 MB 
Two processes can reside in the main memory at the same time. 
Let's say the time for which, one process does its IO is P, 

Then

CPU utilization = (1-P^2) 
let's say P = 70 % 
CPU utilization = (1-0.49) =0.51 = 51 % 

Therefore, we can state that the CPU utilization will be increased if the memory size gets increased.