Operating Systems Memory Management Quiz

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Questions and Answers

What is the primary purpose of memory protection in operating systems?

To enable unrestricted memory access for the OS

To prevent processes from impacting each other's address space (correct)

To increase the speed of memory access

To allow processes to access each other's memory freely

Which registers can only be modified by privileged instructions in kernel mode?

Instruction registers

Data registers

General-purpose registers

Base and limit registers (correct)

At what stage can address binding be delayed until process execution?

Compilation

Execution (correct)

Linking

What type of addresses do source programs contain before they are bound to memory addresses?

Symbolic addresses Signup and view all the answers

What happens if a process tries to access memory outside its address space?

An error is generated if the address exceeds the limit Signup and view all the answers

What is the main function of a linker in the process of address binding?

To bind relocatable addresses to absolute addresses Signup and view all the answers

Which of the following statements best describes a CPU stall?

It occurs when main memory requires multiple cycles Signup and view all the answers

What is the consequence of address binding during the compilation stage?

Absolute addresses are produced if the location is known Signup and view all the answers

What is the primary advantage of using paging in memory management?

It presents a contiguous address space to processes despite physical separation. Signup and view all the answers

How are the page tables used in the paging memory management technique?

To map virtual addresses to physical addresses for memory access. Signup and view all the answers

What aspect of fragmentation is eliminated by the paging technique?

External fragmentation. Signup and view all the answers

What happens when a process requests memory larger than one page?

It is allocated multiple pages, potentially resulting in internal fragmentation. Signup and view all the answers

What characteristics define the address generated by the CPU in a paging system?

Includes a virtual page number and an offset. Signup and view all the answers

What is a typical size for pages in modern paging systems?

4 or 8 KB. Signup and view all the answers

What occurs when a process requiring execution arrives in a system using paging?

The required number of frames is determined based on the process's request. Signup and view all the answers

What is the primary action taken after a page fault occurs when an instruction is fetched?

The instruction is refetched once the page is loaded. Signup and view all the answers

In the event of a page fault, what is the first step that the operating system takes?

Trap to the operating system. Signup and view all the answers

What must be done if a page fault occurs while writing a variable?

All previous steps involved in the instruction execution must be restarted. Signup and view all the answers

What is the purpose of the free-frame list in the context of demand paging?

To maintain a record of available memory frames. Signup and view all the answers

What happens to the user registers and process state when a page fault is triggered?

They are preserved for later restoration. Signup and view all the answers

During the page fault handling process, what occurs while waiting for the page to be read from the disk?

The CPU is allocated to another user. Signup and view all the answers

What technique is used to handle pages in demand paging when a fork() creates a new process?

Copy-on-write. Signup and view all the answers

What is an essential requirement for a page that is brought into memory after a page fault?

It requires a free frame in memory to fit the page. Signup and view all the answers

What is the main advantage of the copy-on-write technique during the fork operation?

It avoids copying all pages by sharing them until a write occurs. Signup and view all the answers

What action should the operating system take when no free frames are available for a new page?

Apply a page replacement algorithm to free up space. Signup and view all the answers

What is the purpose of the dirty bit in a page replacement scenario?

To mark when a page differs from its copy in backing storage. Signup and view all the answers

Which page-replacement algorithm is characterized by selecting the oldest page in memory?

First-In-First-Out (FIFO) Signup and view all the answers

What is the primary performance cost associated with the general page-replacement algorithm?

Two disk operations needed to swap in and out pages. Signup and view all the answers

What characteristic limits the Optimal (OPT) page-replacement algorithm's practical use?

It needs future knowledge to determine the best page to replace. Signup and view all the answers

When a page is considered a 'victim' during replacement, what happens to it?

It is copied to the backing storage if dirty. Signup and view all the answers

What is one of the major drawbacks of using standard swapping instead of page swapping?

It involves moving entire processes rather than just pages. Signup and view all the answers

What is the primary reason for implementing memory management in operating systems?

To manage the hardware resources efficiently Signup and view all the answers

Which of the following topics is introduced during Week 9 of the course?

Memory management Signup and view all the answers

Which reading material is primarily used for understanding memory management concepts in this course?

Operating System Concepts 10th edition Signup and view all the answers

What does the CPU primarily access directly in a computer system?

Main memory Signup and view all the answers

What is the purpose of paging in memory management?

To manage memory more efficiently Signup and view all the answers

Which process involves loading programs from disk to memory?

Memory mapping Signup and view all the answers

During which week do students review the module content in the course?

Week 11 Signup and view all the answers

What does virtual memory allow an operating system to do?

Use disk space as an extension of RAM Signup and view all the answers

What is the purpose of a valid bit in a page table?

It shows that the page is in the logical address space of the process. Signup and view all the answers

What does a page-table length register (PTLR) help to accomplish?

It records the size of the page table for effective memory management. Signup and view all the answers

How does hierarchical paging help in memory management?

It reduces the size of page tables by splitting them into multiple layers. Signup and view all the answers

Which type of code allows for multiple processes to read it simultaneously without modification?

Reentrant code Signup and view all the answers

What is the primary reason for using standard swapping in operating systems?

To reduce the physical memory limitation by swapping less critical processes to disk. Signup and view all the answers

What type of page table solutions are mentioned as alternatives for 64-bit architectures?

Hashed page tables and inverted page tables. Signup and view all the answers

What will happen if a process attempts to access a page marked as invalid?

An error will occur. Signup and view all the answers

Why is the concept of using shared pages beneficial?

It allows processes to execute with minimized memory overhead. Signup and view all the answers

Study Notes

COMP2211 Operating Systems: Memory Management

Course: COMP2211 Operating Systems
Instructor: Mantas Mikaitis
University: University of Leeds
Semester: 1, 2024
Week: 9, Lectures 15 and 16

Objectives

Discuss the need for memory management in operating systems.
Introduce the concept of paging.
Introduce virtual memory.

Reading List

Primary texts are Operating System Concepts (10th edition, 2018) and xv6 (4th edition, 2024).
Specific chapter assignments are provided for each week, and some chapters are to be re-read.

Part I: Description of the Problem

CPU can only directly access registers and main memory with cache in between.
Programs are loaded from disk to memory in a process' memory structure.
CPU requests either address and read requests or address, data, and write requests.
Register access takes 1 clock cycle.
Main memory (e.g., LD/ST instructions) takes multiple cycles, causing stalls in the CPU.
Cache improves performance by reducing stall times.

Introduction

Illustrates a hierarchical memory structure with decreasing access times in a descending order starting with registers and cache, then followed by main memory, nonvolatile memory and secondary storage, then tertiary storage like hard-disk drives, optical disks, and magnetic tapes.

Memory Protection

Processes should only access their address space to prevent them to impact each other (or the operating system).
Each process' memory is limited by a limit register.
Addresses must fall within the base and limit range.
Error occurs if the address goes beyond base+limit range

Base and Limit Registers

These registers can only be accessed by privileged instructions in kernel mode.
Only the OS can modify these registers to protect users from modifying the size of the address space.

Address Binding

Programs on disk must be moved into memory for execution.
Addresses are represented before the placement decision is made.
Source programs have symbolic addresses, which the compiler binds to relocatable addresses relative to a reference address.
The linker or loader binds the relocatable addresses to absolute addresses.
Address binding can occur at compilation time, loading time, or execution time.
Execution time binding is the most common implementation in modern operating systems.

Logical and Physical Address Spaces

Logical address is generated by the CPU.
Physical address is used by the memory unit.
Logical and physical address spaces are separate.
Compile-time and load-time binding result in equivalent logical and physical addresses.
Execution-time binding separates logical and physical spaces.

Memory-Management Unit (MMU)

The MMU uses a relocation register (base register) to translate virtual addresses to physical addresses.
User processes never see the actual physical addresses.
The MMU performs execution-time address binding during memory accesses.

Dynamic Loading

Entire programs do not need to be in memory during execution.
Only parts of a program are loaded when needed.
Improved memory usage by delaying loading of unused code and data.
Programs are stored as relocatable load format on disk.

Dynamic Linking

System libraries and program code are combined into a binary executable at execution-time.
Commonly used with system libraries to load them only when needed.
Allows sharing system libraries among processes (DLLs or shared libraries)
Helps versioning as updated libraries can be used without recompiling programs.

Part II: Contiguous Memory Allocation

OS and processes share memory, requiring contiguous allocation for concurrent execution.
Memory is partitioned into two areas for operating system and processes.
Processes are stored as single, contiguous blocks in memory.
Protection is needed to prevent processes from interfering with each other or the operating system.

Memory Allocation

Track free and occupied partitions.
Initially, all memory is a single free block.
Allocate variable-size partitions as needed.
Memory holes of variable sizes form when processes exit.

Memory Allocation Methods (e.g., first-fit, best-fit, worst-fit)

First-fit: allocate the first available block large enough to satisfy the request.
Best-fit: allocate the smallest block that satisfies the request.
Worst-fit: allocate the largest block that satisfies the request.

Internal Fragmentation

Extra, unused space within an allocated block, arises from fixed partition sizes

External Fragmentation

Memory space not contiguous for satisfying a memory request because of the existence of small, unused memory blocks.

Reducing External Fragmentation (Compaction)

Shuffle memory contents to merge free holes into one large contiguous area.
Requires dynamic relocation and updating relocation registers.

Part III: Paging

Previous methods require contiguous memory, leading to fragmentation.
Paging solves this by allowing processes to view memory contiguously despite fragmented physical memory.
The MMU and OS collaborate to provide this method.

Basic Paging Method

Divide physical memory into fixed-size blocks called frames.
Divide virtual memory into fixed-size blocks called pages.
Allocate frames to map pages to physical memory locations.
Page tables map virtual addresses to physical addresses.
CPU generates page number(p) and offset(d) to access required memory location. -Entry p in page table contains a base address of the corresponding frame.
Offset d locates the specific memory address within the frame.

Hardware Support

MMU performs translation of logical addresses to physical addresses using page tables stored in memory.
The MMU utilizes page tables to perform translations more efficiently.

Paging Example

Demonstrates mapping of a process's logical memory to physical memory frames.
Illustrates the mapping process in a table format.

Page sizes

Pages are commonly 4 or 8 KB in size.
Some systems offer options for larger page sizes known as huge pages.

Frame allocation for Paging

Process arrival with specific page size requirements.
Allocate frames.
Assign frame numbers to pages using the page table.

Paging Implementation

Create page table for each process.
Process Control Block (PCB) stores the page table address.
Hardware page tables use high-speed registers.

Translation Lookaside Buffers (TLBs)

Fast associative memory to store page mappings between virtual and physical addresses.
TLBs speed up address translation, checking for existing mappings before accessing the page table.

Memory Protection with Pages

Page tables include read/write restrictions.
A valid bit indicates the page is in the process' logical address space.
An invalid bit indicates the page is outside of the logical address space.

Protection with Pages: Page-table Length Register (PTLR)

PTLR stores page table size for memory protection.

Shared Pages

Paging allows efficient code sharing between processes.
Reentrant code does not modify itself.
Processes can share pages for the same code.

Hierarchical Paging

Split large page tables into multiple levels to reduce size.
This arrangement is typical for large address spaces.
It uses multiple levels of page tables for improved efficiency.

Hierarchical Paging Address Translation

Illustrates a two-level hierarchical page table structure for 64-bit architectures.
Shows address resolution using multiple page tables.

Part IV: Memory Swapping

Swapping copies processes between main memory and backing store (disk) when memory is full.
Provides more RAM appearance.
Swapping allows process continuity while not simultaneously residing in main memory.

Swapping with Paging

Processes can be swapped between memory and backing store at the page level.
The OS manages page swapping to maintain efficient system operation.

Swapping in mobile systems

Mobile operating systems (e.g., iOS) might not support page swapping due to flash memory limitations.
Limitations include space limitations and flash memory degradation.

Part V: Virtual Memory

Virtual memory extends logical address space beyond physical memory size.
Combines physical memory and backing store.
Creates the illusion of a larger memory space.

Virtual Memory: Introduction

Memory management that makes the logical memory seem larger than the physical memory. Instructions are stored and retrieved as needed from backing storage as requested by processes.
Programs have a larger logical view of memory compared to the physical memory size.
Increased program size, more programs running simultaneously, and less I/O are benefits. Memory is not necessarily contiguous but seen as such by processes.

Virtual Memory and the Heap

Heap grows upwards.
Stacks grow downwards.
Memory holes occur in the space between the stack and heap.

Virtual Memory and Shared Pages

Processes have shared memory in their virtual spaces.
Shared pages are present identically in all participating processes’ physical memory region.

Part VI: Page Replacement

The OS handles page replacement when no free frames are available for the page.
Processes are terminated.
Swapping copies entire processes into memory to free up frames.

General Page Replacement Algorithm

Find desired page location in backing storage.
Identify a victim page.
Copy victim's contents to backing storage.
Update page and frame tables.
Fill free frame with desired page.

Page-replacement Algorithms

FIFO (First-In, First-Out), OPT (Optimal), and LRU (Least Recently Used)

Demand Paging

Strategy to load (bring in) pages into physical memory just when they are needed.
Helps manage memory effectively. Improves efficiency because it loads pages of a program only when needed.

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Description

Test your knowledge on memory protection, address binding, and paging in operating systems. This quiz covers key concepts such as CPU stalls, memory access violations, and the role of the linker. Great for students and professionals looking to solidify their understanding of memory management techniques!