Operating System Overview: Definitions, Functions, Structures, and System Calls

Introduction to Operating Systems

An Operating System (OS) is a program that manages the computer hardware. It acts as an intermediary between the user of a computer and the computer hardware. The primary goals of an OS are to provide an environment in which a user can execute programs in a convenient and efficient manner and to allocate resources (CPU time, memory, I/O devices) fairly among competing processes.

Functions of an OS

1. Resource Management: Acts as a resource allocator, deciding between conflicting requests for efficient resource use.
2. Control Program: Manages the execution of user programs to prevent errors and improper use of the computer.
3. Abstraction: Provides a virtual machine interface that hides the complexities of hardware from the user.

System Calls

A system call is a programmatic way in which a computer program requests a service from the kernel of the operating system. It is the interface between a process and the OS.

How System Calls Work

1. User Mode: The application runs in user mode. When a system call is needed, the process executes a "trap" or "software interrupt."
2. Mode Switch: The CPU switches from user mode to kernel mode.
3. Execution: The kernel examines the system call number, executes the corresponding kernel function, and returns the result.
4. Return: The CPU switches back to user mode, and the application resumes execution.

Common System Call Categories:

• Process Control: fork(), exit(), wait()
• File Management: open(), read(), write(), close()
• Device Management: ioctl(), read(), write()
• Information Maintenance: getpid(), alarm(), sleep()

The Shell

The shell is a command-line interpreter that provides a user interface to the OS. It reads commands from the user and executes them. When a user types a command (e.g., ls or dir), the shell:

1. Parses the command string.
2. Searches for the executable file.
3. Uses a system call (like exec() in Unix) to start the process.
4. Waits for the process to complete or runs it in the background.

Comparison: User Mode vs. Kernel Mode

Computing Environments

• Batch Systems: Jobs are grouped together and processed sequentially without user interaction.
• Multiprogramming: Keeps multiple jobs in memory so the CPU always has one to execute, increasing CPU utilization.
• Time-Sharing (Multitasking): The CPU switches between jobs so frequently that users can interact with each program while it is running.
• Distributed Systems: A collection of physically separate, heterogeneous computer systems networked to provide users with access to various resources.

Worked Example: System Call Flow

Consider a process attempting to read a file:

1. Application: Calls read(fd, buffer, size).
2. Library: The standard C library places the system call number in a specific CPU register.
3. Trap: The library executes a syscall instruction.
4. Kernel: The OS kernel receives the trap, validates the file descriptor (fd), performs the disk I/O, and copies data to the buffer.
5. Return: The kernel clears the trap and returns control to the user process with the number of bytes read.

Exam Tip

In TU exams, questions on this unit often focus on the conceptual flow of system calls and the distinction between OS types.

• When asked to "Define shell and system call," always mention that the shell is the interface and the system call is the mechanism for requesting services.
• If asked about "System Call Handling," draw a simple block diagram showing: User Program -> System Call Interface -> Kernel -> Hardware.
• Be prepared to explain how multiprogramming increases CPU utilization by overlapping I/O operations with CPU processing.