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16.1 Purposes of an Operating System (OS) (3)

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1.

Describe the principles of paging and segmentation as memory management techniques. Compare and contrast paging and segmentation, highlighting their advantages and disadvantages. Include a discussion of how page tables and segment tables are used to manage memory in each technique.

Paging divides both physical and logical memory into fixed-size blocks called pages (logical) and frames (physical). A page table maps logical pages to physical frames. Each process has its own page table. The MMU uses the page table to translate logical addresses to physical addresses. A page hit occurs when the required page is in RAM; a page fault occurs when it's not.

Segmentation divides logical memory into variable-size blocks called segments. Each segment represents a logical unit of the program, such as code, data, or stack. A segment table maps segments to physical memory locations. The segment table contains information about the base address and limit of each segment. The MMU uses the segment table to translate logical addresses to physical addresses. The address is divided into a segment number and an offset within the segment.

Comparison:

Feature | Paging | Segmentation

Memory Allocation | Fixed-size pages | Variable-size segments

Fragmentation | Internal fragmentation (unused space within a page) | External fragmentation (unused space between allocated segments)

Protection | Page table entries can specify access permissions | Segment table entries can specify access permissions

Complexity | Relatively simple | More complex

Advantages of Paging:

Eliminates external fragmentation.

Simple memory allocation.

Disadvantages of Paging:

Internal fragmentation can lead to wasted memory.

Advantages of Segmentation:

Logical organization of memory reflects program structure.

Facilitates sharing of code and data.

Disadvantages of Segmentation:

External fragmentation can lead to wasted memory.

More complex memory management.

2.

Describe the purpose of virtual memory and explain how it allows an operating system to maximise resource utilisation. Include in your answer a discussion of page tables and the concept of swapping.

Virtual memory is a memory management technique that gives processes the illusion of having more memory than is physically available. It allows the OS to run programs that are larger than the available RAM. This is achieved by using a combination of physical memory (RAM) and secondary storage (e.g., hard disk).

How Virtual Memory Maximises Resource Utilisation

Virtual memory allows multiple processes to share the same physical memory. This reduces the overall memory footprint of the system. Furthermore, it enables processes to access memory locations that are not currently in RAM, storing them on disk. This increases the effective memory available to each process.

Page Tables

A page table is a data structure used by the OS to map virtual addresses (used by the process) to physical addresses (in RAM). Each process has its own page table. The page table contains entries that indicate whether a page is in RAM, on disk, or invalid. If a page is on disk, a page fault occurs, and the OS retrieves the page from disk and loads it into RAM.

Swapping

Swapping is the process of moving pages between RAM and disk. When RAM is full, the OS selects pages to be swapped out to disk, making space for new pages. The selection of pages to swap out is based on various algorithms (e.g., Least Recently Used - LRU). Swapping can significantly impact performance due to the slow access time of disk compared to RAM. However, it allows the system to run more processes than would otherwise be possible.

Table: Page Table Entry

Page Number

Present in RAM? (Yes/No)

Physical Frame Number

Trade-offs: While virtual memory increases resource utilisation by allowing larger processes and enabling multitasking, swapping can lead to performance degradation if excessive swapping occurs (thrashing). The OS must carefully manage the swapping process to minimise this impact.

3.

Question 2

Explain the difference between preemptive and non-preemptive scheduling. Discuss the advantages and disadvantages of each approach, considering factors such as response time and CPU utilization. (10 marks)

Preemptive Scheduling: In preemptive scheduling, the operating system can interrupt a running process and switch to another process, even if the running process has not finished its current time slice. This is typically done when a higher-priority process becomes ready to run or when a time slice expires.

Non-Preemptive Scheduling: In non-preemptive scheduling, a process runs until it voluntarily releases the CPU (e.g., by completing its execution or by making a system call that causes it to block). The OS will not interrupt the process.

Advantages and Disadvantages:

Preemptive:
- Advantages: Lower response time (especially for high-priority processes), prevents starvation of high-priority processes, better CPU utilization (can switch to a ready process if the current one is blocked).
- Disadvantages: Overhead associated with context switching (can reduce overall throughput), potential for thrashing (if the system is overloaded with processes).

Non-Preemptive:
- Advantages: Lower overhead (no context switching), simpler to implement.
- Disadvantages: Higher response time (especially for low-priority processes), potential for starvation of low-priority processes, potentially lower CPU utilization (if a long-running process blocks other processes).