14. OCR A Level (H046-H446) SLR4 - 1.2 Paging, segmentation and virtual memory

Summary

This video explores memory management techniques, focusing on paging, segmentation, and virtual memory. It explains how programs are loaded into RAM and the challenges of memory allocation when space is fragmented. Paging divides memory into fixed-size blocks, while segmentation uses variable-sized blocks based on program structure. When RAM is full, virtual memory comes into play, offloading less-used data to the hard disk, creating an illusion of expanded memory and addressing the slowdown caused by limited physical memory.

Takeaways

• 💾 Memory Management: The video discusses memory management techniques, focusing on paging, segmentation, and virtual memory.
• 📈 RAM Usage: It explains how programs are loaded into main memory (RAM) and how memory is managed when programs are opened and closed.
• 🔄 Memory Fragmentation: The issue of memory fragmentation is introduced, where free space is not contiguous, preventing new programs from fitting.
• 🔄 Paging: Paging is a strategy where memory is divided into fixed-size blocks called pages, which are allocated to programs regardless of their size.
• 🔄 Segmentation: Segmentation is another strategy where memory is divided into variable-sized segments based on logical divisions of programs.
• 🔄 Efficiency: Paging can lead to inefficiency by splitting program instructions, while segmentation aims to keep related instructions together.
• 💾 Virtual Memory: When RAM is full, virtual memory is used to extend the available memory space by utilizing the hard disk for storage.
• 🔄 Swapping: The concept of swapping is introduced, where less frequently used parts of a program are moved to the hard disk to free up RAM for other programs.
• 📉 Performance Impact: The video explains that using virtual memory can slow down a computer because accessing data from the hard disk is slower than from RAM.
• 💡 Key Question: The video concludes by posing a key question about how computers handle running out of memory and the associated slowdown.

Q & A

What is the primary function of random access memory (RAM) in a computer?

RAM serves as the main memory where the operating system and other applications are loaded for immediate access and execution by the CPU.

Why is it inefficient to move programs within memory to consolidate free space?

Moving programs within memory is inefficient because it takes a significant amount of time and requires updating all addresses and references to free space, which can disrupt the program's operation.

What are the two main strategies for dividing up memory to solve the problem of non-contiguous free space?

The two main strategies are paging and segmentation. Paging divides memory into fixed-size blocks, while segmentation divides memory into variable-sized sections based on logical divisions of the program.

How does paging differ from segmentation in terms of memory allocation?

Paging allocates memory in fixed-size blocks regardless of the program's structure, which can lead to inefficient memory use. Segmentation, on the other hand, allocates memory in variable-sized segments that correspond to logical divisions of the program, leading to more efficient memory use.

What is the potential downside of paging when it comes to program execution?

The downside of paging is that it can separate instructions within a program, such as those within a loop, into different parts of memory, which can lead to inefficiencies in program execution.

How does segmentation improve upon the limitations of paging?

Segmentation improves upon paging by dividing memory into segments that correspond to complete sections of a program, allowing related instructions to be kept together and enhancing program execution efficiency.

What is virtual memory, and why is it used in modern computers?

Virtual memory is a memory management technique that uses hard disk space to store data and instructions that are not immediately needed in RAM. It is used to give the appearance of having more memory than physically available, allowing more programs to be run simultaneously.

Why does a computer slow down when it runs out of RAM and starts using virtual memory?

A computer slows down when using virtual memory because accessing data from the hard disk is significantly slower than accessing it from RAM. The constant swapping of data between RAM and the hard disk consumes additional time and system resources.

How does the operating system manage the transition of data between RAM and virtual memory?

The operating system manages the transition of data between RAM and virtual memory through a process called swapping. When RAM is full, less frequently used data is moved to the hard disk, and when that data is needed again, other data may be swapped out to make room.

What is the role of the bootstrap program in loading the operating system into RAM?

The bootstrap program is responsible for loading the operating system from the disk into RAM when the computer is turned on, initializing the system for further operations.

Outlines

00:00

💾 Memory Management Techniques

This paragraph introduces the concepts of memory management with a focus on paging, segmentation, and virtual memory. It uses a diagram to illustrate the process of loading programs into main memory and the challenges that arise when there isn't enough contiguous free space for a new program. The paragraph explains the inefficiency of moving programs within memory and introduces two strategies to overcome this: paging and segmentation. Paging divides memory into fixed-size blocks, while segmentation allows for variable-sized sections based on program structure. The paragraph also touches on the limitations of these methods and sets the stage for the discussion of virtual memory in the next paragraph.

05:01

🖥️ Virtual Memory and Its Role in Memory Management

The second paragraph delves into the concept of virtual memory as a solution to the problem of limited RAM. It explains how virtual memory allows the computer to use hard disk space to store instructions that are not immediately needed, thereby giving the illusion of having more memory than physically present. The paragraph describes the process of swapping out less frequently used instructions to the hard disk and swapping in other programs when needed, which helps in managing memory more efficiently. The video concludes by posing a key question about how computers handle running out of memory and the associated slowdown, inviting viewers to consider the practical implications of the discussed memory management techniques.

Keywords

💡Random Access Memory (RAM)

Random Access Memory, or RAM, is the primary memory that a computer uses to store data and instructions for immediate use. It is volatile memory, meaning that it loses its contents when power is turned off. In the video, RAM is discussed as the main memory where the operating system and applications are loaded. The script highlights the limitation of RAM when it is full and how this can affect the loading of new programs, such as program E, which cannot fit into the available spaces in RAM.

💡Paging

Paging is a memory management technique used by operating systems to allocate memory to processes. It involves dividing the memory into fixed-size blocks called pages. The video explains that paging does not consider the structure of the program when allocating memory; it simply divides it into these fixed-size pages. This can lead to inefficiencies, such as splitting instructions within a loop across different pages.

💡Segmentation

Segmentation is another memory management strategy that differs from paging in that it divides memory into variable-sized segments based on the program's logical structure. The video script uses segmentation to illustrate how programs can be split into parts and placed in non-contiguous memory spaces, thus allowing for more efficient memory usage and avoiding the issues associated with paging.

💡Virtual Memory

Virtual memory is a memory management technique that extends the amount of memory available to a process beyond the physical memory (RAM). It does this by using disk space to hold data that is not immediately needed. The video describes how virtual memory allows programs and data to be stored on the hard disk and swapped in and out of RAM as needed, giving the illusion of having more memory than physically exists.

💡Main Memory

Main memory, also known as primary memory, is the memory that a computer uses to store data and instructions that are currently being processed. The video script refers to main memory as the location where the operating system and applications are loaded and where the challenges of memory allocation and management occur.

💡Operating System

The operating system is the software that manages computer hardware and provides a platform for applications to run. In the context of the video, the operating system is loaded into main memory when the computer starts up, and it is responsible for managing resources, including memory allocation and virtual memory.

💡Memory Management

Memory management is the process by which an operating system handles the allocation and deallocation of memory. The video script discusses various memory management strategies, such as paging and segmentation, which are used to efficiently use the limited space in main memory and to handle situations where there is not enough contiguous free space for a new program.

💡Physical Memory

Physical memory refers to the actual hardware components used to store data, such as RAM. The video script contrasts physical memory with virtual memory, highlighting that while physical memory is limited and directly accessible by the CPU, virtual memory extends the usable memory space by utilizing disk storage.

💡Logical Divisions

Logical divisions refer to the way memory is organized based on the program's structure and functionality. In segmentation, memory is divided into logical divisions that correspond to the program's natural structure, such as code, data, and stack segments. The video script explains that this approach is more efficient than arbitrary physical divisions used in paging.

💡Contiguous Memory Allocation

Contiguous memory allocation is a method where a program is loaded into a single, continuous block of memory. The video script points out the limitations of this approach when there is not enough contiguous free space in main memory, leading to the need for memory management techniques like paging and segmentation.

💡Efficiency

Efficiency in the context of the video refers to the effective use of memory resources to minimize wasted space and optimize performance. The video discusses how paging can lead to inefficiencies by splitting program instructions across different pages, while segmentation aims to improve efficiency by aligning memory allocation with the program's logical structure.

Highlights

Introduction to memory management with a focus on paging, segmentation, and virtual memory.

Explanation of random access memory (RAM) and its role in loading the operating system and applications.

Challenge of fitting new programs into fragmented memory spaces.

The inefficiency of moving programs within memory and the need for a better solution.

Introduction to paging as a strategy for dividing memory into fixed-size sections.

Discussion on the limitations of paging due to its disregard for program structure.

Segmentation presented as an alternative with variable-sized memory divisions based on program logic.

Advantages of segmentation in managing memory more efficiently by keeping related program parts together.

Comparison of paging and segmentation in terms of memory division strategies.

The concept of virtual memory as a solution to running out of physical RAM.

Description of how virtual memory allows more programs to be managed than can fit in RAM.

Explanation of how virtual memory uses the hard disk to store less frequently used instructions.

The process of swapping programs between RAM and virtual memory to optimize memory usage.

The illusion of having more memory than physically available through virtual memory.

Key question posed: How does a computer handle running out of memory and why does it slow down?

Encouragement for viewers to pause and take notes on the similarities and differences between paging and segmentation.

Transcripts

in this video we discuss memory

management

with particular reference to paging

segmentation

and virtual memory

[Music]

so in order to understand this topic

we're going to use

a diagram on the left here so

effectively what we've got

is random access memory main memory

now when your computer first starts up

it loads the operating system

and then you interact with other

applications and programs

and they get loaded into memory so here

programs a b c and d have all been

loaded

program c has now been closed and the

memory has been released

program e arrives but as we can see it

won't fit

into either of the three spaces

available in main memory

now there is enough total free space

available it just doesn't have to be

located together

in a single block

now one solution could be to move the

location of program d

in memory so the free space is all

together

although this looks really simple to do

with our abstract diagram here

it's not actually very desirable to

operate like this in practice

it takes a lot of time to move a program

in memory or on disk

all the addresses and free space

references need updating

we actually need another solution now

there are two main strategies

both of which essentially evolve

dividing up memory

into smaller sections they are paging

and segmentation

so with pages the memory is divided into

fixed sizes all the same

pages are made to fit sections of memory

these pages are considered to be

physical divisions

and the programs are split up to fit

into these given number of pages

now paging takes no account of how

it splits the program only that it

splits it

into these fixed size pages it's quite

possible that it could separate the

instructions inside say a looping

condition

so they're in different parts of memory

and of course this is not very desirable

as it become quite inefficient

it would be better to keep those kind of

structures all together

well another option is segmentation

here the segments are different sizes

they're complete sections of program

so now we're dividing up memory based on

logical divisions

and not arbitrary physical divisions

so here program b has been closed

and the memory has been released

a new program e is loaded there's plenty

of free space so it just goes in

now program f arrives is too big to fit

in either of these three spaces but

based on logical and sensible divisions

segmentation can split the program into

two parts

and it places them in the available free

space as shown

leaving two smaller areas of free space

so as a summary here are the

similarities and differences

with paging versus segmentation you

might like to pause the video at this

point and take some notes before moving

on so after a while it's obviously

perfectly reasonable to have a situation

where main memory has become

full as is shown here now job

g ideally would like to run it would

seem we have to prevent the program from

loading at all

now this situation is far from ideal the

solution

is to make use of what we call virtual

memory

when programs and data are being fetched

and executed by the cpu

they are stored in ram however this

assumes that the memory is large enough

to hold all the programs being executed

in modern computers it's common to have

more than one program open at once

however you can store significantly more

programs on the hard disk compared to

ram

because byte for byte hard disk space is

much cheaper than ram

when you turn your computer on the

bootstrap program will load the

operating system from the disk into ram

when you use a computer every time you

double click an icon to load a program

it transfer those instructions into ram

but what happens when you run out of ram

as instructions are fetch one at a time

that means that some of the instructions

are not likely to be fetched in the near

future

therefore one solution is to transfer

instructions that are not being used

to a space on the hard disk and this is

known

as virtual memory

when these instructions are needed again

a different program can be swapped out

of ram to virtual memory

to make room for the instructions that

are now needed

this gives the impression that a

computer has more memory

than it actually has

having watched this video you should be

able to answer the following key

question

how does a computer handle running out

of memory and why does it slow down

[Music]

you