GEORGE MASON UNIVERSITY
Computer Science Department
CS 571 – Operating Systems
Fall 1995
Mid-term Exam
DUE DATE – Nov 15
1. (a) Writepseudo-code forimplementingSolaris mutexesandcondition variablesusingsemaphores.
Specifically, you have to describe (i) what data structures would used to implement condition
variables and mutexes (assuming the existence of a semaphore data structure) and (ii) how the
mutex lock,mutex unlock,cond wait and cond signal calls would be implemented. 10
(b) Why is a cond wait call in Solaris always embedded in a while loop in which the conditionthat
caused the wait is being tested? How and why is this different from the way wait operations on
condition variables in a monitor are used? Give an example whichillustratesthe problems that
can occur if the cond wait call is not embedded in a while loop. 5
2. Consider the following snapshot of a system with 6 instances of resource
R
1, 3 instances of resource
R
2, 4 instances of resource
R
3, and 2 instances of resource
R
4.
Allocation Need Available
R
1
R
2
R
3
R
4
R
1
R
2
R
3
R
4
R
1
R
2
R
3
R
4
P
03011 1100 1020
P
20100 0112
P
31110 3100
P
41101 0010
P
50000 2110
Here the
Allocation
matrix shows the number of instances of a resource type allocated to a particular
process. Similarly, the
Need
matrix shows the number of instances of a resource that could be
requested by a particular process (in addition to the resources are already allocated to it). The
Available
vector shows the number of instances of each resource type that are currently free.
(a) The system is currently in a safe state. Explain why.
(b) Suppose process
P
5now requests an instance of resource
R
1. If the system is using a deadlock
avoidance approach, should this request be granted? Explain why or why not. (5)
3. (a) How long does it take to fork off a child process under the following conditions: text size = 100
Kbytes, data size = 20 Kbytes, stack size = 10 Kbytes, process table structure = 1 Kbytes, user
structure = 5 Kbytes. Reading and writing a 32-bit word from memory take 200 nanoseconds
each, and a kernel trap and return take 1 ms each. Assume that text segments are shared, and
that any other operations that are required take negligible time. 4
(b) How would the vfork call reduce this time? What is the disadvantage of using vfork?3
(c) What is copy-on-write? How would it reduce the time for forking a process? 3
4. A pure paging system has a page size of 512 words, a virtual memory of 512 pages numbered 0
through 511, and a physical memory of 10 page frames numbered 0 through 9. The 18-bit virtual
1
