Editing Memory management unit (section)

== Benefits ==

An MMU can provide [[memory protection]], so that attempts by code to access or modify data to which it does not have access results in a failure; this catches both [[software bug]]s and deliberate attempts to access or modify that data. Typically, an operating system assigns each program its own virtual address space.<ref name="ucsd-lecture" />

An MMU can also be used to expand the size of the physical address when the virtual address is too small. For instance, the [[PDP-11]] originally had a 16-bit address that made it too small as memory sizes increased in the 1970s. This was addressed by expanding the physical memory bus to 18 bits, and using an MMU to add two more bits based on other pins on the [[processor bus]] to indicate which program was accessing memory.<ref>{{cite web |url=https://gordonbell.azurewebsites.net/Digital/Bell_Retrospective_PDP11_paper_c1998.htm |first=Gordon |last=Bell |title=A Retrospective on What We Have Learned From the PDP-11}}</ref>