Operating Systems

1.4 What is the purpose of interrupts? How does an interrupt differ from a trap? Can traps be generated intentionally by a user program? If so, for what purpose?

An interrupt is a hardware‐generated change‐of‐flow within the system. An interrupt handler is summoned to deal with the cause of the interrupt; control is then returned to the interrupted context and instruction. A trap is a software‐generated interrupt. An interrupt can be used to signal he completion of an I/O to obviate the need for device polling. A trap can be stu ed to call operating system routines or to catch arithmetic errors.

2.13 Would it be possible for the user to develop a new command interpreter using the system-call interface provided by the operating system?

An user should be able to develop a new command interpreter using the system-call interface provided by the operating system. The command interpreter allows an user to create and manage processes and also determine ways by which they communicate (such as through pipes and files). As all of this functionality could be accessed by an user-level program using the system calls, it should be possible for the user to develop a new command-line interpreter.

1.5 How does the distinction between kernel mode and user mode function as a rudimentary form of protection (security) system?

Answer: The distinction between kernel mode and user mode pro- vides a rudimentary form of protection in the following manner. Certain instructions could be executed only when the CPU is in kernel mode. Similarly, hardware devices could be accessed only when the program is executing in kernel mode. Control over when interrupts could be en- abled or disabled is also possible only when the CPU is in kernel mode. Consequently, the CPU has very limited capability when executing in user mode, thereby enforcing protection of critical resources.

2.5 What is the main advantage of the layered approach to system design? What are the disadvantages of the layered approach?

As in all cases of modular design, designing an operating system ina modular way has several advantages. The system is easier to debug and modify because changes affect only limited sections of the system rather than touching all sections of the operating system. Information is kept only where it is needed and is accessible only within a defined and restricted area, so any bugs affecting that data must be limited to aspecific module or layer

3.8 Describe the actions taken by a kernel to context-switch between pro- cesses.

1. In response to a clock interrupt, the OS saves the PC and user stack pointer of the currently executing process, and transfers control to the kernel clock interrupt handler, 2.The clock interrupt handler saves the rest of the registers, as well as othermachine state, such as the state of the floating point registers, in the processPCB. 3.The OS invokes the scheduler to determine the next process to execute, 4.The OS then retrieves the state of the next process from its PCB, and restores the registers. This restore operation takes the processor back to the state in which this process was previously interrupted, executing in user code with user mode privileges.

1.8 Some CPUs provide for more than two modes of operation. What are two possible uses of these multiple modes?

Answer: Although most systems only distinguish between user and kernel modes, some CPUs have supported multiple modes. Multiple modes could be used to provide a finer-grained security policy. For example, rather than distinguishing between just user and kernel mode, you could distinguish between different types of user mode. Perhaps users belonging to the same group could execute each other's code. The machine would go into a specified mode when one of these users was running code. When the machine was in this mode, a member of the group could run code belonging to anyone else in the group. Another possibility would be to provide different distinctions within kernel code. For example, a specific mode could allow USBdevice drivers to run. This would mean that USB devices could be serviced without having to switch to kernel mode, thereby essentially allowing USB device drivers to run in a quasi-user/kernel mode.

2.19 What is the main advantage of the microkernel approach to system design? How do user programs and system services interact in a micro- kernel architecture? What are the disadvantages of using the microker- nel approach?

Benefits typically include the following: (a) adding a new service does not require modifying the kernel, (b) it is more secure as more operations are done in user mode than in kernel mode, and (c) a simpler kernel design and functionality typically results in a more reliable operating system. User programs and system services interact in a microkernel architecture by using interprocess communication mechanisms such as messaging. These messages are conveyed by the operating system. The primary disadvantages of the microkernel architecture are the overheads associated with interprocess communication and the frequent use of the operating system's messaging functions in order to enable the user process and the system service to interact with each other.

1.10 Give two reasons why caches are useful. What problems do they solve? What problems do they cause? If a cache can be made as large as the

Caches are useful when two or more components need to exchange data, and the components perform transfers at differing speeds. Caches solve the transfer problem by providing a buffer of intermediate speed between the components. If the fast device finds the data it needs in the cache, it need not wait for the slower device. The data in the cache must be kept consistent with the data in the components. If a component has a data value change, and the datum is also in the cache, the cache must also be updated. This is especially a problem on multiprocessor systems where more than one process may be accessing a datum. A component may be eliminated by an equal-sized cache, but only if : the cache and the component have equivalent state-saving capacity ( that is, if the component retains its data when electricity is removed, the cache must retain data as well) , and the cache is affordable, because faster storage tends to be more expensive.

1.12 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?1

Clustered systems are typically constructed by combining multiple computers into a single system to perform a computational task distributed across the cluster. Multiprocessor systems on the other hand could be a single physical entity comprising of multiple CPUs. A clustered system is less tightly coupled than a multiprocessor system. Clustered systems communicate using messages, while processors in a multiprocessor system could communicate using shared memory. In order for twomachines to provide a highly available service, the state on the two machines should be replicated and should be consistently updated. When one f the machines fail, the other could then take‐over the functionality of the ailed machine

2.8 How could a system be designed to allow a choice of operating systems from which to boot? What would the bootstrap program need to do?

Consider a system that would like to run both Windows XP and three different distributions of Linux (e.g., RedHat, Debian, and Mandrake). Each operating system will be stored on disk. During system boot-up, a special program (which we will call the boot manager) will determine which operating system to boot into. This means that rather initially booting to an operating system, the boot manager will first run during system startup. It is this boot manager that is responsible for determining which system to boot into. Typically boot managers must be stored at certain locations of the hard disk to be recognized during system startup. Boot managers often provide the user with a selection of systems to boot into; boot managers are also typically designed to boot into a default operating system if no choice is selected by the user.

1.26 Describe some distributed applications that would be appropriate for a peer-to-peer system.

Essentially anything that provides content, in addition to existing services such as file services, distributed directory services such as domain name services, and distributed e-mail services.

2.7 Why do some systems store the operating system in firmware, while others store it on disk?

For certain devices, such as handheld PDAs and cellular telephones, a disk with a file system may be not be available for the device. In this situation, the operating system must be stored in firmware.

3.15 Consider the RPC mechanism. Describe the undesirable consequences that could arise from not enforcing either the "at most once" or "exactly once" semantic. Describe possible uses for a mechanism that has neither of these guarantees.

If an RPC mechanism cannot support either the "at most once" or "at least once" semantics, then the RPC server cannot guarantee that a remote procedure will not be invoked multiple occurrences. Consider if a remote procedure were withdrawing money from a bank account on a system that did not support these semantics. It is possible that a single invocation of the remote procedure might lead to multiple withdrawals on the server. For a system to support either of these semantics generally requires the server maintain some form of client state such as the timestamp described in the text. If a system were unable to support either of these semantics, then such a system could only safely provide remote procedures that do not alter data or provide time-sensitive results. Using our bank account as an example, we certainly require "at most once" or "at least once" semantics for performing a withdrawal (or deposit!). However, an inquiry into an account balance or other account information such as name, address, etc. does not require these semantics

2.2 What is the purpose of the command interpreter? Why is it usually separate from the kernel? to do?

It reads commands from the user or from a file of commands and executesthem, usually by turning them into one or more system calls. It is usuallynot part of the kernel since the command interpreter is subject to changes.

2.16 Contrast and compare an application programming interface (API) and an application binary interface (ABI).

Make it simple, API(Application programming interface) is used in traditional computer languages, where applications, libraries or server application expose interface to others to invoke. While ABI(Application binary interface) is used in blockchain such as Ethereum, where one contract exposes interface to other contract or frontend to invoke. These two can be considered to be the same concept but with different names and used in different region

2.17 Why is the separation of mechanism and policy desirable?

Mechanism and policy must be separate to ensure that systems are easy to modify. No two system installations are the same, so each installation may want to tune the operating system to suit its needs. With mechanism and policy separate, the policy may be changed at will while the mechanism stays unchanged. This arrangement provides a more flexible system

2.15 What are the two models of interprocess communication? What are the strengths and weaknesses of the two approaches?

Message passing- strengths: program structures better separated, dangerous operations firewalled- weaknesses: message passes more slowly, for symmetrical copy operations are to be made• Shared memory- strengths: fast and direct- weaknesses: unexpected behavior when unauthentic programs wrongly accesses the shared memorysegments

2.11 Describe how you could obtain a statistical profile of the amount of time a program spends executing different sections of its code. Discuss the importance of obtaining such a statistical profile.

One could issue periodic timer interrupts and monitor what instructions or what sections of code are currently executing when the interrupts are delivered. A statistical profile of which pieces of code were active should be consistent with the time spent by the program in different sections of its code. Once such a statistical profile has been obtained, the programmer could optimize those sections of code that are consuming more of the CPU resources.

3.5 When a process creates a new process using the fork() operation, which of the following states is shared between the parent process and the child process? a. Stack b. Heap c. Shared memory segments

Only the shared memory segments are shared between theparent process and the newly forked child process. Copies of the stackand the heap are made for the newly created process.

1.27 Identify several advantages and several disadvantages of open-source operating systems. Identify the types of people who would find each aspect to be an advantage or a disadvantage.

Open source operating systems have the advantages of having many people working on them, many people debugging them, ease of access and distribution, and rapid update cycles. Further, for students and programmers, there is certainly an advantage to being able to view and modify the source code. Typically open source operating systems are free for some forms of use, usually just requiring payment for support services. Commercial operating system companies usually do not like the competition that open source operating systems bring because these features are difficult to compete against. Some open source operating systems do not offer paid support programs. Some companies avoid open source projects because they need paid support, so that they have some entity to hold accountable if there is a problem or they need help fixing an issue. Finally, some complain that a lack of discipline in the coding of open source operating systems means that backward compatibility is lacking making upgrades difficult, and that the frequent release cycle exacerbates these issues by forcing users to upgrade frequently.

1.4 Consider the various definitions of operating system. Next, consider whether the operating system should include applications such as Web browsers and mail programs. Argue both that it should and that it should not, and support your answers.

Point. Applications such as web browsers and email tools are performing an increasingly important role in modern desktop computer systems. To fulfill this role, they should be incorporated as part of the operating system. By doing so, they can provide better performance and better integration with the rest of the system. In addition, these important applications can have the same look-and-feel as the operating system software. Counterpoint. The fundamental role of the operating system is to man- age system resources such as the CPU, memory, I/O devices, etc. In ad- dition, it's role is to run software applications such as web browsers and email applications. By incorporating such applications into the operating system, we burden the operating system with additional functionality. Such a burden may result in the operating system performing a less-than- satisfactory job at managing system resources. In addition, we increase the size of the operating system thereby increasing the likelihood of system crashes and security violations.

1.20 Consider an SMP system similar to the one shown in Figure 1.8. Illustrate with an example how data residing in memory could in fact have a different value in each of the local caches.

Say processor 1 reads dataAwith value 5 from main memory into itslocal cache. Similarly, processor 2 reads dataAinto its local cache as well.Processor 1 then updatesAto 10. However, sinceAresides in processor1's local cache, the update only occurs there and not in the local cacheforprocessor2.

1.2 We have stressed the need for an operating system to make efficient use of the computing hardware. When is it appropriate for the operating system to forsake this principle and to "waste" resources? Why is such a system not really wasteful?

Single-user systems should maximize use of the system for the user. A GUI might "waste" CPU cycles, but it optimizes the user's interaction with the system. GUI

2.1 What is the purpose of system calls?

System calls allow user-level processes to request services of the operat-ing system

2.4 What is the purpose of system programs?

System programs can be thought of as bundles of useful system calls.They provide basic functionality to users so that users do not need to write their own programs to solve common problems.

1.11 Distinguish between the client-server and peer-to-peer models of dis- tributed systems.

The client-server model firmly distinguishes the roles of the client and server. Under this model, the client requests services that are provided by the server. The peer-to-peer model doesn't have such strict roles. In fact, all nodes in the system are considered peers and thus may act as either clients or servers - or both. A node may request a service from another peer, or the node may in fact provide such a service to other peers in the system. For example, let's consider a system of nodes that share cooking recipes. Under the client-server model, all recipes are stored with the server. If a client wishes to access a recipe, it must request the recipe from the specified server. Using the peer-to-peer model, a peer node could ask other peer nodes for the specified recipe. The node (or perhaps nodes) with the requested recipe could provide it to the requesting node. Notice how each peer may act as both a client (i.e. it may request recipes) and as a server (it may provide recipes.)

1.8 Many SMP systems have different levels of caches; one level is local to each processing core, and another level is shared among all processing cores. Why are caching systems designed this way?

The different levels are based on access speed as as well as size. Ingeneral, the closer the cache is to theCPU, the faster the access. However,faster caches are typically more costly. Therefore, smaller and fastercaches are placed local to eachCPU, and shared caches that are larger,yet slower, are shared among several different processors.

1.6 Which of the following instructions should be privileged? a. Set value of timer. b. Read the clock. c. Clear memory. d. Issue a trap instruction. e. Turn off interrupts. f. Modify entries in device-status table. g. Switch from user to kernel mode. h. Access I/O device.

The following operations need to be privileged: Set value of timer, clear memory, turn off interrupts, modify entries in device-status table, access I/O device. The rest can be performed in user mode.

1.24 Describe some of the challenges of designing operating systems for mobile devices compared with designing operating systems for tradi- tional PCs.

The greatest challenges in designing mobile operating systems include: Less storage capacity means the operating system must manage memory carefully. The operating system must also manage power consumption carefully. Less processing power plus fewer processors mean the operating system must carefully apportion processors to applications.

1.3 What is the main difficulty that a programmer must overcome in writing an operating system for a real-time environment?

The main difficulty is keeping the operating system within the fixed time constraints of a real-time system. If the system does not complete a task in a certain time frame, it may cause a breakdown of the entire system it is running. Therefore when writing an operating system for a real-time system, the writer must be sure that his scheduling schemes don't allow response time to exceed the time constraint.

3.6 Consider the "exactly once"semantic with respect to the RPC mechanism. Does the algorithm for implementing this semantic execute correctly even if the ACK message sent back to the client is lost due to a net- work problem? Describe the sequence of messages, and discuss whether "exactly once" is still preserved.

The"exactly once"semantics ensure that a remore procedure will be executed exactly once and only once. The general algorithm for ensuring this combines an acknowledgment (ACK) scheme combined with timestamps (or some other incremental counter that allows the server to distinguish between duplicate messages).The general strategy is for the client to send the RPC to the server along with a timestamp. The client will also start a timeout clock. The client will then wait for one of two occurrences: (1) it will receive an ACK from the server indicating that the remote procedure was performed, or (2) it will time out. If the client times out, it assumes the server was unable to perform the remote procedure so the client invokes the RPC a second time, sending a later timestamp. The client may not receive the ACK for one of two reasons: (1) the original RPC was never received by the server,or (2) the RPC was correctly received—and performed—by the server but theACK was lost. In situation (1), the use of ACKs allows the server ultimately to receive and perform the RPC. In situation (2), the server will receive a duplicate RPC and it will use the timestamp to identify it as a duplicate so as not to perform the RPC a second time. It is important to note that the server must send a second ACK back to the client to inform the client the RPC has been performed.

1.6 Direct memory access is used for high-speed I/O devices in order to avoid increasing the CPU's execution load. How does the CPU interface with the device to coordinate the transfer? How does the CPU know when the memory operations are com- plete? The CPU is allowed to execute other programs while the DMA controller is transferring data. Does this process interfere with the execution of the user programs? If so, describe what forms of interference are caused.

To initiate a DMA transfer, the CPU first sets up the DMA registers, which contain a pointer to the source of a transfer, a pointer to the destination of the transfer, and a counter of the number of bytes to be transferred. Then the DMA controller proceeds to place addresses on the bus to perform transfers, while the CPU is available to accomplish other work. b. Once the entire transfer is finished, the DMA controller interrupts the CPU. c. Both the CPU and the DMA controller are bus masters. A problem would be created if both the CPU and the DMA controller want to access the memory at the same time. Accordingly, the CPU should be momentarily prevented from accessing main memory when the DMA controller seizes the memory bus. However, if the CPU is still allowed to access data in its primary and secondary caches, a coherency issue may be created if both the CPU and the DMA controller update the same memory locations

3.10 Explain the role of the init (or systemd) process on UNIX and Linux systems in regard to process termination.

When a process is terminated, it briefly moves to the zombie state and remains in that state until the parent invokes a call to wait(). When this occurs, the process id as well as entry in the process table are both released. However, if a parent does not invoke wait(), the child process remains a zombie as long as the parent remains alive. Once the parent process terminates, the init process becomes the new parent of the zombie. Periodically, the init process calls wait() which ultimately releases the pid and entry in the process table of the zombie process.

2.6 List five services provided by an operating system, and explain how each creates convenience for users. In which cases would it be impossible for user-level programs to provide these services? Explain your answer.

a. Program execution. The operating system loads the contents (or sections) of a file into memory and begins its execution. A user-level program could not be trusted to properly allocate CPU time. b. I/O operations. Disks, tapes, serial lines, and other devices must be communicated with at a very low level. The user need only specify the device and the operation to perform on it, while the system converts that request into device- or controller-specific commands. User-level programs cannot be trusted to access only devices they Practice Exercises 7 should have access to and to access them only when they are otherwise unused. c. File-system manipulation. There are many details in file creation, deletion, allocation, and naming that users should not have to perform. Blocks of disk space are used by files and must be tracked. Deleting a file requires removing the name file information and freeing the allocated blocks. Protections must also be checked to assure proper file access. User programs could neither ensure adherence to protection methods nor be trusted to allocate only free blocks and deallocate blocks on file deletion. d. Communications. Message passing between systems requires messages to be turned into packets of information, sent to the network controller, transmitted across a communications medium, and reassembled by the destination system. Packet ordering and data correction must take place. Again, user programs might not coordinate access to the network device, or they might receive packets destined for other processes. e. Error detection. Error detection occurs at both the hardware and software levels. At the hardware level, all data transfers must be inspected to ensure that data have not been corrupted in transit. All data on media must be checked to be sure they have not changed since they were written to the media. At the software level, media must be checked for data consistency; for instance, whether the number of allocated and unallocated blocks of storage match the total number on the device. There, errors are frequently process- independent (for instance, the corruption of data on a disk), so there must be a global program (the operating system) that handles all types of errors. Also, by having errors processed by the operating system, processes need not contain code to catch and correct all the errors possible on a system.

2.20 What are the advantages of using loadable kernel modules?

it is difficult to predict what features an operating system will need when it is being designed. The advantage of using loadable kernel modules is that functionality can be added to and removed from the kernel while it is running. There is no need to either recompile or reboot the kernel.

2.3 What system calls have to be executed by a command interpreter or shell in order to start a new process on a UNIX system?

n Unix systems, a fork system call followed by an exec system call need to be performed to start a new process. The fork call clones the currently executing process, while the exec call overlays a new process based on a different executable over the calling process.

1.1 What are the three main purposes of an operating system?

• To provide an environment for a computer user to execute programs on computer hardware in a convenient and efficient manner. • To allocate the separate resources of the computer as needed to solve the problem given. The allocation process should be as fair and efficient as possible. • As a control program it serves two major functions: (1) supervision of the execution of user programs to prevent errors and improper use of the computer, and (2) management of the operation and control of I/O devices. EAC environment - for USERS to execute programs allocate control