CS 213 Spring '01
Lab 2

Lab Assignment L2 ( L2.ps, L2.pdf )

You can check the final bomb stats to see how you and your classmates did.

Hints

• ddd
  To run ddd on a FISH machine, add the following two lines to your ~/.cshrc file:
  setenv LD_LIBRARY_PATH /afs/cs/user/dkoes/lesstif/lib
setenv PATH /afs/cs.cmu.edu/academic/class/15213-s01/bin:$PATH

  Documentation about ddd is available from the official ddd home page.

  Just a few pointers on using ddd:

  • Use the stepi (not step) function. step and next are designed for working with source files. You don't have the source file. You will not like the effect of trying to step into a phase. Use stepi (and you might want to stay away form nexti as I've noticed it can be unpredictable).
  • To view the machine code (and you want to view the machine code) go to Source->Display Machine Code.
  • It's possible to observe the contents of registers and memory using the appropriate menu options (under Data I think).
  • If you've set a bunch of breakpoints and want to keep for the next time you run ddd you can save a session (under the File menu). ddd doesn't autosave sessions so make sure you save before quitting.
  • ddd is far from perfect software. If things get flaky quit and restart.
  • If you have any questions about ddd post to the bboard since other people probably have the same questions.
• gdb
  To single-step through your binary bomb in gdb, you will need to use the nexti and stepi commands (which step through binary instructions) rather than the next and step commands (which step through C instructions).
• In our experience, the best overall strategy for defusing a bomb has two steps: (i) Disassemble the bomb using objdump -d and study the printout to get an overall view of things. (ii) Use gdb or ddd to single-step through each phase, inspecting register values and memory values along the way. Use your printout from (i) to help you keep your place.
• Sometimes it is easier to debug a problem starting from the end and working backwords. You may find this true for some of your bomb phases as well.
• To avoid those painful explosions, always set a breakpoint at the routine that explodes the bomb.
• Your bomb is only defused when all the inputs to the bomb are correct. So yes, you could use your debugger to alter the contents of the registers and memory and then jump around the code to generate the phase defused messages. BUT you only get points for defusing a stage if the input you used is valid, so I wouldn't recommend doing this.
• There is a rumor going around that the bomb has a secret 7th phase. If this phase does indeed exist, there will be no extra points awarded for defusing it. However, to show our appreciation, we will set up a "Wall of Fame" on the Web page listing in perpetuity the students who successfully defuse it.
• All explosions count, even if you are working on the alleged secret phase. No guts, no glory!

Revisions and Updates:

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