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Review What is an operating system • The first program • A program that lets you run other programs • A program that provides controlled access to resources: – CPU – Memory – Display, keyboard, mouse – Persistent storage – Network Tanenbaum & Bo, Modern Operating Systems:4th ed., (c) 2013 Prentice-Hall, Inc. All rights reserved. Operating System Structure www.cs rutgers.edu/~pxk kernel • Core component of the operating system: the central program that manages resources and scheduling – Controls execution of programs – Schedules – Allocates memory – Allows programs to controlled access to devices Processor Modes • Mode bit: Supervisor or User mode • Supervisor mode – Can execute all machine instructions – Can reference all memory locations • User mode – Can only execute a subset of instructions – Can only reference a subset of memory locations Operating Systems: A Modern Perspective, Chapter 3 System Calls other stuff Kernel portion of address space kernel stack kernel text trap into kernel User portion of address space write(fd, buf, len) Interrupt Vector • Table set up by OS kernel; pointers to code to run on different events System Boot In the beginning… • Memory is initialized • Initial program loaded from boot device – Early days switches – ROM • BIOS : Basic Input/Output System – Low level software for accessing basic I/O – Code to load stage 1 boot loader When the computer is booted: • BIOS is started – Checks RAM size – Checks devices present – Determines the boot device • Typically one of several disks tagged as such – Reads block 0 from that device into memory at a fixed location and jumps there. First Stage of Boot Loader • This first block(0) is called the Master Boot Record (MBR) • Contains first stage of boot loader • Contains a program that normally examines the partition table at the end of sector – Loads the Volume Boot Record (VBR) Second Stage of Boot Loader • The secondary boot loader is read from the partition identified in the VBR • The loader reads in the operating system from the active partition and starts it. • May offer user a choice of operating systems Transfer of Control • When the boot loader finishes loading the OS, it transfer control to it • The OS will initialize itself and load various modules as needed. • The OS queries the BIOS to get information about the devices. Good-bye BIOS • Univided Extensible Firmware Interface (UEFI) – Or EFI • Created for 32bit and 64 bit architectures • Goal – Create a successor to BIOS • Not restrictions in addressing UEFI • BIOS Components preserved – Power management – System management (date, etc) • Support for larger disks • No need to start up in 16-bit (real) mode – Access to all memory • Boot manager lets you select OS – No need for MBR Processes Main Points • Process Concept • Creating and managing processes – fork, exec, wait • Communicating between processes • Example: implementing a shell Objectives • To introduce the notion of a process -a program in execution, which forms the basis of all computation • To describe the various features of processes, including scheduling, creation and termination, and communication Process vs. Program • Program – Code and static data stored in a file • Process is an instance of an executing program. – – – – Has its own address pace Memory map Open files/ pending signals Processor state • Program counter • CPU registers • Relationship: – Many processes can be running the same program Process Concept • An operating system executes a variety of programs: – Batch system – jobs – Time-shared systems – user programs or tasks • Textbook uses the terms job and process almost interchangeably Operating System View of Process • At start of process, the Operating System’s duties include: – Load program into memory – Allocate memory for program data – Set up kernel bookkeeping: Process ID Priority User IDs Process State Address of executing instruction Address of return instruction Typical Process Layout • Libraries provide the glue between user processes and the OS – libc linked in with all C programs – Provides cout, malloc, and a whole slew of other routines necessary for programs Activation Records Stack OBJECT1 OBJECT2 Heap HELLO WORLD GO BIG RED CS! Data cout.write( “string”, size) { create the string to be printed SYSCALL 80 } malloc() { … } strcmp() { … } Library Text main() { cout <<“HELLO WORLD”; ! Program Stack and Stack Frames square (int x) STACK { return x*x } doCalc(int val) { printf (“square is %d\n”, square(val) } Frame for square Frame for doCalc main (int x, int y) { key = 9999 doCalc (key) } Frame for main Frames for C run-time start up functions Process in Memory Supervisor and User Memory Supervisor Process Supervisor Space User Process User Space Operating Systems: A Modern Perspective, Chapter 3 Full System Layout • The OS is omnipresent and steps in where necessary to aid application execution Kernel Activation Records USER OBJECT1 OBJECT2 LINUX syscall_entry_point() { … } – Typically resides in high memory • When an application needs to perform a privileged operation, it needs to invoke the OS OS Stack OS Heap OS Data OS Text Activation Records OBJECT1 OBJECT2 HELLO WORLD GO BIG RED CS! printf(char * fmt, …) { Stack Heap Data Library main() { … } Program The Process Model One physical program counter Figure 2-1. (a) Multiprogramming of four programs in memory. Tanenbaum & Bo,Modern Operating Systems:4th ed., (c) 2013 Prentice-Hall, Inc. All rights reserved. Processes in a Multitasking Environment • Each process has a unique identifier PID • Asynchronous events(interrupts) may occur • Processes may have requests running that take a long time • OS Goal: • Always have some process running. • Context saving/switching • Processes may be suspended and resumed • Need to save/restore state about process The Process Model A logical program counter stored in the process and loaded into physical counter Figure 2-1. (b) Conceptual model of four independent, sequential processes. Tanenbaum & Bo,Modern Operating Systems:4th ed., (c) 2013 Prentice-Hall, Inc. All rights reserved. Diagram of Process State Process State • As a process executes, it changes state – new: The process is being created – running: Instructions are being executed – waiting: The process is waiting for some event to occur – ready: The process is waiting to be assigned to a processor – terminated: The process has finished execution Process Concept • Process: an instance of a program, running with limited rights – Process control block: the data structure the OS uses to keep track of a process – Two parts to a process: • Thread: a sequence of instructions within a process – Potentially many threads per process (for now 1:1) – Thread aka lightweight process • Address space: set of rights of a process – Memory that the process can access – Other permissions the process has (e.g., which procedure calls it can make, what files it can access) Process Control Block (PCB) Process Control Blocks PID Terminated children Link Return code PID Terminated children Link Return code PID Terminated children Link Return code Process Control Block Process Control Block (PCB) Bookkeeping/Management of a process Includes: • • • • • • • • • Process ID Program Counter CPU Registers CPU scheduling information Priority Process state Memory management information Accounting information List of I/O devices allocated to process PCB Figure 2-4. Some of the fields of a typical process table entry. Tanenbaum & Bo,Modern Operating Systems:4th ed., (c) 2013 Prentice-Hall, Inc. All rights reserved. Environment Variables • Every process has an environment block that contains a set of environment variables and their values. • There are two types of environment variables: • user environment variables (set for each user) • system environment variables (set for everyone) • Some library functions allow their behavior to change based on environment variables. Microsoft Developer Resources http://linuxcourse.rutgers.edu/rute/node12.html Diagram of Process State Process Creation Four principal events that cause processes to be created: 1. System initialization. 2. Execution of a process creation system call by a running process. 3. A user request to create a new process. 4. Initiation of a batch job. Tanenbaum & Bo,Modern Operating Systems:4th ed., (c) 2013 Prentice-Hall, Inc. All rights reserved. Process Creation Parent process create children processes, which, in turn create other processes, forming a tree of processes UNIX examples fork system call creates new process Child process is almost an exact duplicate of parent process exec system call used after a fork to replace the process’ memory space with a new program Process Creation (Cont) • Generally, process identified and managed via a process identifier (pid) • Resource sharing – Parent and children share all resources – Children share subset of parent’s resources – Parent and child share no resources • Execution – Parent and children execute concurrently – Parent waits until children terminate • Address space – Child duplicate of parent – Child has a program loaded into it UNIX Process Management • UNIX fork – system call to create a copy of the current process, and start it running – No arguments! • UNIX exec – system call to change the program being run by the current process • UNIX wait – system call to wait for a process to finish • UNIX signal – system call to send a notification to another process Process Creation Parent my perform other actions here Child statuses passed to parent Memory of parent copied to child Kernel restarts parent C Program Forking Separate Process #include < iostream> # include <sys/types.h> #include <unistd.h> int main() { pid_t pid; /* fork another process */ pid = fork(); if (pid < 0) { /* error occurred */ cout << “ Fork Failed” << endl; exit(-1); } else if (pid == 0) { /* child process */ execlp("/bin/ls", "ls", NULL); } else { /* parent process */ /* parent will wait for the child to complete */ wait (NULL); cout << Child Complete << endl; exit(0); } } UNIX Process Management Question: What does this code print? int child_pid = fork(); if (child_pid == 0) { // I'm the child process cout << “ I am process “ << getpid() << endl; return 0; } else { // I'm the parent process cout << “ I am parent of process “<<child_pid<<endl; return 0; } Implementing UNIX fork Steps to implement UNIX fork – Create and initialize the process control block (PCB) in the kernel – Create a new address space – Initialize the address space with a copy of the entire contents of the address space of the parent – Inherit the execution context of the parent (e.g., any open files) – Inform the scheduler that the new process is ready to run Implementing UNIX exec • Steps to implement UNIX fork – Load the program into the current address space – Copy arguments into memory in the address space – Initialize the hardware context to start execution at ``start'' Windows CreateProcess • System call to create a new process to run a program – Create and initialize the process control block (PCB) in the kernel – Create and initialize a new address space – Load the program into the address space – Copy arguments into memory in the address space – Initialize the hardware context to start execution at ``start'’ – Inform the scheduler that the new process is ready to run Windows CreateProcess API (simplified) if (!CreateProcess( NULL, // No module name (use command line) argv[1], // Command line NULL, // Process handle not inheritable NULL, // Thread handle not inheritable FALSE, // Set handle inheritance to FALSE 0, // No creation flags NULL, // Use parent's environment block NULL, // Use parent's starting directory &si, // Pointer to STARTUPINFO structure &pi ) // Pointer to PROCESS_INFORMATION structure ) Diagram of Process State System Call Using the trap Instruction … fork(); … Trap Table fork() { … trap N_SYS_FORK() … } Kernel sys_fork() sys_fork() { /* system function */ … return; } Operating Systems: A Modern Perspective, Chapter 3 Context Switch • When 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 • Context of a process represented in the PCB • Context-switch time is overhead; the system does no useful work while switching • Time dependent on hardware support CPU Switch From Process to Process CPU Switch From Process to Process Diagram of Process State Process Termination Typical conditions which terminate a process: 1. Normal exit (voluntary). 2. Error exit (voluntary). 3. Fatal error (involuntary). 4. Killed by another process (involuntary). Tanenbaum & Bo,Modern Operating Systems:4th ed., (c) 2013 Prentice-Hall, Inc. All rights reserved. Process Termination • Process executes last statement and asks the operating system to delete it (exit) – Output data from child to parent (via wait) – Process’ resources are deallocated by operating system • Parent may terminate execution of children processes (abort) – Child has exceeded allocated resources – Task assigned to child is no longer required – If parent is exiting • Some operating system do not allow child to continue if its parent terminates – All children terminated - cascading termination Process Termination Parent Process It is useful for the parent to know how and when the child terminates Waiting on a child process: Wait() system call waits for one (any one) of the child processes to end Returns the process ID (PID) of the child being terminated Returns status of the child Kernel adds the process CPU times and resource statistic to the running total for all children of the parent process Releases resources held by child to system pool SIGCHLD signal Q.E.D. Orphans and Zombies Orphan Parents ends before Child process Who becomes the parent? INIT, the ancestor of all processes. INIT will periodically clean up orphans Zombie Child terminates before the parent performs wait() Kernel turns process into ZOMBIE Most resources held by child are returned to the system pool Un-killable processes – Are waiting for their parent to issue wait() Wait for parents to exit when they will become orphans Big problem for daemons. The UNIX Architecture Interactive User Libraries Commands Application Programs … OS System Call Interface … Device Driver Operating Systems: A Modern Perspective, Chapter 3 Trap Table Driver Interface Device Driver Device Driver Monolithic Kernel Module •Process Management •Memory Management •File Management •Device Mgmt Infrastructure Introduction to Interprocess Communcaiton Interprocess Communication • Processes within a system may be independent or cooperating • Cooperating process can affect or be affected by other processes, including sharing data • Reasons for cooperating processes: – – – – Information sharing Computation speedup Modularity Convenience • Cooperating processes need interprocess communication (IPC) • Two models of IPC – Shared memory – Data Transfer Cooperating Processes • Independent process cannot affect or be affected by the execution of another process • Cooperating process can affect or be affected by the execution of another process • Advantages of process cooperation – Information sharing – Computation speed-up – Modularity – Convenience Examples of IPC System PIPES Data streaming Direct communication Used to connect 2 processes Ordinary pipes Producer – Consumer fashion Cannot be accessed outside the process that creates it. Usually Parent – Child communication Deleted when process terminates Named pipes Alias: FIFO in unix Appear as typical files in the system Pipes • • • • 2 processes set up pipes in advance Communication is in one direction Byte stream … show example Implementing a shell System Call : fork() • Create a child process • Returns to both the child and parent System Call : exec() • Run application in current process exec(prog, args) Execl( pathname, arg[0], arg[1]…. 0) Excelp(arg[0], arg[1]…. 0) http://www.yolinux.com/TUTORIALS/ForkExecPr ocesses.html System Call: wait() • Pause until child process has exited wait(): Blocks calling process until the child process terminates. If child process has already teminated, the wait() call returns immediately. if the calling process has multiple child processes, the function returns when one returns. waitpid(): Options available to block calling process for a particular child process not the first one • http://www.yolinux.com/TUTORIALS/ForkExecProcesse s.html System call : system() • Invokes the command processor to execute a command. • Uses fork() exec() and wait() • The call "blocks" and waits for the task to be performed before continuing. system(“ls –l”); UNIX Signal • Facility for one process to send another instant notification • Send an interrupt to a process signal(processID, type) • Signal Handling example – http://www.yolinux.com/TUTORIALS/C++Signals.h tml System Call : dup2() • Replace the tofileDesc file descriptor with a copy of the fromfiledesc file descriptor. Used for replacing stdin or stdout or both in a child process dup2(fromFileDesc, toFileDesc) • Example: http://www.cs.loyola.edu/~jglenn/702/S2005/Exam ples/pipe.html Shell • A shell is a job control system – Allows programmer to create and manage a set of programs to do some task – Windows, MacOS, Linux all have shells • Example: to compile a C program cc –c sourcefile1.c cc –c sourcefile2.c ln –o program sourcefile1.o sourcefile2.o Implementing a Shell char *prog, **args; int child_pid; // Read and parse the input a line at a time while (readAndParseCmdLine(&prog, &args)) { child_pid = fork(); // create a child process if (child_pid == 0) { exec(prog, args); // I'm the child process. Run program // NOT REACHED } else { wait(child_pid); // I'm the parent, wait for child return 0; } } POSIX • POSIX, an acronym for "Portable Operating System Interface", is a family of standards specified by the IEEE for maintaining compatibility between operating systems. POSIX defines the application programming interface (API), along with command line shells and utility interfaces, for software compatibility with variants of Unix and other operating systems. http://en.wikipedia.org/wiki/POSIX