Comp111  Quiz 2 Review Answers
Dec 8, 2009  closed book
Name:__Answers__
Login:____couch_____
Please fill in the blanks. Each blank is worth 5 points.
 There are three important statistical distributions
important in modelling operating systems.
The
____exponential____
distribution models interarrival times for events.
Its cumulative distribution function is
____y=1e^{(λx)}____
The
____exponential____
distribution models service times for
individual processing requests.
These distributions are both
____memoryless____, e.g.,
service times for
____prior/past____
requests do not affect the service time for the next request.
 In Kendall notation, an M/M/infinity queue is one in which
arrival times are ____poisson____,
service times are ____exponential____,
and there are an infinite number of ____servers____.
 Suppose we have an M/M/1 queue with arrival factor λ and
service factor μ. This queue has a stable behavior if
____λ/μ____<1, but
____saturates/overfills/etc____
when
____λ/μ____>=1. (both blank 9's have same answer).
In the latter case, the queue length
_______increases____ without bound.
 A monitor differs from a semaphore in two ways:
 It does not ____buffer/queue____ signals; uncaught
signals are ____ignored/dropped____.
 It is bound (via a programming language) to a specific
____data structure____. It is not possible to erroniously
utilize a monitor to access a different one.
 A(n) ____interrupt____ tells the operating system that
attention is needed for a device or peripheral; a(n)
____signal____ tells a process that attention is needed.
The operating system cannot receive a(n)
____signal____ any more than a process can handle
a(n)
____interrupt____
 Process memory is naturally divided into
____segments____ that contain
____pages____, each of which
contains a fixed number of bytes. A typical byte of memory can
be divided into three bit fields including a(n)
____segment____ address,
____page____ address,
and word address.

Inverted page tables provide a(n)
____more____ efficient mechanism for memory mapping
than
____hierarchical____ page tables, because the number of
pages kept in memory is often far less than the total number of
virtual pages available.