memory-management

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Memory management

Memory management is a fundamental role of the operating system, dealing with the allocation and deallocation of the computer’s primary memory
When a user opens an application, its data is loaded from storage into active memory so that it can run smoothly
When a user opens a file from the file system, e.g. word document, the CPU loads this file data, as well as application data, into the primary memory
Primary memory is a limited resource in the system, so it needs careful management
Benefits of memory management are:
- Efficient allocation of memory enables multitasking, allowing multiple programs to run at once
- Memory management maintains security, it does not let programs access memory reserved for other programs
Memory management is made more efficient through 3 techniques:
- Paging
- Segmentation
- Virtual memory

Links between different types of memory

The main benefit of memory management is to make computer systems run smoothly. Most users don’t realise that as they effortlessly move between applications, it’s made possible because the OS is rapidly reallocating memory depending on the task that the user is completing.
Make sure you can name one benefit and one drawback for each memory management method in this revision note.

In A Level Computer Science, paging is a method of chunking the primary memory into equal-sized blocks
Data stored in memory will lead to the smooth running of applications
When an application is launched, data will be moved from the hard disk into Pages for faster access
As users move between applications, memory is dynamically allocated
Pages will be taken away from applications not in active use and granted to applications that are in active use
Paging can lead to internal fragmentation
If a 200KB file is divided into four 64KB Pages, the last Page would have 56KB of unused space
- First 64KB → Full page (64KB used, 0KB unused)
- Second 64KB → Full page (64KB used, 0KB unused)
- Third 64KB → Full page (64KB used, 0KB unused)
- Fourth 64KB → Partial page (8KB used, 56KB unused)
Unused space in a Page is wasteful because other unrelated data cannot be stored in this Page
Over time, more pockets of wasted space will exist across the memory; this process is called internal fragmentation
The image below shows a single 64KB Page with 4KB of unoccupied space
The box below this shows many Pages, each with varying sizes of internal fragments

Internal fragmentation

Segmentation is a method of chunking memory into blocks that correspond to different types of data needed by an application
A video editing application may have a Segment for video data, audio data and special effects
Segments are not all the same size; they are sized depending on their allocated data
Segmentation is space-efficient due to only allocating space depending on the amount an application needs
Segmentation can lead to external fragmentation
As Segments fill up the memory, physical gaps reduce the maximum size of new Segments that can be allocated
Below (left) shows different application data assigned to a Segment
The arrangement of data in the segment becomes more fragmented over time because as blocks are taken away it’s not possible to guarantee a new block will occupy the same amount of space
Below (right) shows a defragmented version of the Segment to highlight the total unused space

External fragmentation

If a computer is running low on primary memory, it can make secondary storage act as an ‘extension’ of the main memory
The operating system can offload data from the primary memory into virtual memory
Virtual memory creates an illusion of a larger memory and enables applications to continue to multitask
However, accessing data in virtual memory is considerably slower compared to RAM
Solid-state drives are faster than traditional hard-disk drives, but neither are as fast as RAM
Over-reliance on virtual memory can lead to performance issues

Memory Management Technique	Description	Example	Benefits	Drawbacks
Paging	Divides memory into fixed-sized blocks (pages)	A process needing 200KB of memory is divided into four 64KB pages, leaving 8KB unused in the last page	Facilitates efficient memory management and enables the use of virtual memory	This can lead to internal fragmentation
Segmentation	Divides memory into variable-sized segments based on logical parts of a process	In a video editing application, different segments may be created for video data, audio data, effects, and UI elements	Allows for intuitive and efficient memory access	This can result in external fragmentation
Virtual Memory	Uses hard drive space as an ‘extension’ of RAM	When memory-intensive applications exceed the available RAM, the OS moves less frequently accessed pages to the hard disk	Allows more extensive programs to be run and facilitates effective multitasking	Slower to access than physical memory, which degrades performance if overused

Virtual memory, paging and segmentation are used in memory management.

(a) Explain what is meant by virtual memory.[3]

(b) State one difference between paging and segmentation in the way memory is divided.[1]

Answers

(a)

Disk / secondary storage is used to extend the RAM / memory available [1 mark]
… so the CPU appears to be able to access more memory space than the available RAM [1 mark]
Only the data in use needs to be in main memory so data can be swapped between RAM and virtual memory as necessary [1 mark]
Virtual memory is created temporarily [1 mark]

(b)

Paging allows the memory to be divided into fixed size blocks and Segmentation divides the memory into variable sized blocks [1 mark]
The operating system divides the memory into pages, the compiler is responsible for calculating the segment size [1 mark]
Access times for paging is faster than for segmentation [1 mark]