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733.

Consider the relation examinee (regno, name, score), where regno is the primary key to score is a real number.
(a) Write a relational algebra using (∏,σ,ρ,×) to find the list of names which appear more than once in examinee.
(b) Write an SQL query to list the regno of examinees who have a score greater than the average score.
(c) Suppose the relation appears (regno, centr_code) specifies the center where an examinee appears. Write an SQL query to list the centr_code having an examinee of score greater than 80.
[5 marks]

asked in Computer Science And Engineering, 2001

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734.

Consider a disk with the 100 tracks numbered from 0 to 99 rotating at 3000 rpm.
The number of sectors per track is 100. the time to move the head between two successive tracks is 0.2 millisecond.
(a) Consider a set of disk requests to read data from tracks 32, 7, 45, 5 and 10. Assuming that the elevator algorithm is used to schedule disk requests, and the head is initially at track 25 moving up (towards larger track numbers), what is the total seek time for servicing the requests?
(b) Consider an initial set of 100 arbitrary disk requests and assume that no new disk requests arrive while servicing these requests. If the head is initially at track 0 and the elevator algorithm is used to schedule disk requests, what is the worst case time to complete all the requests?
[5 marks]

asked in Computer Science And Engineering, 2001

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735.

Two concurrent processes P1 and P2 want to use two resources R1 and R2 in a mutually exclusive manner. Initially, R1 and R2 are free. The programs executed by the two processes are given below.
Program for P1:
S1: While (R1 is busy) do no-op;
S2: Set R1 <- busy;
S3: While (R2 is busy) do no-op;
S4: Set R2 <- busy;
S5: Use R1 and R2;
S6: Set R1 <- free;
S7: Set R2 <- free;
Program for P2:
Q1: While (R1 is busy) do no-op;
Q2: Set R1 <- busy;
Q3: While (R1 is busy) do no-op;
Q4: Set R1 <- busy;
Q5: Use R1 and R2;
Q6: Set R2 <- free;
Q7: Set R1 <- free;

(a) Is mutual exclusion guaranteed for R1 and R2? If not, show a possible interleaving of the statements of P1 and P2 such that mutual exclusion is violated (i.e., both P1 and P2 use R1 or R2 at the same time).
(b) Can deadlock occur in the above program? If yes, show a possible interleaving of the statements of P1 and P2 leading to deadlock.
(c) Exchange the statements Q1 and Q3 and statements Q2 and Q4. Is mutual exclusion guaranteed now? Can deadlock occur?
[5 marks]

asked in Computer Science And Engineering, 2001

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736.

(a) Remove left-recursion from the following grammar:
S -> Sa| Sb | a | b
(b) Consider the following grammar:
S -> aSbS| bSaS |∈
Construct all possible parse trees for the string abab. Is the grammar ambiguous?
[5 marks]

asked in Computer Science And Engineering, 2001

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737.

The syntax of the repeat-until statement is given by the following grammar
S -> repeat S₁ until E
Where E stands for expressions, S and S₁ stand for statement. The non-terminals S and S1 have an attribute code that represents generated code. The nonterminal E has two attributes. The attribute code represents generated code to evaluate the expression and store its truth value in a distinct variable, and the attribute varName contains the name of the variable in which the truth value is stored? The truth-value stored in the variable is 1 if E is true, 0 if E is false. Give a syntax-directed definition to generate three-address code for the repeatuntil statement. Assume that you can call a function newlabel( ) that returns a distinct label for a statement. Use the operator ‘\\’ to concatenate two strings and the function gen(s) to generate a line containing the string s. [5 marks]

asked in Computer Science And Engineering, 2001

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738.

Consider the following grammar with terminal alphabet ∑{a, (,), +, *}and start
symbol E. The production rules of the grammar are:
E -> aA
E -> (E)
A -> +E
A -> *E
A -> ∈
(a) Compute the FIRST and FOLLOW sets for E and A.
(b) Complete the LL(1) parse table for the grammar.
[5 marks]

asked in Computer Science And Engineering, 2001

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