CSI3 131 – Operating Systems Tutoring 1 – Solution
Operating Systems | Network | network代写 | 作业shell – 本题是一个利用Operating Systems进行练习的代做, 对Operating Systems的流程进行训练解析, 是比较有代表性的Operating Systems/Network/network/shell等代写方向
Tutoring 1 – Solution
1. What are the three main roles of the operating system?
Hardware abstraction: To provide an environment in which a computer user can run programs in various
hardware easily and efficiently.
Resource allocation: To allocate the various computer resources as required by the programs. The
allocation should be fair and effective as much as possible.
Control program: A control program has two main functions: (1) overseeing the execution of user programs
to prevent errors and misuse of the computer; and (2) operation management and control of I/O devices.
2. Consider the various definitions of the operating system. Consider including apps such as WEB
browsers or email programs in the operating system. Debate the pros and cons of this issue with
justification.
For: Applications such as web browsers and email tools are playing an increasingly important role in the use of
the modern PC. To meet these needs, these applications could be incorporated into the operating system. This
could lead to better performance and integration with the rest of the system. In addition, these important
applications could have the same look and feel of the system software of the OS.
Cons: The fundamental role of the operating system is to manage system resources such as CPU, memory, I/O
devices, etc. In addition, its role is to run software applications such as WEB browsers and email applications.
By incorporating such applications into the OS, it is weighed down with these additional functions. This
additional burden may result in an OS performing its system resource management task less well. In addition,
the size of the OS is increased, increasing the possibility of the OS crashing and breaches of safety.
3. How does the distinction between kernel and user mode serve the rudimentary protection (security)
of the system?
The distinction between the core and the user mode offers a rudimentary form of protection in the following
way. Some instructions can only be executed when the CPU is in kernel mode. Also, devices can only be
accessed by a program when it runs in kernel mode (i.e. within a system call). The activation and deactivation
control of interruptions is only done when the CPU is in kernel mode. Therefore, the CPU is quite limited when
it runs in user mode, and thus protects critical resources while running user code.
4. Which instructions should require kernel privileges?
a. Timer update.
b. Read the clock.
c. Clear the memory.
d. Run a software interrupt (this is a CPU instruction).
e. Turn off interruptions.
f. Change entries in a device state table.
g. Switch between user mode and kernel mode.
h. Access an I/O device.
5. A timer can be used to determine the current time. Give a short description of how to accomplish
this task.
A program can use the following approach to determine current time with timer interruptions. The program
sets up the timer to generate an interruption after a specific time and then be suspended (fall asleep). When
activated by the interrupt, it can update a variable whose value reflects the number of interruptions received to
date. These steps are repeated continuously, i.e., set up the interruption and update the variable when the
interruption is produced. The variable can thus track time count, especially if it is initialized to a significant
value. UNIX OSs uses such an approach by counting the number of seconds since Jan 1, 1970, with a 32-bit
counter (overflow will take place on Jan 19, 2038).
6. What is the purpose of system calls?
System calls allow user processes to make requests for services to the operating system. Note that system calls are made with software interruptions. Be aware that the OS works on interruptions and that it may receive an interruption of hardware AND software (i.e. user programs) as a request to complete any action.
7. What are the five main activities of the operating system when managing processes?
a. The creation and termination of user and system processes.
b. Suspension and resumption of processes.
c. The provision of mechanisms for process synchronization.
d. The provision of mechanisms for communication between processes.
e. The provision of mechanisms to deal with deadlocks.
8. What are the three main activities of the operating system when managing memory?
a. Consider which parts of the memory are used and by what processes.
b. Decide which processes are loaded into memory when memory space becomes available.
c. The allocation and release of memory space as needed.
9. What are the three main activities of the operating system when managing secondary storage
memory?
a. The management of free space.
b. Secondary memory allocation.
c. The disk scheduling.
10. What is the purpose of the Command Interpreter (CLI)?
It reads user commands or a command file is either running them directly or by launching another process to
run a separate program.
11. What system calls are run by the command interpreter or shell to start a new process?
In the UNIX system, the system calls fork followed by exec must be made to initiate a new process. The call fork clones the process that makes the call, while the exec call replaces the program in the process with a new program. This is a difficult concept – what needs to be taken into account is that fork is run by the original process (parent), while the exec is run by the launched process (child). We will be reviewing these calls this week.
12. What is the purpose of system programs?
System programs can be seen as system call software. It offers basic functions to users to avoid forcing them to
write programs to solve common problems. Common systems programs include utilities to manipulate files and
directories, shells to run programs (including utility systems), utilities to monitor the user base and network (e.g.
UNIX who shows which users have an open session in the local system and remote systems), process
management, etc.
13. What is the main advantage of designing a system with layers? What are the disadvantages of this
approach?
As in all modular design cases, the design of an operating system with a modular approach has several
advantages. The system is easier to debug and modify since the changes only affect limited parts instead of
affecting all sections of the OS. Data is maintained only where necessary and accessible from a well-defined and
limited region, which means that bugs affecting data are limited to a specific module or layer.
14. For each of the following five services offered by an operating system, explain how each of them is
convenient for the user. Explain when it would be impossible to offer these services with user
programs.
Program execution. The operating system loads the content (or parts of the content) of an executable file
into primary memory and initiates its execution. It allocates resources to the program during its
implementation (e.g. CPU). It is not possible to trust a user program to make a good allocation of resources
such as CPU.
I/O operations. Communication with discs, magnetic tapes, serial lines of communication, and other
devices must be done at a very low level. The user program only specifies the device (in principle as a file)
and the operations to be done with the device, and the OS converts these requests into specific instructions to
the device controller. It is not possible to trust a user program to access the devices to which they are
entitled, and when they are free.
Manipulating the file system. There are many details for file creation, file deletion, file space allocation,
and file name definition. The disk is organized into blocks of data that need to be managed. Protection must
ensure proper access to the files. It is impossible to trust user programs to adhere to the protection rules;
allocate only free blocks to the creation and updating of files; and release the blocks when deleting the file.
Communications. The exchange of messages between systems (and processes) requires that messages be
transformed into data packets that are sent to a communication controller, transmitted via a transmission
medium, and reassorted into the remote system. Packets need to be ordered and corrected as well. Again, a
user program might not make a good access to the communication device controller or could take packages
for other processes.
Error detection. Error detection is done at the hardware and logical level. At the hardware level, all data
transfers must be inspected to ensure that the data was not corrupted during transit. All supporting data
must be verified to ensure that they have not changed since registering on the support. At the logical level,
the support must be checked for data deterioration; for example, ensuring that the number of blocks
allocated and free blocks of secondary memory correspond to the total number of blocks of the device.
Errors in these cases are often independent of specific processes (for example, data corruption on a disk),
and therefore a global program (the OS) has to deal with this type of error. Also, by having the OS deal
with this type of problem, it is not necessary to include in programs users of code to deal with all possible
errors in the system.
In addition to the reasons given above against having user programs deal directly with these services, imagine the work to create even a simple program if the programmer would have to deal with all the issues discussed above.