netw 3005. reading for this lecture, you should have read chapter 2 (sections 1-9). netw3005...

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.


This lecture

• Hierarchical Structure in Operating Systems

• System calls and interrupts

• Representing processes in Operating Systems

• Overview of process scheduling.


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.



Application programs

System programs

Kernel operations

Device drivers

Terminal drivers

Memory manager


• Ones that ordinary users interact with:– Word-processors– Database packages– Web browsers– Compilers, editors, IDEs, etc– . . .


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.


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.

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.


MS-DOS

NETW3005 (Operating Systems) 11COSC 243 (Operating Systems) 11

System programs

Kernel operations

Device drivers

Terminal drivers

Memory manager


Lecture 02 - System Structure & Processes

Talking to the kernel: system calls


System Program

Kernel

Hardware

‘‘read from the keyboard’’ ‘‘open a file’’‘‘write to the screen’’

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.


A simple system program — copy

• cp file1 file2

• Open file1; Create file2.

• Loop: Read from file1; Write to file2.

• Close file1; Close file2.


Types of system call (1)

• Process control– create, terminate, suspend, resume, abort.

• File manipulation– open, close, read, write

• Device manipulation– request, release, read, write.


Types of system call (2)

• Housekeeping– get/set time or date– get/set attributes (process, device, file)

• Communications– set up link,– send/receive message


InterruptsHow 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).


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.


Implementation issues

• How do we guarantee that the interrupt-handing routine won’t affect the interrupted process?

• What happens if an interrupt occurs while an interrupt-handling routine is executing?


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.



processes

hardware

kernel

processes processes processes

kernel1 kernel2 kernel3

hardware

VM1 VM2 VM3

virtual-machine implementation

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.


Problems with virtual machines

• Speed: virtual machines are slower.

• Implementation is very difficult, e.g. resource allocation (particularly disc).


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.


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.


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.


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.


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


An Example from MacOS X

oucs1046: chandley$ ps –ax


Macos X processes (1)


PID TTY TIME CMD 1 ?? 1:26.15 /sbin/launchd10 ?? 0:03.03 /usr/libexec/kextd11 ?? 1:06.37 /usr/sbin/DirectoryService12 ?? 0:27.57 /usr/sbin/notifyd13 ?? 15:11.87 /usr/sbin/syslogd14 ?? 2:08.74 /usr/sbin/configd15 ?? 0:18.55 /usr/sbin/distnoted16 ?? 25:23.28 /usr/sbin/mDNSResponder –launchd20 ?? 0:03.56 /usr/sbin/securityd -i



24 ?? 1:24.42 /usr/sbin/ntpd -n -g -p …25 ?? 0:02.00 /usr/sbin/cron26 ?? 17:58.14 /usr/sbin/update27 ?? 0:00.01 /sbin/SystemStarter31 ?? 0:00.03 /System/Library/CoreServices/…


• Produced 75 different processes.

• Several distinct classes– Daemons – security, cron, update, etc.– Core Services – Dock, Finder, etc.– Network and communications.– Application programs – Preview, Power-

Point, Word, Adobe Acrobat, etc.


Scheduling: an overview


Disk Job2, Job2, Job3, Job4, …

CPU

Main Memory

O.S.

Process 1

Process 2

Process 3

Process 4

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.


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.)


The Flow of Processes in an OS


ready queue CPU

I/O request

I/O device queue

I/O device queue

I/O

I/O

fork

interrupt mechanism

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.


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).


A process tree (1)


root

daemons init

user1 user2 user3

system proc application

child child

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.


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.


Next Lecture

Threads and Data SharingChapter 4 (Sections 1-4)

netw 3005. reading for this lecture, you should have read chapter 2 (sections 1-9). netw3005...

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