Operating System

🖥️ Operating System (OS)

: A program that runs in the kernel space of memory. Its primary role is to manage and allocate available system resources to user applications

🧩 Resources

: Resources are essential components required for running programs. They include:

• CPU
• Memory
• Secondary memory
• I/O devices

👤 User Interface

The OS provides user interfaces(not part of the kernel space) to interace with the system

• GUI(Graphic User Interface)
• CLI(Command Line Interface)

⚙️ Dual Mode: User Mode & Kernel Mode

To protect critical hardware access and system functions, modern computers operates in two modes:

• Kernel Mode
  : The OS kernel runs here and has full access to the hardware

  System call: software interrupt occured by applications to run interrupt code in kernel space

• User Mode: Applications run in this mode. They can't access directly hardware or priviledged instructions
  💡 When a program needs system-level access(e.g. I/O, memory), it uses a system call, which triggers a software interrupt to transition into kernel mode.
  

📋 Main Roles of the Operating System

1. Resource Management & Allocation

• CPU: Uses scheduling algorithms in the kernel to assign CPU time to processes
• Memory: Allocate sapce in RAM for running programs
• I/O devices: Handles requests via Interrupt service routine

2. File system

🖥️ Virtual Machine & Hypervisor Mode

• A Virtual machine(VM) is a program that runs in user mode, but it has its own operating system (guest OS)
• VMs run in user mode, so they can't access priviledged instructions directly
• To solve this, CPUs introduces a special mode called Hypervisor Mode
• Hosted Hypervisor: placed between the VM and the host OS
  The hypervisor intercepts any system-level requests from the VM and either:
  - Directly accesses hardware (in bare-metal hypervisors), or
  - Requests the host OS to perform the priviledged operation