COP 6611 – Operating Systems
Assignment 1
Sridhar Godavarthy (U32086851)
1.8) Which of the following instructions should be privileged?
a. Set value of timer
c. Clear Memory
e. Turn off interrupts
f. Modify entries in a device-status table
g. Switch from user to kernel mode.
h. Access I/O Device.
1.9) Some early computers protected the operating system by placing it in a memory partition that could not
be modified by either the user job or the operating system itself. Describe two difficulties that you think could
arise with such a scheme.
a. Any new critical data created by the OS would have to be anyway placed on the writable partition –
nullifying its use.
b. The operating system can never be updated / patched.
1.13) In a multiprogramming and time-sharing environment, several users share the system simultaneously.
This situation can result in various security problems.
a) What are two such problems?
i) A single process may exhaust all resources unless proper resource management is performed.
ii) User programs may overwrite others address spaces without proper memory management.
iii) Users may be able to access others file and data without proper file management.
iv) User programs may result in fatal system errors which will affect all other users.
b) Can we ensure the same degree of security in a time-shared machine as in a dedicated machine?
No. No matter how secure an OS is, there are always loopholes and an OS can never be 100% secure. Even if
the OS provides for reasonable restrictions, the administrators must fully understand and utilize these resources
– which is a very complex task.
1.14) The issue of resource utilization shows up in different forms in different types of Uses. List what
resources must be managed carefully in the following settings:
COP 6611 – Operating Systems
Assignment 1
Sridhar Godavarthy (U32086851)
a) Mainframe or minicomputer systems – CPU time, Memory, I/O
b) Workstations connected to servers – Network bandwidth, server utilization, file servers, compute servers
and print servers
c) Hand held computers – Power consumption, Memory usage.
1.17) Describe the differences between symmetric and asymmetric multiprocessing. What are three advantages
and one disadvantage of multiprocessor systems?
A) In symmetric systems, all processors are peers – in software and hardware. i.e all processors perform all
tasks.
In asymmetric systems, the processors differ from each other( in software- where has control software and
controls the other CPUs. In Hardware- one processor is designed to be good at control and others good at
computations). One CPU acts as the master and assigns tasks to the other processors.
Advantages: Increased throughput (better performance); Economy of scale (reduction in costs by resource
sharing); Increased reliability.
Disadvantage: Increased complexity
1.18) How do clustered systems differ from multiprocessor systems? What is required for two machines
belonging to a cluster to cooperate to provide a highly available service?
A) Clustered systems are two or more individual systems coupled together and may share storage or have a
faster interconnect.
Multiprocessor systems on the other hand are much more closely coupled and generally use shared memory to
communicate. The processors generally share the same system resources.
High availability can be achieved by adding a level of redundancy to the system. A software known as the
cluster software runs on the system and each machine is monitored ( keep track of process information) by one
or more machines. When a machine fails, the monitoring machine takes up its tasks.
1.20) Consider a computer cluster consisting of two nodes running a database. Describe two ways in which the
cluster software can manage access to the data on the disk. Discuss the benefits and disadvantages of each.
A) Asymmetric clustering – The nodes can be implemented in the hot standby mode with one node performing
the database operation and the other node monitoring the first node keeping track of its state. If the first node
fails, the second node takes up the operation from where the first one failed. However, in this case, only one
node is utilized while the other node’s computation power is wasted
Symmetric clustering – In this mode, both nodes perform the operation, and monitor each other. When one
fails, the other machine takes up both the operations. In this case, the computation power is utilized, but the
system design is very complex – to handle parallel operations and proper locking and synchronization
mechanisms between the two nodes.
COP 6611 – Operating Systems
Assignment 1
Sridhar Godavarthy (U32086851)
1.21) How are network computers different from traditional personal computers? Describe some usage
scenarios in which it is advantageous to use network computers.
A) Personal computers have all the required resources local to the machine and are capable of handling all
requests locally. Network computers have very minimal resources locally( in some cases- known as dumb
terminals- only the memory and a processor) and a minimalistic operating system too. They depend on a
network server for all their resource requirements.
Systems that have a centralized storage that needs to be shared between several clients is a good scenario to
use such a setup. For example, a database class whose students need to work on a central database server.
1.22) What is the purpose of interrupts. What are the differences between a trap and an interrupt? Can traps be
generated intentionally by a user program? If so, for what purpose?
A) Interrupts are used to indicate events ( changes in state, interesting scenarios, freeing of resources, I/O
completions, errors) to the Operating System, which usually results in a change in the operating system
performing an action. Traps are software generated interrupts, whereas an interrupt is hardware generated.
Yes. Traps can be generated intentionally by user programs – All system calls are called by traps. Also,
exception handling mechanisms can generate traps to allow the O/S to handle errors.
1.26) Consider an SMP system similar to what is shown in figure Illustrate with an example how data residing
in memory could in fact have two different values in each of the local caches.
A)
Time
Operation
CPU 1 Cache
CPU 2 Cache
Memory Location G
T
BEGIN
X
X
32
T+t
CPU 1 Read G
32
X
32
T + 2t
CPU 2 Read G
32
32
32
T + 3t
CPU 1 Write G
0
32
0
Assume write through caches. Initially both caches do not cache memory location G. At T + t, the 1st CPU
issues a read on G, This causes a miss and fetches the value 32 from main memory into the cache. At T + 2t,
CPU2 issues a read on G which again causes a miss and fetches the value 32 from main memory. Now, at T +
3t, CPU 1 issues a write to memory location G. This is a hit and hence the value is updated in cache-1 and also
written back to memory. In the absence of mechanisms to notify the Cache of CPU2 of its dirty value, the
Cache of CPU2 now has a wrong value – the two caches have two different values for the same memory
location. This is shown in red at T + 3t
COP 6611 – Operating Systems
Assignment 1
Sridhar Godavarthy (U32086851)
1.27) Discuss, with examples, how the problem of maintaining coherence of cached data manifests itself in the
following processing environments:
a) Single-processor systems – In a multitasking environment, this is not very complicated. Mechanisms should
be in place to provide the data from the cache even when requested from a different process. All other
principles of caching remain the same.
b) Multiprocessor systems – As each processor will have its own cache, there must be a synchronization
between the caches of all processors. However, writing back at each access will cause performance
degradation. Instead, each cache must communicate with the others and when one cache is invalidated, all
other caches must be notified. Another method is to have a central cache control such as a snoopy cache which
will manage the synchronization. Only when the block is being removed from the cache should the block be
written-back ( for ideal caching- but this is seldom the scenario).
c) Distributed systems – As each node has its own local memory, caching is not an issue in this scenario.
However, file caching discrepancy is a scenario. To overcome this, any update to a file in any system will
result in updates to all replicas.
1.28) Describe a mechanism for enforcing memory protection in order to prevent a program from modifying
the memory associated with other programs
Allocate memory in a fixed size chunk. If more memory is requested by a process, allocate another chunk.
Keep track of chunks allocated to a process and do not allow access outside the allocated memory.
A simpler, but performance degrading method would be track each and every block of memory allocated to a
process and check the bounds at every access.
1.30) Define the essential properties of the following types of OS:
a) Batch – batch operating systems are those where several jobs that are dependent on the output of the
previous job –or- completely independent jobs are queued to run on a computer. Earlier, this was done by
operators. Such programs usually do not interact with users. Such a system provides a high CPU utilization in
single process operating systems.
b) Interactive – An OS where each job needs several inputs from the user at different points in time making it
unfit for batch processing. Generally, GUIs which require a lot of user interactions utilize this kind of OS.
c) Time sharing – The OS is accessed by multiple users simultaneously. It is capable of servicing several users
by rapid switching between the various processes. It, handles synchronization of resources and also handles
locking mechanisms for parallel access of memory.
d) Real time – Such OSs are usually used for a specific purpose. They read information from sensors and must
provide a response/action to the sensors within a specified time. The time limit is a hard limit and cannot be
broken.
COP 6611 – Operating Systems
Assignment 1
Sridhar Godavarthy (U32086851)
e) Network – The OS and resources reside on a network server and a minimalistic OS is present on other
machines. The central OS may either control or support the other machines.
f) Parallel – Capable of handling multiple processor scenarios. It is designed to synchronize the processors and
handle shared access of resources. Each CPU has the same OS running on it. All resources are common and
shared except for memory.
g) Distributed – Several machines exist with each system having the same OS. They generally communicate
over a LAN. The OS is designed to split up a specific task into multiple components. Each system performs an
individual task- generally on the same data. There are no common resources
h) Clustered – Several individual nodes are combined into a single system. Each node has its own OS running
on it. The OS communicate with each other. They can be designed to monitor each other, operate in parallel or
as a hot standby. There are no common resources.
I) Handheld – This is a very simple OS which runs on very low power, low memory, very little resources and
small display sizes and provide for longer battery life to performance.
1.32) Identify several advantages and several disadvantages of open-source operating systems. Include the
types of people who would find each aspect to be an advantage or a disadvantage.
A)
Advantage For
Disadvantage For
Advantages:
Build on others ideas
Independent developers, Students,
small businesses
Peer reviewed
Developers of the software
Better quality
Community
Competitors
Less expensive
Independent developers, Students,
small businesses
Competitors
Availability of source code to
tailor modules to individual
needs
Anybody who needs to customize
the product
Competitors, Consultants,
Independent developers
Improved security as flaws are
analyzed by "many eyes"
Community
Hackers, Cyber thieves
Several vendors for same OS
Independent developers, small
scale businesses
Competitors
Portable with wider hardware
Independent developers, Students,
Competitors,
COP 6611 – Operating Systems
Assignment 1
small businesses
Easier licensing
Independent developers, Students,
small businesses
More support in developing
stage
Independent developers, Students,
small businesses
Ability to and satisfaction from
contributing
Independent developers, Students
Sridhar Godavarthy (U32086851)
manufacturers of OS
which work on only their
H/W
Competitors
Disadvantages:
Comes without warranty or
guarantee
Competitors
Small businesses
Chances for intellectual property
infringements
Competitors, advocates
Small businesses,
Independent developers
Projects can be abandoned
Competitors
Small businesses,
Independent developers
Might not get support when
required as there is no obligation
Competitors
Small Businesses
REFERENCES:
[1] Silberschatz. Galvin and Gagne, Operating System Concepts 7ed, ©2004, John Wiley & Sons Inc.
[2] John L. Hennessy and David A. Patterson, Computer Architecture: A Quantitative Approach 4ed, © 2006,
Morgan Kaufmann