Transcript system programs

NETW 3005
Reading
• For this lecture, you should have read
Chapter 2 (Sections 1-9).
NETW3005 (Operating Systems)
Lecture 02 - System Structure & Processes
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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.
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This lecture
• Hierarchical Structure in Operating
Systems
• System calls and interrupts
• Representing processes in Operating
Systems
• Overview of process scheduling.
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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.
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Application programs
System programs
Kernel operations
Device
drivers
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Terminal
drivers
Memory
manager
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Application programs
• Ones that ordinary users interact with:
– Word-processors
– Database packages
– Web browsers
– Compilers, editors, IDEs, etc
– ...
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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.
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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.
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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.
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MS-DOS
Application programs
System programs
Kernel operations
Device
drivers
NETW3005
COSC 243 (Operating
(Operating Systems)
Systems)
Terminal
drivers
Memory
manager
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Talking to the kernel: system calls
System Program
‘‘read from the keyboard’’
‘‘open a file’’
‘‘write to the screen’’
Kernel
Hardware
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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.
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A simple system program — copy
• cp file1 file2
• Open file1; Create file2.
• Loop: Read from file1; Write to file2.
• Close file1; Close file2.
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Types of system call (1)
• Process control
– create, terminate, suspend, resume, abort.
• File manipulation
– open, close, read, write
• Device manipulation
– request, release, read, write.
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Types of system call (2)
• Housekeeping
– get/set time or date
– get/set attributes (process, device, file)
• Communications
– set up link,
– send/receive message
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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).
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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.
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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?
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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.
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processes
processes processes processes
kernel
hardware
kernel1
kernel2
kernel3
VM1
VM2
VM3
virtual-machine implementation
hardware
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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.
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Problems with virtual machines
• Speed: virtual machines are slower.
• Implementation is very difficult, e.g.
resource allocation (particularly disc).
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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.
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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.
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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.
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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.
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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
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An Example from MacOS X
oucs1046: chandley$ ps –ax
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Macos X processes (1)
PID
1
10
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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
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Macos X processes (2)
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?? 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/…
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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.
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Scheduling: an overview
Disk Job2, Job2, Job3, Job4, …
Main Memory
O.S.
CPU
Process 1
Process 2
Process 3
Process 4
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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.
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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.)
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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
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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.
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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).
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A process tree (1)
root
daemons
user1
init
user2
system proc
application
child
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user3
child
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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.
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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.
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Next Lecture
Threads and Data Sharing
Chapter 4 (Sections 1-4)