COMP 310: Problem Set 1: Answers
All problems from OS Concepts by Silberschatz, Galvin, and Gagne
4.3 A DECSYSTEM-20 computer has multiple register sets. Describe the actions of a context
switch if the new context is already loaded into one of the register sets. What else must happen if
the new context is in memory rather than in a register set, and all the register sets are in use?
During a context switch, all the information about the current process (P1) must be saved to that
process’s PCB, and the information about the next process (P2) must be loaded. If P2’s state is
already loaded into registers, then the values of P1’s registers can remain in place, but P1 needs to
record which register set it was using (if it hasn’t done so already). P1’s state should be changed
to Ready (assuming it got swapped out because of a scheduler interrupt). Then, P2’s address space
must be swapped in. This includes restoring access to any files P2 was working with, access to
it’s call stack, etc. Finally, the system simply needs to switch to using P2’s registers as the current
register set and set P2’s state to running.
If P2’s registers are not already loaded in, then P1’s registers must be saved to P1’s memory (P1’s
PCB) before P2’s address space replaces P1’s. Then, P2’s registers must be loaded into this register
set from P2’s PCB before P2 starts running.
Note that in a context switch, the address space swap (not the register copy) is the most time
consuming step (think about the difference between thread context switching and process context
switching). So even though having P2’s data already loaded into registers saves some time, it does
not save that much time relative to the total time of the context switch.
5.1 Provide two programming examples of multithreading that improve performance over a single-
threaded solution.
Some possible answers include:
•A text recognition program. The user’s strokes could be captured and rendered as they are
drawn in one thread while the recognition proceeds in the other so that the strokes may be
rendered in real time even if the recognition lags behind.
•A database lookup program. Several threads could for information in the database, starting
at different points, to find the information more quickly.
•Computer games. Separate input, rendering, calculations.
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