Virtual Memory (Cambridge (CIE) O Level Computer Science)
Revision Note
Written by: James Woodhouse
Reviewed by: Lucy Kirkham
Virtual Memory
What is virtual memory?
Virtual memory is an extension of primary storage (RAM) located on secondary storage
In situations where RAM is close to being full, virtual memory can be used so that the computer remains operational
Programs and data not currently being executed are transferred to virtual memory
Programs and data are transferred back to RAM when they are needed
Programs are stored as pages of data
Pages of data are transferred between RAM and virtual memory when needed
Virtual memory is much slower than RAM and its use will impact negatively on system performance
To avoid the use of virtual memory, increase the size of the RAM
Example
In the example above, the computer is turned on and the elements of the operating system are loaded into RAM
The user opens some spreadsheet and video editing software but RAM is full
The program not currently being used (video editing) is transferred to virtual memory
When the user wants to use the video editing software again, it is transferred back from virtual memory to RAM
Case Study
A student uses software to create 3D models.
3D modelling often means processing large amounts of data
This takes up a large amount of RAM
If there is not enough physical RAM, virtual memory is used to:
Extend the RAMs capacity
Ensure the software does not freeze/crash when RAM gets full
Worked Example
A computer only has 4GB of RAM, Fynn says that virtual memory can be used instead of adding more RAM.
Explain how virtual memory can compensate for the lack of RAM in Fynn's computer [3]
Explain why it would be beneficial for Fynn to get more RAM instead of relying on virtual memory [2]
How to answer these questions
What is the definition of virtual memory? how does it work?
What is the downside of using virtual memory? turn this into an advantage of adding more RAM
Answers
How can Virtual Memory compensate
Virtual Memory is used when RAM is full
...part of the secondary storage used as (temporary) RAM
Data from RAM is moved to the secondary storage/Virtual Memory (to make space in RAM)
RAM can then be filled with new data
When data in Virtual Memory is needed it is moved back to RAM
Benefit of more RAM
More RAM will improve the performance of the computer // More RAM will speed up the access to data
Virtual Memory is slower to access than RAM direct (because it has to go back to RAM first)
Moving data between RAM and VM takes processor time
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