Operating Systems

================ Start Lecture #21 ================

NOTE: Lab4 is available.

4.8: Segmentation

Up to now, the virtual address space has been contiguous.

Homework: 37.

** Two Segments

Late PDP-10s and TOPS-10

** Three Segments

Traditional (early) Unix shown at right.

** Four Segments

Just kidding.

** General (not necessarily demand) Segmentation

** Demand Segmentation

Same idea as demand paging, but applied to segments.

The following table mostly from Tanenbaum compares demand paging with demand segmentation.

Consideration Demand
Programmer aware NoYes
How many addr spaces 1Many
VA size > PA size YesYes
Protect individual
procedures separately
Accommodate elements
with changing sizes
Ease user sharing NoYes
Why invented let the VA size
exceed the PA size
Sharing, Protection,
independent addr spaces

Internal fragmentation YesNo, in principle
External fragmentation NoYes
Placement question NoYes
Replacement question YesYes

** 4.8.2 and 4.8.3: Segmentation With (demand) Paging

(Tanenbaum gives two sections to explain the differences between Multics and the Intel Pentium. These notes cover what is common to all segmentation+paging systems).

Combines both segmentation and demand paging to get advantages of both at a cost in complexity. This is very common now.

Although it is possible to combine segmentation with non-demand paging, I do not know of any system that did this.

Homework: 38.