Operating Systems: Virtual Memory Concepts

Podcast

Play an AI-generated podcast conversation about this lesson

Questions and Answers

What is the main advantage of using mmap in relation to page tables?

It maps files directly to process virtual address space. (correct)

It creates multiple copies of the same process.

It allows for slower memory access.

It disables sharing of physical memory.

How many memory lookups are needed for each memory access in a system using page tables?

One lookup to the page table.

Three lookups to find both the physical and virtual addresses.

Two lookups: one for the page table entry and one for the physical address. (correct)

No lookups are needed for memory access.

What is the relationship between address sizes and the addressable space?

Address sizes are not related to the size of addressable space.

Larger address sizes decrease addressable space.

Smaller address sizes result in larger addressable space.

Address sizes determine the limit of the addressable space directly. (correct)

Which formula correctly indicates the physical address composition?

Physical Address = PPN + Offset Signup and view all the answers

What is the benefit of allowing multiple processes to share the same physical memory?

It reduces memory redundancy and increases efficiency. Signup and view all the answers

What does virtualization provide each process in terms of resources?

The illusion of having its own separate resource Signup and view all the answers

How does the operating system virtualize the CPU?

By using time sharing and scheduling techniques Signup and view all the answers

What is the purpose of the Memory Management Unit (MMU) in virtual memory?

To translate virtual addresses to physical addresses Signup and view all the answers

What does each process's view of memory represent?

The unique address space for that process Signup and view all the answers

What is the starting point of a process's virtual address space referred to as?

Base Address Signup and view all the answers

What can occur if two processes are allocated overlapping address ranges?

Data loss due to overwriting Signup and view all the answers

What is a primary challenge when trying to support multiple process address spaces?

Creating non-overlapping address regions Signup and view all the answers

What happens when a context switch occurs during CPU processing?

The current process's state is saved and the new process begins execution Signup and view all the answers

What is the purpose of paging in memory management?

To allow for flexible memory allocation in fixed-sized chunks. Signup and view all the answers

What mechanism is used to map virtual page numbers (VPN) to physical page numbers (PPN)?

The page table. Signup and view all the answers

How does demand paging work?

Physical memory is allocated only when the pages are accessed. Signup and view all the answers

What determines the location of a byte within a physical page?

The offset bits. Signup and view all the answers

What size virtual addresses are required for a 64-bit address space with 16-byte pages?

6 bits Signup and view all the answers

How are physical addresses represented when using a 128-byte physical memory space?

7 bits Signup and view all the answers

What role does the Memory Management Unit (MMU) play in address translation?

It translates virtual page numbers into physical page numbers. Signup and view all the answers

Which of the following is NOT a characteristic of a page table?

It is managed by the hardware. Signup and view all the answers

What does the Resident bit in a page table entry indicate?

It indicates if the page is present in physical memory. Signup and view all the answers

When a page fault occurs, what is the first action taken by the MMU?

It invokes the OS page fault handler. Signup and view all the answers

What is the purpose of the dirty bit in a page table entry?

To show if the page has been modified in memory. Signup and view all the answers

In a system with limited memory, what strategy can help manage memory efficiently?

Allocating pages only on demand. Signup and view all the answers

Which page replacement policy is mentioned as being applied in the discussed system?

LRU (Least Recently Used) Signup and view all the answers

If a page is not present in physical memory and is needed for access, what occurs?

A page fault occurs. Signup and view all the answers

What does the OS do when it runs a new process regarding the BASE and BOUND registers?

It saves the current BOUND register before changing it. Signup and view all the answers

What does the term 'locality' refer to in the context of memory management?

The tendency of processes to access a localized set of pages. Signup and view all the answers

What is the role of the free list in memory management?

It helps find free physical memory space for new processes. Signup and view all the answers

In a virtual memory system, what is the purpose of mapping a Virtual Page Number (VPN) to a Physical Page Number (PPN)?

To manage memory efficiently and provide abstraction. Signup and view all the answers

What is one benefit of dynamic relocation in memory management?

It allows the OS to optimize the free space in memory. Signup and view all the answers

How does the MMU contribute to the property of isolated address spaces?

It enforces hardware-level memory protection during address translation. Signup and view all the answers

What is a key characteristic of internal fragmentation in memory systems?

It occurs when allocated memory is not fully utilized. Signup and view all the answers

What is the proposed solution to internal fragmentation in memory management?

Utilize segmentation to allocate regions of memory separately. Signup and view all the answers

What problem does the OS encounter when managing physical memory for processes?

There can be overhead due to memory translation. Signup and view all the answers

What happens when a process accesses an address beyond its BOUND register?

The memory access is denied, and an exception is triggered. Signup and view all the answers

Study Notes

Operating System Part Two: Virtual Memory

Virtual memory is a core concept in operating systems.
It gives the illusion that each process has its own dedicated memory space.
This is achieved by dividing physical memory into fixed-sized chunks called "pages."
Virtual memory pages are mapped to physical memory pages.

Virtualization

Virtualization allows each process to have its own illusion of a separate, non-shared device.
It means a physical shared device is given to each process.
This results in each process believing they have their own device, when in fact they are sharing just one.

Virtualization of the CPU:

The operating system virtualizes a CPU via time-sharing and scheduling.
Time slicing is used to give each process a small slice of CPU time.
Each process gets time on the CPU.

Virtualization of Memory

The operating system gives each process the illusion of its own address space using memory virtualization.
Each process's view of memory is isolated from other processes.
Processes can't access another process's memory.

Physical Memory

Physical memory is composed of memory devices directly connected to the CPU through address and data buses.
It is organized as an array of byte-sized locations.
Each location can hold a single 8-bit value.
The number of locations is proportional to the installed physical RAM units.
Physical address space is the range of locations in physical memory.

Process's View of Memory

Processes have their own address space, ranging from 0x00 to 0xFF.
This depends on the size.
All assembly instructions a process uses are virtual addresses, meaning they are not the actual physical addresses.

The Problem with Non-overlapping Memory Regions

Attempts to place processes into non-overlapping memory regions to support multiple processes with a single physical memory fail because this leads to memory fragmentation.
Physical memory is not fully used by processes.
There is wasted space between processes.
Processes need to be non-overlapping.

Address Translation Using MMU

MMU (Memory Management Unit) is a hardware component.
The MMU translates virtual addresses to physical addresses.
The MMU converts a process's virtual address into a corresponding physical address to access data.
Virtual address = physical address + base
The MMU verifies that the translation is within the boundaries of the allocated address space.

OS Management of Address translation

The operating system maintains registers.
The base register stores a starting address for a process’s memory.
The bound register stores the size of a process’s memory.
When a process accesses memory, the MMU calculates the physical address.
If the address is within the bounds, the MMU allows access; otherwise, it's denied.

Free Space Management

The operating system maintains a free list to track available physical memory.
Dynamic relocation is a technique that moves processes’ addresses around to make free spaces larger.
The OS reconfigures addresses to improve memory utilization.

Good Properties of Address Translation

The MMU performs address translation quickly.
Processes are isolated from each other, and can't access another's memory.
Processes don't know that their addresses are being translated.

Paging

Paging is a memory management technique.
Virtual memory is broken up into fixed-sized chunks called pages, and physical memory is as well.
It maps virtual pages to physical pages dynamically as needed.
Only allocates physical memory when actually used.
The operating system manages page tables, storing mappings of virtual page numbers to physical page numbers.

How to Do Address Translation

MMU takes a VPN (virtual page number) and finds the corresponding PPN (physical page number) from the page table to convert a virtual address to a physical address.
The process happens in hardware and is very fast.
A 64-bit address space requires representing 0 to 63 using binary notation.
128 bytes of physical memory might need 7-bit physical addresses.

Page Tables

Page tables store the mapping between virtual page numbers and physical page numbers.
The OS manages the page table's contents.
The MMU has a register that points to the page table.
Page table entries record details like the resident (or present/valid) bit, dirty bit, showing if a page has been changed, and permission bits.

Page Swapping

Pages that aren’t in use but are needed are swapped out to disk and back in as needed.
This helps expand the accessible memory for processes.
Only pages actually needed by the process are loaded in memory during time-sharing/scheduling.

Page Fault

It occurs when a process tries to access a page not in physical memory.
The operating system handles the fault by swapping the required page.
OS loads the page from the disk into memory, replacing an already allocated page if needed.

Replacement Policy (LRU)

The least recently used (LRU) replacement policy chooses the page not used for a while.
This page is usually swapped out for a new one.

Page Tables : Neat Bonuses

The mmap system call allows for mapping files on disk to locations in a process’s virtual address space (VAS).
Processes can share the same physical memory page for greater resource efficiency.

Next Time

The study notes highlight several memory access considerations for the next study period.

Examining Virtual Memory Through Examples

Address sizes correlate directly to describing/accessing addressable data/memory spaces.
Number of bits in an address dictates the amount of bytes in memory you can address. (e.g. 10-bit addresses = 2^10 bytes)

The Math of Page Tables

The size of virtual and physical addresses depends on the number of pages and each page's size.
Calculating these sizes involves using logarithms, relating them to the amount of memory and page sizes a system can support.

Studying That Suits You

Use AI to generate personalized quizzes and flashcards to suit your learning preferences.

Description

This quiz delves into key concepts of virtual memory in operating systems, focusing on the advantages of mmap, page table operations, and the function of the Memory Management Unit (MMU). Explore how virtualization affects resource allocation and manage multiple process address spaces effectively.