Download Mid-Semester Examination for CS551/CS651: Dependable Computing - Air-Bag System - Prof. Jo and more Exams Computer Science in PDF only on Docsity!
. This is a closed book, closed notes examination. . Write your answers on the examination paper in ink or legible pencil. . If your answer cannot be read or understood , or if your answer is vague or confused, it will be marked wrong. . In any question, make any assumptions that you need to but document your assumptions. . The number in parentheses after each question is the number of points allocated to that ques- tion.
NAME ( Print Legibly. All Capitals ):
PLEDGE ( Write Out In Full And Sign ):
Graduate Students (CS651) Sections A and B
Undergraduate Students (CS551) Section A
CS 551/CS
DEPENDABLE C OMPUTING
MID-S EMESTER EXAMINATION
FALL 2003
Time Limit - 75 Minutes
The following system description is used in some of the questions in this examination:
A hypothetical air-bag system for an automobile uses the following design. A variety of sensors monitor vehicle conditions and supply their readings to a computer system. The computer system attempts to detect a crash situation by comparing sensor values with pre- defined thresholds and with sensor data patterns (how the data has changed in the recent past). A small forward looking radar attempts to locate obstacles such as other vehicles in the road ahead. If the computer determines that a crash is happening based on sensor val- ues, it sends a signal to the air-bag inflation charge to cause air-bag inflation. The system performs numerous self checks on the hardware whenever a crash is not tak- ing place. If a self check fails, the system is disabled and a light is illuminated to inform the driver of the system’s non-operational status. It is not considered dangerous for the system to fail provided the failure does not occur during an accident and provided the driver’s warning light is illuminated when the problem is detected and remains illumi- nated. The following figure illustrates the system design:
Three axis vehicle speed sensor
Three axis vehicle acceleration sensor
Passenger position sensor
Forward-looking radar
Dashboard warning light
Computer system
Computer system
Computer system
Air bag inflation charge
Voting logic
Sensor data distribution
Page score
- Explain how the air-bag system could be engineered to be very safe but nothing else (e.g., not reliable). (Hint: Do not confuse safety of the air-bag system with overall safety of the car). (10)
- In what part of the air-bag system might Byzantine faults be expected to occur? (10)
- Define the dependability requirements that you think need to be met by the air-bag deploy- ment system. To make your answer as realistic as possible, make up any numbers that you think you need. (30)
Page score
- Give an example of a hazard to the passengers of a car that would arise because of a defect in the air-bag system. (10)
- Draw the top two levels of the fault tree for the air-bag system. The top level is to be the haz- ard you identified in the previous question and the second level is to have the appropriate event content. (20)
- Assume that a set called files exists and contains all the files in a file Unix system and a func- tion called sizeof() exists that supplies the size in bytes of its operand (a file).
(a) Write a set definition of the set of all files that are larger than 100 MBytes. (10)
(b) Write a predicate that is true if there is a file in the file system that is of length zero. (10)
