Open course ware

A few announcements at the beginning today. This is 3. 91. For those of you who are new to the class, welcome. A couple of administrative things. When you joined 3.091, regardless of what registration took place outside, please check in with my office. I want you to check in with either Hillary or Lori who will assign you to a lecture section. There is a lecture now Monday, Wednesday and Friday at 11:00. There is also a lecture Monday, Wednesday and Friday at 1:00. And we are trying to level out the populations so that they balance the seating capacity. And also, likewise with the recitations. We have recitations that run all day long Tuesdays and Thursdays. The earliest one is at 9:00, the latest one is at 4:00. And, again, we are trying to balance the occupancy there so that we have good access to the recitation instructor. So please don't go squatting. We need to know what the populations are. And that office is just down the hall here in 8-201. There it is. They a...

There are a few MMU implementation details we can tweak for more efficiency or functionality. In our simple page-map implementation, the full page map occupies some number of physical pages. Using the numbers shown here, if each page map entry occupies one word of main memory, we'd need 2^20 words (or 2^10 pages) to hold the page table. If we have multiple contexts, we would need multiple page tables, and the demands on our physical memory resources would start to get large. The MMU implementation shown here uses a hierarchical page map. The top 10 bits of virtual address are used to access a "page directory", which indicates the physical page that holds the page map for that segment of the virtual address space. The key idea is that the page map segments are in virtual memory, i.e., they don't all have to be resident at any given time. If the running application is only actively using a small portion of its virtual address space, we may only need a hand...

PROFESSOR: Let's define a few familiar and standard operations on sets. So here's a picture of two sets A and B, where the idea is that the circle represents the points in A. The other circle represents the points in B. The overlapping area, this lens-shaped region, are the points that are in both A and B. And the background are the points that are in neither A and B. So this sort of a general picture allows you to classify points with respect to And B, and it's called a Venn diagram, in this case for two sets. It's still useful for three sets. It gets more complicated for four sets. And after that point, they're not really very useful. But a lot of the basic operations can be illustrated nicely in terms of the Venn diagram for two sets, and that's what we're about to do. So the first operation is union. It's the set of points shown here in magenta. It's the set of points that are in either A or B, all of them. And so if we were definin...