What are System Calls?

System calls provide the interface between a running program and the operating system. They are the primary way a user-level program requests a service from the OS kernel.

How System Calls Work

When a program needs a service (like reading a file or creating a process), it executes a specific instruction that triggers a trap to the kernel. This switches the CPU from user mode to kernel mode.

Types of System Calls:

1. Process Control: end, abort, load, execute, create process, terminate process.
2. File Management: create file, delete file, open, close, read, write.
3. Device Management: request device, release device, read, write, get/set device attributes.
4. Information Maintenance: get time or date, get/set system data.
5. Communications: create/delete communication connection, send/receive messages.

Operating System Structures

The design and implementation of operating systems vary significantly. Over the years, several architectural approaches have been developed.

1. Simple / Monolithic Structure

In a monolithic system, the entire OS runs as a single, large program in kernel mode. All services are bundled together.

• Example: Early versions of MS-DOS and original UNIX.
• Pros: High performance due to low overhead in communication.
• Cons: Difficult to debug, maintain, and extend. A crash in any service can bring down the entire system.

2. Layered Approach

The OS is broken down into a number of layers (levels), each built on top of lower layers. The bottom layer (layer 0) is the hardware; the highest (layer N) is the user interface.

• Rule: A layer can only use functions and services of lower-level layers.
• Pros: Simplicity of debugging and system verification.
• Cons: Performance can be slow due to the overhead of passing through multiple layers.

3. Microkernel Structure

This method structures the OS by removing all non-essential components from the kernel and implementing them as system and user-level programs.

• Kernel Role: Provides minimal services like communication (IPC), memory management, and CPU scheduling.
• Example: Mach (used in macOS), QNX.
• Pros: High reliability and security. If a service (like a file server) fails, the rest of the OS remains unaffected.
• Cons: Performance overhead due to increased system-function communication (message passing).

4. Modules (Hybrid Approach)

Most modern operating systems (like Linux and Windows) use a hybrid approach. They use a monolithic kernel but have Loadable Kernel Modules (LKMs) that can be added or removed while the system is running.