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January 18, 2008 / Marco

What is “demand paging”?

You might very well have heard the term “demand paging” when you read about handsets like the Nokia N95 8GB, the Nokia N82 and a few other cell phones. So what’s demand paging, and why is it so praised?

According to Wikipedia, demand paging is an application of virtual memory. In a system that uses demand paging, the operating system copies a disk page into physical memory only if an attempt is made to access it (i.e., if a page fault occurs). It follows that a process begins execution with none of its pages in physical memory, and many page faults will occur until most of a process’s working set of pages is located in physical memory. This is an example of lazy loading techniques.

What does all this rather technical discussion mean? Essentially, in a system which supports demand paging, if you open, say, 6 applications and work with one of them, just the app which is in the foreground uses RAM and other apps which are in the background don’t consume RAM. This results in a highly economical way of preserving the RAM.

Currently the N95 8GB and the N82 support demand paging. If you update your N95 to the latest 20.XX firmware, it’ll also take advantage of demand paging, and this is also true for the N73 ME, albeit with a less noticeable increase in the amount of available RAM. The N82, for instance, offers about 93MB of RAM as it restarts. The N95 with the latest firmware offers less than 30MB of RAM. Do you know of any other handsets in the market that offer demand paging?

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