15-410 Homework 2
This homework assignment is due Friday, April 29th at 23:59:59. As we intend to make solutions available on the web site immediately thereafter, .
Homework must be submitted (online) in either PostScript or PDF format (not:
Microsoft Word, Word Perfect, Apple Works, LaTeX, XyWriter, WordStar,
etc.). Except as otherwise directed (in the crypto question), turn
in your answers as either
As usual, you may discuss this assignment with others, but you must then go off by yourself to write up the solution.
Question 1 - Public Key Practicum
This question is not hard, but it does take some time to do it right. Please don't leave this question to the last minute, and think carefully about what the various steps accomplish.
Follow the directions in pgp.html to generate
a PGP key ring,
containing public and private keys
for digital signature and encryption purposes.
Do not turn the key ring in to your
Question 2 - Virtual Memory
Consider this virtual-to-physical mapping data structure, which could be used for an x86-like processor instead of the traditional x86 page-directory/page-table structure.
When the translation unit needs to translate from a virtual
address to a physical address, it applies a (documented)
In particular, the TU compares the
virtual page address being translated to the virtual page address
field in the zeroth PME of the bucket. If they match and at least
one of the two valid bits in the PME is on, the PME represents the
translation for this page. The frame address, permission bits,
and flags are then checked, applied, and updated as appropriate.
If the virtual page address doesn't match
If the virtual address has a translation but the permission bits are inappropriate, a permission fault occurs. If the virtual address does not have a translation, a translation fault occurs. If the number-of-buckets field in the MDR is zero, your manager will yell at you.
In what circumstances do you believe the x86 mapping structure is superior to this one? Explain your reasoning.
Can you argue that this mapping structure is superior to the x86's? Explain.
Question 3 - Storage
Imagine you are interviewing for a summer internship at Seagate Research in downtown Pittsburgh. Your hiring manager asks you to explore two possible modifications to their well-known Barracuda line of disks:
Your interviewer asks you to suggest which application market segments would be most receptive to disks with each of these modifications and to suggest which one you think Seagate should pursue. Your answer should be approximately three to five paragraphs.
By the way, if you think you are having AFS permission problems,
try running the program located at
[Last modified Saturday April 23, 2005]