Sample Exam Questions

These are examples of a few different types of questions that have been asked on other mid-terms, along with examples of full-credit, 2/3 credit, and 1/3 credit answers to those questions. The purpose of these examples is to show you:

What kinds of questions I am likely to ask
What kinds of answers I am expecting to see

General considerations

For full credit I am generally looking to see that:

You understand the question, including a clear understanding of the meanings of all terms used in the question.
You understand the concepts and issues that the question is exploring.
You understand or are able to determine what the most important factors are.
You can clearly express your understanding in a way that directly responds to the question.

While the details are highly problem specific, partial credit will generally be awarded in proportion to how well you were able to demonstrate how many of these things. In the examples below:

2/3 credit was given for answers that clearly responded to the question, but showed little depth or mastery of detail.
1/3 credit was given for answers that, while correct, were vague and superficial.
0 credit would be given for answers that were incorrect, or that failed to respond to the question.

"Principles" question

Why is it important to keep interface definitions distinct from implementations?
Give an example of a bad thing that could happen if this principle were violated.

Full Credit Answer

If the interface is an implementation-agnostic specification of expected behavior, it will be much easier to change the implementation, while maintaining interface compatability. Otherwise, it may be difficult to know what the actual interface is.

If the interface definition was "the current behavior of module X":

individual clients are left to examine behavior and try to guess the interface specifications. This greatly increases the likelihood that future changes will break old clients.
implementors are likely to find it impossible to change the implementation without breaking some clients. This will greatly constrain their ability to evolve their software.

2/3 Credit Answer

Without a clear interface specification, nobody will be sure what the right interface is, and a much greater chance that future implementations will break some clients.

1/3 Credit Answer

Better interace definitions, and fewer fewer upwards compatability problems.

"Description" question

List the primary types of segments found in a process' virtual address space and two characteristics (not contents) of each.

Full Credit Answer

The key segments in a process' address space are:

text ... executable, read-only
data ... read/write size, can change at run-time
stack ... read/write, grown/shrunk by os as needed
DLLs/shared libs ... executable, read-only other code segments loaded at load- or run-time
thread stacks ... like program stacks, but are probably allocated in the data segment and may not be managed by OS

2/3 Credit Answer

text ... executable code
data ... read/write program data
stack ... read/write, grows down

1/3 Credit Answer

code
data
stack

"Process" question

Describe or illustrate (in detail) the sequence of operatins involved in the processing of an illegal-address trap from a user-mode program with a registered segmentation fault handler.

Full Credit Answer

illegal address raises fault in CPU
CPU loads new PC/PS from associated trap vector
CPU pushes PC/PS at time of trap onto supervisor mode stack
first level handler saves registers and status, forwards to 2nd level handler
2nd level handler determines it was a user-mode segmentation fault and that there is a registered signal handler
push PC/PS at time of trap onto user mode stack, and changes the return PC (on the supervisor mode stack) to be the signal handler.
2nd level handler returns to 1st level handler
1st level handler restores registers and returns to user mode
user mode execution resumes in the registered signal handler

2/3 Credit Answer

CPU loads new PC/PS from associated trap vector
CPU pushes PC/PS at time of trap onto supervisor mode stack
OS trap handler saves registers and identifies cause of trap
Trap handler finds signal handler and changes stack to return to it
Restore registers, and return (to user-mode signal handler)

1/3 Credit Answer

trap into operating system
OS identifies cause
OS identifies handler
OS invokes signal handler

"Comparison" question

(a) List two key similarities between a (trap) system call and a procedure call.
(b) List two key differences between a (trap) system call and a procedure call.

Full Credit Answer

System calls are like procedure calls in that they pass parameters, perform a service, and return a result when the operation is complete.
System calls run with different privileges, in a different address space. Procedure calls are made with direct call instructions, while system calls are a requested with an illegal instruction that is recognized by the trap handler. Consquently, system calls are much more expensive than procedure calls.

2/3 Credit Answer

parameter passing and return values
subroutines are fast, system calls go into OS, which is slow

1/3 Credit Answer

both are made from subroutine calls in C
subroutines are in libraries, system calls are in OS