CSE 221: Homework 1
Hardcopy due Thursday, February 5, 2015 at the start of class (8am)
Answer the following questions. For questions asking for short
answers, there may not necessarily be a "right" answer, although some
answers may be more compelling and/or much easier to justify. But I
am interested in your explanation as much as the answer itself. Also,
do not use shorthand: write your answers using complete sentences.
- A fundamental aspect of protection in operating systems is rights
amplification. Rights amplification enables a more privileged
protection domain to perform an operation on behalf of a less
privileged protection domain in a controlled fashion without violating
protection in the system.
For each of the following operating systems, state (a) the
protection domain that they support, (b) the mechanism for crossing
protection domains, (c) how rights are represented, (d) how rights are
amplified crossing domains, and (e) how the OS determines whether to
allow the domain crossing.
Support your answers with a bit of explanation, such as a concise
summary explanation in your own words (a quote of a phrase or sentence
from the papers is fine as well). For instance, two possible answers
to part (a) for Hydra are:
In other words, we're looking for more than just "local name space"
— but at the same time your answers don't have to be lengthy
discussions. The balance in the example above is fine.
A protection domain in Hydra is the "local name space" (LNS). An
LNS represents the current set of objects and rights to which a
process has access, and those objects and rights change when a process
moves from one LNS to another.
A protection domain in Hydra is the "local name space" (LNS): "At
any instant, the execution environment (domain) of a program is
defined by an LNS object associated with it...the rights lists in each
capability define the permissible access rights of this program at
this instant." (Hydra p. 341).
- Operating systems go to great lengths to provide isolation and
protection among processes executing on the system. Process
debugging, however, represents a necessary, interesting feature that
is made more difficult by process isolation and protection, and
requires support from the operating system to function correctly.
To the extent possible and where appropriate, when answering the
following questions support your answers with approaches for debugging
support found in the papers you have read to this point (e.g., Tenex,
Lampson Protection, Pilot).
- Why must a traditional operating system like Unix explicitly
provide support for process debugging?
- List two distinct operations that a debugger must perform that
require support from the operating system.
- Because processes are protected and isolated from each other,
operating systems must also provide support for communication and
coordination among processes. Why can't debuggers just use the
support that operating systems already provide for process
communication and coordination?
- Do language runtime environments like Java and Perl require
operating system support for debugging programs in those languages?
Why or why not?
- When working on an operating system, developers also need
to use a debugger on the operating system itself. Why is debugging
the kernel of an operating system more challenging than debugging a
user-level process? What is one option for where to run a kernel
- Some of the systems we have read about and discussed use
specialized hardware to facilitate their implementation. Choose one
such instance, describe the hardware that was used, and what advantage
it gave the system implementors and designers. What is one drawback of
relying upon specialized hardware? Do we still use hardware of this
- Plan 9 used a three-tier storage model. The third tier, an
optical write-once read-many jukebox, was used for daily dumps of the
entire file system — and no data was ever deleted. Hard disk
and memory were used solely as caches to data stored on the jukebox.
At this point in time, their experience was that capacity was not an
issue: "Technology has created storage faster than we can use it."
Do you think that this statement still holds today? If so, why?
If not, why not? Try to incorporate how technology has changed since
Plan 9 was developed (hardware and/or software), or how users have
changed how and for what they use computers (workloads), in your
answer. Does your answer depend on whether the users you consider are
users at home, departments/enterprises, or Internet services like
Google and Amazon?
- Pilot made a strong and persuasive argument for tailoring the
design and implementation of operating systems to personal computers.
We have also seen commercial operating systems
like MSDOS, Windows
before NT, and
tailored towards personal computers as well. Why do you think we
still run multi-user timesharing systems like Unix — which the
Plan 9 developers refer to as "old UNIX warhorses" — on our PCs
today? (Even Android, one of the most popular operating systems for
smartphones, uses Linux!) Consider, for example, the requirements we
have of the systems that we use.