The software that controls internal computer operations ( reading data From input devices transmitting, processed information to the output devices, checking system components, converting data/instruction to computer understandable form etc. )
But before you can run any Application software on your system, The OS which contains an instruction that coordinate the activities among computer hardware devices, so, first of all, you need to load hard disk into computer’s memory.
There can be two subcategories of this OS type:
Single Task operating system software:-
This OS supports/runs single application at a time e.g., OS for mobiles, MS-DOS etc.
Each active user program get a fair share of CPU time,if the time elapses or an I/O operation requested, CPU shifts over to the next job waiting and the previous program is put to wait (in case Time is over ) or handed over to I/O manager (in case I/O request is made).