Operating System Structures

Types of Operating Systems

Definition: Placing multiple programs in the main memory to utilize the CPU efficiently.
Main Memory Division:
- Operating System
- User Programs
Example:
- Assumptions: Programs P1, P2, P3 in main memory.
- Execution Process:
  - CPU begins executing P1.
  - If P1 needs I/O operation, CPU becomes idle.
  - Operating system allocates CPU to another process (P2).
  - Repeat the above steps if P2 needs I/O operation.
  - Allocate CPU to P3.
Process: A program under execution.
Efficiency: Executing processes concurrently to utilize CPU effectively.
Speed: CPU is faster than main memory; compensating speed difference by keeping more programs in memory.
Memory Limitation: Only currently executing programs are kept in main memory due to its smaller size compared to the hard disk.
Advantage: Improved CPU performance by keeping multiple programs in main memory.

Definition: Extension of multi-programming where CPU time is shared among available processes.
CPU Time Sharing:
- CPU time is divided into time slots allocated to each process.
- Example:
  - Processes P1, P2, P3 in main memory.
  - P1 needs 4ms, P2 needs 6ms, P3 needs 8ms for execution.
  - Operating system allocates 2ms time slot to each process initially.
  - P1 executes for 2ms, remaining 2ms.
  - P2 executes for 2ms, remaining 4ms.
  - P3 executes for 2ms, remaining 6ms.
  - The CPU then cycles back to P1 and repeats.
Frequent Switching: CPU frequently switches between processes, enhancing interactivity.
User Perspective: Appears as if their process is being executed constantly.

Multi-programming and Time-sharing systems aim to optimize CPU utilization.
Both types involve managing multiple processes but differ in how CPU time is allocated.
Multi-programming focuses on keeping the CPU busy by loading several programs into memory.
Time-sharing ensures fair CPU time distribution among processes, thus improving user interactivity.