Editing Memory paging (section)

===Main memory smaller than virtual memory===
In most systems, the size of a process's virtual address space is much larger than the available main memory.<ref name="buzbee">{{cite web|author=Bill Buzbee|title=Magic-1 Minix Demand Paging Design|url=http://www.homebrewcpu.com/demand_paging.htm|access-date=December 9, 2013|url-status=live|archive-url=https://web.archive.org/web/20130605134128/http://www.homebrewcpu.com/demand_paging.htm|archive-date=June 5, 2013}}</ref> For example:
* The [[address bus]] that connects the CPU to main memory may be limited. The [[Intel 80386#The i386SX variant|i386SX CPU]]'s 32-bit internal addresses can address 4&nbsp;GB, but it has only 24 pins connected to the address bus, limiting installed physical memory to 16&nbsp;MB. There may be other hardware restrictions on the maximum amount of RAM that can be installed.
* The maximum memory might not be installed because of cost, because the model's standard configuration omits it, or because the buyer did not believe it would be advantageous.
* Sometimes not all internal addresses can be used for memory anyway, because the hardware architecture may reserve large regions for I/O or other features.