Mon, 3 October 2016, 22:09

My students,

In case you are studying old exams and have looked at Problem 4b from the
Spring 2009 exam, Siavash found an error in the solution.  The number of
resident pages in the process page table is 2, not 3.

Since PBR is x90, and PLR is 9, There are 9 PTEs in the process page table
that are stored at system virtual addresses x90, x91, x92 .... x98. In
other words, PTEs of user space are in pages x04, x05, and x06 of system
space.

Using SBR = x18. We can calculate that the PTEs of the 3 aforementioned
pages are stored at physical addresses x1C, x1D, and x1E. Based on the
table provided, only x1C and x1E contain PTEs with valid bit of 1.
Therefore only pages x04 and x06 are resident, i.e., page x05 is not
resident.

Thus, even though 3 pages are "required" based on the value of PLR, the
total number of pages of process page table "that are resident in physical
memory" is 2.

At some point we will post this correction to the website, but in the
meantime, I wanted to be sure it did not mislead any of you.

Good luck on the exam on Wednesday.



Yale Patt