16.1 Purposes of an Operating System

An operating system (OS) is a fundamental piece of software that manages computer hardware and software resources. It provides a platform for applications to run and acts as an intermediary between the user and the hardware. This section explores the key purposes of an operating system, with a focus on memory management techniques like virtual memory, paging, and segmentation.

Core Purposes of an Operating System

Resource Management

The OS is responsible for managing the computer's resources, including:

• CPU Management: Allocating CPU time to different processes, scheduling processes for execution.
• Memory Management: Allocating and deallocating memory to processes, managing virtual memory.
• Input/Output (I/O) Management: Controlling and coordinating I/O devices.
• File Management: Organizing and managing files and directories on storage devices.

User Interface

The OS provides a user interface (UI) that allows users to interact with the computer. This can be a graphical user interface (GUI) or a command-line interface (CLI).

Process Management

The OS creates, manages, and terminates processes. It ensures that processes can run concurrently and that they do not interfere with each other.

Security and Protection

The OS provides security features to protect the system from unauthorized access and malicious software. This includes user authentication, access control, and protection mechanisms.

Memory Management: Virtual Memory, Paging, and Segmentation

Memory management is a crucial function of the OS. It involves allocating memory to processes and ensuring that processes do not interfere with each other's memory spaces. Modern operating systems heavily rely on virtual memory to extend the available memory beyond the physical RAM.

Virtual Memory

Virtual memory is a memory management technique that gives processes the illusion of having more memory than is physically available. It uses a combination of RAM and disk space (swap space) to create a larger address space for each process.

When a process tries to access a memory location that is not currently in RAM, the OS brings the required data from the disk (swapping) into RAM. This process is called swapping.

Paging

Paging is a memory management scheme that divides both physical memory and the logical address space of a process into fixed-size blocks called pages and frames, respectively.

Concept	Description
Logical Address Space	The address space seen by the process.
Physical Address Space	The actual physical memory.
Page	A fixed-size block of the logical address space.
Frame	A fixed-size block of the physical address space.
Page Table	A data structure that maps logical pages to physical frames.

The page table is used to translate logical addresses to physical addresses. If a page is not in RAM, a page fault occurs, and the OS loads the page from disk.

Segmentation

Segmentation is another memory management technique that divides the logical address space of a process into logical units called segments. Each segment represents a distinct part of the program, such as code, data, and stack.

Concept	Description
Segment	A logical unit of the address space (e.g., code, data, stack).
Segment Table	A data structure that maps segments to physical memory locations.

Segmentation allows for non-contiguous allocation of memory, which can be more efficient than paging in some cases. However, it can also lead to external fragmentation.

Comparison of Paging and Segmentation

Feature	Paging	Segmentation
Memory Allocation	Fixed-size pages	Variable-size segments
Fragmentation	Internal fragmentation (within a page)	External fragmentation (between segments)
Complexity	Simpler to implement	More complex to implement

In summary, the operating system plays a vital role in managing computer resources and providing a platform for applications. Virtual memory, paging, and segmentation are key memory management techniques that enable modern operating systems to efficiently utilize memory and support multitasking.