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NETW 3005 Reading • For this lecture, you should have read Chapter 2 (Sections 1-9). NETW3005 (Operating Systems) Lecture 02 - System Structure & Processes 2 Last lecture • History and types of operating systems: – batch systems, multiprogramming systems, time sharing systems, etc. • Operating system tasks: – process management, storage management, I/O device management, user interface. NETW3005 (Operating Systems) Lecture 02 - System Structure & Processes 3 This lecture • Hierarchical Structure in Operating Systems • System calls and interrupts • Representing processes in Operating Systems • Overview of process scheduling. NETW3005 (Operating Systems) Lecture 02 - System Structure & Processes 4 Hierarchical structure in OS • An operating system, like any complex computer system, must be carefully designed. • One central requirement is modularity. • Distinction between system programs and application programs. NETW3005 (Operating Systems) Lecture 02 - System Structure & Processes 5 Application programs System programs Kernel operations Device drivers NETW3005 (Operating Systems) Terminal drivers Memory manager Lecture 02 - System Structure & Processes 6 Application programs • Ones that ordinary users interact with: – Word-processors – Database packages – Web browsers – Compilers, editors, IDEs, etc – ... NETW3005 (Operating Systems) Lecture 02 - System Structure & Processes 7 System programs • Provide a general-purpose lower-level function. System functions include: – file manipulation: create, delete, copy etc. – status info: date, available memory. – program loading and execution. – communication between processes. – Command interpreters (a.k.a. shell programs). The set of system programs define the user interface. NETW3005 (Operating Systems) Lecture 02 - System Structure & Processes 8 Other system functions • Not strictly part of the OS, but often packaged with it. • Programs to modify files (text editors, search, transform). • Compilers, assemblers and interpreters. The set of system programs defines the user interface. NETW3005 (Operating Systems) Lecture 02 - System Structure & Processes 9 Degrees of modularity • Different operating systems enforce different degrees of modularity. • Ideally you want to oblige all system and application programs to talk to the hardware via the kernel. NETW3005 (Operating Systems) Lecture 02 - System Structure & Processes 10 MS-DOS Application programs System programs Kernel operations Device drivers NETW3005 COSC 243 (Operating (Operating Systems) Systems) Terminal drivers Memory manager Lecture 02 - System Structure & Processes 11 Talking to the kernel: system calls System Program ‘‘read from the keyboard’’ ‘‘open a file’’ ‘‘write to the screen’’ Kernel Hardware NETW3005 (Operating Systems) Lecture 02 - System Structure & Processes 12 System calls • Written in the same language as kernel (typically C). • Available to Assembler and (at least) some HLLs — C, Perl, etc. • The set of system calls is termed the programmer interface to the system. NETW3005 (Operating Systems) Lecture 02 - System Structure & Processes 13 A simple system program — copy • cp file1 file2 • Open file1; Create file2. • Loop: Read from file1; Write to file2. • Close file1; Close file2. NETW3005 (Operating Systems) Lecture 02 - System Structure & Processes 14 Types of system call (1) • Process control – create, terminate, suspend, resume, abort. • File manipulation – open, close, read, write • Device manipulation – request, release, read, write. NETW3005 (Operating Systems) Lecture 02 - System Structure & Processes 15 Types of system call (2) • Housekeeping – get/set time or date – get/set attributes (process, device, file) • Communications – set up link, – send/receive message NETW3005 (Operating Systems) Lecture 02 - System Structure & Processes 16 Interrupts How system calls are implemented • CPU responds to interrupts no matter what else it happens to be doing. • An interrupt transfers control to an appropriate module in the kernel. • A system call transfers control to the kernel by generating an interrupt (sometimes called a trap in this context). NETW3005 (Operating Systems) Lecture 02 - System Structure & Processes 17 Responding to an interrupt • Effectively a Jump to Subroutine: – current instruction address (PC) is saved – control transferred to fixed address, depending on the interrupt. • The interrupt vector is an array of locations that hold the addresses of these routines, usually held in low memory. NETW3005 (Operating Systems) Lecture 02 - System Structure & Processes 18 Implementation issues • How do we guarantee that the interrupthanding routine won’t affect the interrupted process? • What happens if an interrupt occurs while an interrupt-handling routine is executing? NETW3005 (Operating Systems) Lecture 02 - System Structure & Processes 19 Virtual machines • System calls allow the OS to hide the low-level hardware from application programs. • In a virtual machine these system calls are executed by a program which emulates the hardware. • This hardware may, or may not be the same as the actual hardware. NETW3005 (Operating Systems) Lecture 02 - System Structure & Processes 20 processes processes processes processes kernel hardware kernel1 kernel2 kernel3 VM1 VM2 VM3 virtual-machine implementation hardware NETW3005 (Operating Systems) Lecture 02 - System Structure & Processes 21 Benefits of virtual machines • Protection: users aren’t even aware there are other users. • Good for operating systems R & D. (No down-times: just give a system programmer her own virtual machine.) • A way of solving system-compatibility problems. NETW3005 (Operating Systems) Lecture 02 - System Structure & Processes 22 Problems with virtual machines • Speed: virtual machines are slower. • Implementation is very difficult, e.g. resource allocation (particularly disc). NETW3005 (Operating Systems) Lecture 02 - System Structure & Processes 23 Java • Compiled Java code is called byte-code. • It is designed to run on the Java Virtual Machine (JVM). • Think about the command-line process of compiling and running a Java program as opposed to compiling and running a C++ program. NETW3005 (Operating Systems) Lecture 02 - System Structure & Processes 24 Processes • Recall: a process is not just a program – it is a dynamic entity. • A given program (e.g. emacs) could be executing many times on a given machine – the machine must represent each execution as a separate process. NETW3005 (Operating Systems) Lecture 02 - System Structure & Processes 25 Components of a process (1) • Code section: the program code itself • Data section: any global variables used by the program • Process stack: any local variables currently being used (subroutine parameters, return addresses, etc.) • Program counter: a pointer to some place in the program code. NETW3005 (Operating Systems) Lecture 02 - System Structure & Processes 26 Components of a process (2) • Contents of CPU registers. • Memory management information. • Accounting information: who owns the process, how long it’s been running, how much CPU time it’s used so far, etc. NETW3005 (Operating Systems) Lecture 02 - System Structure & Processes 27 Process state • The operating system keeps track of the state of each process. • A process can be in any of the following states: – new – running – waiting/blocked – ready – terminated NETW3005 (Operating Systems) Lecture 02 - System Structure & Processes 28 An Example from MacOS X oucs1046: chandley$ ps –ax NETW3005 (Operating Systems) Lecture 02 - System Structure & Processes 29 Macos X processes (1) PID 1 10 11 12 13 14 15 16 20 TTY TIME CMD ?? 1:26.15 /sbin/launchd ?? 0:03.03 /usr/libexec/kextd ?? 1:06.37 /usr/sbin/DirectoryService ?? 0:27.57 /usr/sbin/notifyd ?? 15:11.87 /usr/sbin/syslogd ?? 2:08.74 /usr/sbin/configd ?? 0:18.55 /usr/sbin/distnoted ?? 25:23.28 /usr/sbin/mDNSResponder –launchd ?? 0:03.56 /usr/sbin/securityd -i NETW3005 (Operating Systems) Lecture 02 - System Structure & Processes 30 Macos X processes (2) 24 25 26 27 31 ?? 1:24.42 /usr/sbin/ntpd -n -g -p … ?? 0:02.00 /usr/sbin/cron ?? 17:58.14 /usr/sbin/update ?? 0:00.01 /sbin/SystemStarter ?? 0:00.03 /System/Library/CoreServices/… NETW3005 (Operating Systems) Lecture 02 - System Structure & Processes 31 Macos X processes (3) • Produced 75 different processes. • Several distinct classes – Daemons – security, cron, update, etc. – Core Services – Dock, Finder, etc. – Network and communications. – Application programs – Preview, PowerPoint, Word, Adobe Acrobat, etc. NETW3005 (Operating Systems) Lecture 02 - System Structure & Processes 32 Scheduling: an overview Disk Job2, Job2, Job3, Job4, … Main Memory O.S. CPU Process 1 Process 2 Process 3 Process 4 NETW3005 (Operating Systems) Lecture 02 - System Structure & Processes 33 Types of scheduler • Long-term scheduler (batch systems): decides which jobs loaded onto the disk should be moved to main memory. • Short-term scheduler (a.k.a. CPU scheduler): chooses how to allocate the CPU between the processes which are ready to execute. NETW3005 (Operating Systems) Lecture 02 - System Structure & Processes 34 Scheduling queues • The relationships between processes are represented by the O/S as queues. – Job queue: all the processes in the system (including those on disk). – Ready queue: all the processes which are ready to execute. – Device queue: all the processes waiting to use a particular device. (One queue per device.) NETW3005 (Operating Systems) Lecture 02 - System Structure & Processes 35 The Flow of Processes in an OS ready queue I/O I/O device queue I/O I/O device queue CPU I/O request fork interrupt mechanism NETW3005 (Operating Systems) Lecture 02 - System Structure & Processes 36 Process creation • A process is created (‘spawned’) by another process – we talk of parent and child processes. • When you launch an application, the terminal process spawns the application process. • Processes are able to ‘call’ other processes, just as a program can call functions. NETW3005 (Operating Systems) Lecture 02 - System Structure & Processes 37 Differences • A child process can (and often does) run concurrently with its parent. • A child process needn’t be constrained to use the resources of its parent (although it may be, and often is). NETW3005 (Operating Systems) Lecture 02 - System Structure & Processes 38 A process tree (1) root daemons user1 init user2 system proc application child NETW3005 (Operating Systems) user3 child Lecture 02 - System Structure & Processes 39 A process tree (2) • Note that users are treated as processes by the operating system. • How does that work? • Answer: users are really represented to the system as shell processes. • An important notion: the root user. NETW3005 (Operating Systems) Lecture 02 - System Structure & Processes 40 Homework • The Unix command ps displays information about processes on the system. Read the man page for ps (i.e. do “man ps”), and try out some of the options. NETW3005 (Operating Systems) Lecture 02 - System Structure & Processes 41 Next Lecture Threads and Data Sharing Chapter 4 (Sections 1-4)