ACSC271 Operating Systems Answer Sheet: 3 Semester: Fall 2017 Instructor:

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1 ACSC271 Operating Systems Answer Sheet: 3 Semester: Fall 2017 Instructor: Christos Markides 1) Informally, a process is a program in execution. A process is more than the program code, which is sometimes known as the text section. It also includes the current activity are represented by the value of the program counter and the contents of the CPU s registers. A program is a passive entity such as the contents of a file stored on a disk, whereas a process is an active entity. It might be the case that there are two instances of a web browser program invoked by the same user. An example of a process is the execution of an editor program and the associated program i.e. notepad.exe. 2) Only the shared memory segments are shared between the parent process and the newly forked child process. Copies of the stack and the heap are made for the newly created process. 3) The Process Control Block contains information associated with a specific process such as the process state, the program counter, the CPU registers, the memory limits and the associated list of open files. Finally there is a pointer structure indicating the next process in the list. The following diagram illustrates this: 4) One basic piece of information held by the PCB is the process state and a pointer. (a) The process state if it was just admitted would be ready. (b) The process state upon finalising execution would be terminated and the pointer would point to the next process, to be admitted. Page 1 of 5

2 5) The following are defined as the state that a process may be in: New: The Process is created, Running: Instructions are being executed, Waiting: The process is waiting for some event to occur, usually I/O event, Ready: The Process is waiting to be assigned to a processor and Terminated: The process has finished executing. 6) When the CPU switches to another process, the system must save the state of the old process and load the saved state for the new process via a context switch. The context of a process is represented in the PCB, which is stored in memory. The Context-switch time is overhead (i.e. the time that it takes to unload and load another process, while the CPU is idle, thus the system does no useful work while switching). This is time dependent on hardware support. 7) The processes entering the system are put into a job queue. The processes waiting to be executed or are ready to execute are put in the ready queue. The processes waiting for an I/O device are put in the device queue. Processes migrate from one queue to another. 8) The long-term scheduler (or job scheduler) selects which processes should be brought into the ready queue. The short-term scheduler (or CPU scheduler) selects which process should be executed next and allocates CPU. The Short-term scheduler is invoked very frequently (milliseconds and must be fast). The long-term scheduler is invoked very infrequently (seconds, minutes and may be slow). The long-term scheduler controls the degree of multiprogramming. Page 2 of 5

3 9) The parent process can create children processes, which, in turn can create other processes, forming a tree of processes. In this case each process is generally identified and managed via a process identifier (PID). Processes can share resources using the following schemes (a) Parent and children share all resources, or (b) Children share subset of parent s resources, and (c) Parent and child share no resources. In terms of execution the parent and children can execute concurrently, or the parent waits until all children terminate. In terms of the address space in memory the child process is a duplicate of the parent process, or the child process has a program loaded into it. In UNIX the system calls used are: fork system call which creates a new process; and the execv system call used after a fork to replace the process memory space with a new program. The functions getppid() and getpid() return the parent and child PID respectively. 10) Zombie processes are created, when the parent process has terminated before the child, or the child has seized to respond to the operating system s system calls. In UNIX a zombie process appears with a Z letter in the status and the <defunct> (dysfunctional) prefix in the name of the process. 11) At system startup the first two processes aren t real processes, because they are not running a user mode.exe (executable), and no user-mode address space is allocated. The data structures for these processes (and their initial threads) are pre-created in NtosKrnl.Exe and loaded along with the code. The first (Idle) Process id 0 is part of the loaded system image and serves as home for idle thread(s) (neither a real process nor real threads). This is called System Process in many displays. The second (System) Process id 4 in 7/10 is part of the loaded system image and home for kerneldefined threads (not a real process). This Process is the Thread 0 (routine name Phase 1 of Initialization), which launches the first real process, running smss.exe (Session Manager). The Process then becomes the zero page thread. The smss.exe (Session Manager) is the first created process. It takes parameters from \HKEY_LOCAL_MACHINE\System\CurrentControlSet\Control\Session. The Session Manager launches the required Windows Subsystems (csrss.exe) and then the logon process (winlogon.exe). The Logon process (winlogon.exe) launches the Service Controller (services.exe) and the Local Security Authentication Server (lsass.exe). The Logon Process presents the first login prompt and when someone logs in, launches applications as stored in \Software\Microsoft\Windows NT\WinLogon\Userinit. The Service Controller (services.exe) is also home for many Windowssupplied services and is responsible for starting services based on the registry entries in \Registry\Machine\System\CurrentControlSet\Services. The Logon Process (winlogon.exe) also launches the user shell (userinit.exe) which is started after logon and starts the Desktop (explorer.exe) and exits (hence Explorer appears to be an orphan without a parent PID). (see \Software\Microsoft\Windows NT\CurrentVersion\WinLogon\Shell) Page 3 of 5

4 12) The Svchost.exe file is located in the %SystemRoot%\System32 folder. At startup, Svchost.exe checks the services part of the registry to construct a list of services that it must load. Multiple instances of Svchost.exe can run at the same time. Each Svchost.exe session can contain a grouping of services. Therefore, separate services can run, depending on how and where Svchost.exe is started. This grouping of services allows for better control and easier debugging. On Windows 7/10, at least four Svchost processes are created: SYSTEM, SYSTEM (2nd instance for RPC), LOCAL SERVICE, and NETWORK SERVICE. To view a list of services running under Svchost type Tasklist /SVC in the command prompt. 13) In Windows services are defined in the registry location HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services. Each key represents an installed service. The mandatory information kept on each service is the type of service (Windows, Driver,...) the imagename of service.exe (the executable), and the start type (automatic, manual, or disabled). In recent versions of Windows each service also keeps a description, associated dependencies (files, drivers, etc) and the account & password to run under. Some services also have other parameters passed as subkeys such as run at startup. At install time, the Setup application tells the Service Controller about the service to be installed and the associated information. Setup Application CreateService Registry At System boot and initialization the Service Control Manager (SCM) reads the registry, starts the services as directed. In terms of Management and maintenance, the Control panel can start and stop services as well as change the startup parameters (Automatic, Manual, Disabled) Control Panel Service Controller/ Manager (Services.Exe) Service Processes 14) A process is created in Windows by first initialising the strartup information and the process information memory space. The CreateProcess() system call is then issued with all the parameters needed to create a new process. If the process creation is successful a handle (pointer) is returned on the startup information and the process information memory space. The Process Handle Table is also updated, which contains the handles for each process (unique for each process). The Process Handle Tables is in system address space, hence cannot be modified from user mode. Security checks are made when handle table entry is created after i.e. a CreateXxx system call is issued. The Handle table entry indicates the validated access rights to the object (Read, Write, Delete, Terminate, etc). Now both the parent and the child continue executing. The parent process will wait for the child to finish executing with the WaitForSingleObject() system call. After the child has finished executing all handles can then close on the startup and process information memory space. The figure below illustrates how a process is created and also a thread can be created within a process. (a) Windows 32-bit (b) Windows 64-bit Page 4 of 5

5 15) Windows Error Reporting (WER) is a sophisticated mechanism that automates the submission of both user-mode process crashes as well as kernel-mode system crashes in Windows 7/10. Windows Error Reporting can be configured by going to Control Panel, choosing Action Center, When an unhandled exception is caught by the unhandled exception, it builds context information (such as the current value of the registers and stack) and opens an Advanced Local Procedure Calls (ALPC) port connection to the WER service. This service begins to analyse the crashed program s state and performs the appropriate actions to notify the user. As described previously, in most cases this means launching the WerFault.exe program, which executes with the current user s credentials and, unless the system is configured not to, displays a message box informing the user of the crash. On default configured systems, an error report (a minidump and an XML file with various details, such as the DLL version numbers loaded in the process) is sent to Microsoft s online crash analysis server. Eventually, as the service is notified of a solution for a problem, it will display a tooltip to the user informing her of steps that should be taken to solve the problem. An entry will also be displayed in the Action Center. Furthermore, the Reliability Monitor will also show all instances of application and system crashes. In environments where systems are not connected to the Internet or where the administrator wants to control which error reports are submitted to Microsoft, the destination for the error report can be configured to be an internal file server. Microsoft provides to qualified customers a tool set called Corporate Error Reporting that understands the directory structure created by Windows Error Reporting and provides the administrator with the option to take selective error reports and submit them to Microsoft. Page 5 of 5

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