What does 'RTU' mean? While I was reading about Base &amp; Bounds memory protection mechanism, I noticed the use of the abbreviation RTU as indicated in the slide below:

However I didn't know what it means.
So after searching for 'RTU', I found only this article on Wikipedia; But I'm not sure whether it matches the term.
Does anyone have any idea?
RTU stands for Return To User. This is invoked when the OS first accesses a physical memory range, and upon doing so will return the memory range address (code).