CSE 221: Homework 1
Due Thursday, January 28 at 11:59pm
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 (the "why") as much as the answer
itself. Also, do not use shorthand: write your answers using complete
When grading homeworks, we will grade one question in detail and
assign full credit for technical answers to the others.
Submit your homework by uploading it to Gradescope (see Piazza
for the entry code).
- 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 (e.g., system calls) 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
- Whether or not a file system should incorporate versioning as a
fundamental feature provided by the file system is a long-standing
question in operating system design. Some operating systems, like
Tenex and Plan 9, implement file systems that incorporate
versioning in the file system itself; the NetApp filer is a modern
example. (Note that with file versioning, a file system need only
data blocks of a file that are changed between two versions of a
file.) Yet others, notably Unix and the popular operating systems
we use today (Linux, Windows, MacOS), implement file systems that
do not; instead, we use applications like version control systems
(svn, git) or cloud storage systems (Dropbox) on top of the file
system that keep track of file versions.
- First argue why a file system should incorporate versioning
as an explicit feature, and justify your argument.
- Then argue why a file system should not incorporate
versioning, also justifying your argument.
- For you personally, in terms of your computing needs, which
argument do you find more compelling?