Virtual Memory Then And Now

Windows 3.1

Back in the good old days of DOS 6.22 and Windows 3.1, everyone knew that creating a permanent swapfile was the key to optimal swapfile performance. This was because Windows 3.1 only creates permanent swapfiles that are contiguous.

A contiguous swapfile is a swapfile that consists of an uninterrupted block of hard disk space. When a swapfile is contiguous, the read-write heads of the hard disk can read and write data on the swapfile in a continuous fashion.

In Windows 3.1, if the swapfile was set up as a temporary swapfile which is created every time Windows 3.1 boots up, it will end up at the end of the hard disk and fragmented too. So, every time the swapfile is read from or written to, the hard disk heads have to seek all over the platters to conduct those operations.

Needless to say, this greatly erodes the performance of the swapfile. That's why it was important to make the swapfile permanent in Windows 3.1 - so that the swapfile will become contiguous.

Windows 95 And Above

From Windows 95 onwards, Microsoft encouraged the use of its new dynamic virtual memory system. Of course, there is nothing new about the virtual memory part but the keyword in this new technique is dynamic.

The new dynamic virtual memory system no longer relies on a fixed-size swapfile but a paging file that dynamically resizes itself according to need. When the computer runs out of memory, more memory is created by increasing the size of the paging file. Once the virtual memory is freed up, theoretically the paging file diminishes in size.

Microsoft claims that while its dynamic virtual memory system will create a fragmented paging file, it is still faster than Windows 3.1's static virtual memory system. As a bonus, no hard disk space will be tied up in a permanent paging file.

However, this dynamic virtual memory system does have a big disadvantage - it cannot be moved to the outer tracks of the hard disk platters.