15-410, F'13
1
Debugging
Sep. 9, 2013
Dave Eckhardt
Dave Eckhardt
Todd Mowry
Todd Mowry
L06a_Debug
15-410
“Experience is what you get...
...when you don't get what you want.”
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Debugging - Operating System Design and Implementation - Lecture Slides, Slides of Operating Systems
This lecture is from Operating System Design and Implementation course. Its key words are: Debugging, Plot Summaries, Critical Question, Pinpointing Problempinpointing the Problem, Measuring, Measurement Techniques, Debugger, Writing Codewriting Code
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2013/2014
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15-410, F'
Debugging
Sep. 9, 2013
Dave Eckhardt
Dave Eckhardt
Todd Mowry
Todd Mowry
L06a_Debug
“Experience is what you get...
...when you don't get what you want.”
15-410, F'
What is “Debugging”?
Debugging is resolving a clash between stories
Debugging is resolving a clash between stories
Your hopeful story of achievement
The world's sad tale of woe
The stories look alike!
The stories look alike!
At the beginning, they both start with main()...
Key step: finding the divergence
Stories are fractal
Stories are fractal
You can zoom in on them and get more detail each time
The divergence is typically a tiny detail
●
You will need to zoom in quite a lot
15-410, F'
Telling Your Story
When I install my keyboard handler...”
When I install my keyboard handler...”
What do you really hope?
15-410, F'
Telling Your Story
When I install my keyboard handler...”
When I install my keyboard handler...”
What do you really hope?
Hardware notices key press/release
Hardware posts interrupt
CPU recognizes interrupt as keyboard interrupt
CPU responds to (vs. ignores) keyboard interrupt
CPU stores trap frame
CPU vectors through your IDT entry
Your wrapper is run
Wrapper calls C code
C code does ...
15-410, F'
Matching Phenomena to the Story
Keypress
Keypress ⇒
crash” tells you quite a bit
crash” tells you quite a bit
✔ Hardware notices keyboard event
✔ Hardware posts interrupt
✔ CPU recognizes interrupt as keyboard interrupt
✔ CPU responds to (vs. ignores) keyboard interrupt
? CPU stores trap frame
? CPU vectors through your IDT entry
? Your wrapper is run
? Wrapper calls C code
? C code does ...
What now?
What now?
15-410, F'
Measuring
How can you
How can you measure
measure the other steps?
the other steps?
? CPU stores trap frame
? CPU vectors through your IDT entry
? Your wrapper is run
? Wrapper calls C code
? C code does ...
15-410, F'
Measurement Techniques
Writing code
Writing code
Breakage of a complex data structure is, well complex
Probably need code to check invariants
Doing it by hand is fun at most once
15-410, F'
Asking for Help
Plot summary” is not enough
Plot summary” is not enough
We probaby have no idea what's wrong
Really!
Please see “triple fault” web page
You should always have a measurement plan
You should always have a measurement plan
What is the next thing to measure?
How would I measure it?
You may reach the end of your rope
You may reach the end of your rope
Some things are genuinely tricky to debug
Things in this class may occasionally qualify
This is a good learning experience
15-410, F'
Summary
Debugging is about reconciling two stories
Debugging is about reconciling two stories
“Plot summaries” aren't stories (you must zoom in)
“If you don't know where you are going, you will wind up
somewhere else.” — Yogi Berra
Measure multiple things, use multiple mechanisms
Measure multiple things, use multiple mechanisms
You should “always” have a next measurement target
You should “always” have a next measurement target
When you see us, bring a long story
When you see us, bring a long story
...which you will naturally be an expert on the first part of
Try to know why each register has the value it does