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OSOS St
t
St
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OSOS Structure
Structure
COMP755 Advanced OS
The OS is a BIG
program
- The operating system is a large program
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composed of many modules.
- Writing and updating an OS requires
careful software engineering techniques.
- Errors in the OS can cause the whole
system to failsystem to fail.
System Implementation
- Once written in assembly language,
operating systems are now written inhigher level languageshigher-level languages
- Code written in a high-level language:
- Can be written faster– Is more compact.– Is easier to understand and debug
Is easier to understand and debug
- An operating system is far easier to
port
(move to some other hardware) if it iswritten in a high-level language
System Design Goals
•^
User goals
- operating system should be
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convenient to use, easy to learn, reliable,safe, and fast
-^
System goals
be easy to design, implement, andmaintain, as well as flexible, reliable, error-maintain, as well as flexible, reliable, errorfree, and efficient
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System Components
- Process management• Memory management• File management• I/O management• Security manager• Shell
OS kernel
- The kernel of the OS is that portion
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providing privileged high priority functions.– Dispatcher– Interrupt handling– I/O control– Memory management
Memory management
- Some parts of the operating system do not
reside in the kernel.
Traditional UNIX System Structure
Microkernel System Structure
- Moves as much from the kernel into “
user
space
- Communication takes place between user
modules using message passing
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Shell Features
- Reads user input and executes programs.• Provides the prompt.• Implements redirection• Implements parallel execution with &• May implement simple commands• Shells may provide a graphical user
interface.
Human Interface
- The shell is the part of the OS that is most
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important to most users.
- Considerable effort has gone into trying to
make the interface intuitive and easy to use
Popular Shells
- command.com in DOS• C shell in Unix• Korn shell in Unix• Borne shell in Unix• explorer.exe – Windows user interface• Cygwin – Unix shell for Windows
Shell Outline
do forever {
print prompt;read command;if (fork()==0) { // child process
use exec() to load program;
} wait for child process to terminate;
}
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execv
function
**int _execv( const char ***
cmdname
,
**const char ***
argv[ ]
);
g [ ]
where:•^
cmdname
is the name of the program file.
-^
argv
is an array of pointer to strings
containing the parameters. By conventionargv[0] = cmdname; The last argv entry mustbe NULL. The execv function causes the specified
program to overlay the calling program. Thisfunction does not return if successful.
Simple C Program
int main ( int argc,
char
*argv[]
)
{{
...return 5;
}^ The
argv
array passed to the main function when
you start a C programs is the
argv
array passed to
execv
by the shell.
Standard I/O Streams
When a program is started, it has three I/O
t streams open:0 stdin – Standard input, usually keyboard1 stdout – Standard output, usually screen2 stderr – Standard error, usually screen
Redirection
Standard I/O streams can be redirected from
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the command line
ls
myfile.txt
myprog
usualstuff.txt
cc
nogood.c
allmsg.txt
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Scripts
- The shell can execute script files to
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produce commands for the shell.
- Scripts are a program whose output is a
series of commands to the shell.
- DOS batch files are a simple example of
scriptingscripting.
- Unix scripts can be much more elaborate.
Utilities
- An operating system is more than just the
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tiliti
kernel, there are many utilities.
- The utilities are just user applications.• Different user interfaces require different
utilities
DOS Commands
attrib
find
set
cd or chdir
format
setver
chkdsk
help
share
cls
join
smartdrive
copy
label
sort
ctty
md or mkdir
sys
defrag
mem
time and date
del or erase
more
tree
del or erase
more
tree
deltree
move
type
dir
rd or rmdir
xcopy
echo
ren
fdisk
scandisk
Unix Commands
at
env
nice
cat
grep
ps
cd
head
rm
chmod
kill
rmdir
chown
lp
sort
cmp
make
tail
compres
man
touch
p
cp
mkdir
uncompress
date
more
who
du
mv
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Windows Utilities
In Windows the user interface is
called the Explorer. It runs in
25%
25%
25%
25%
K
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. Kernel mode 2. High priority OS3. Low priority OS4. User mode
Kernel
mode
High
priority
OS
Low
priority
OS
User
mode
Process Management
• A
process
is a program in execution
- A process needs certain resources,
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CPU i
fil
d I/O
including CPU time, memory, files, and I/Odevices, to accomplish its task
- The operating system is responsible for
the following activities in connectionwith process management
p
g
deletion
resumption
communication
Main-Memory Management
-^
Memory
is a large array of words or
bytes, each with its own address
- RAM is a volatile storage device
It loses
RAM is a volatile storage device. It losesits contents without power.
- The operating system is responsible for
the following activities in connection withmemory management
Keep track of which parts of memory are
- Keep track of which parts of memory are
currently being used and by whom
- Decide which processes to load when
memory space becomes available
- Allocate and deallocate memory space as
needed
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Priorities
- The many different parts of the OS may
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run at different priorities– Non-process– High priority– Lower priority– User level
User level
- The Intel Pentium supports 4 levels of
privilege.
OS Services
- Program Execution• I/O control• Communications• Error detection• Resource allocation• Protection
Supervisor Calls
- Supervisor Calls (SVC) are a means of
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ll t
OS
i
making a function call to an OS service.
- The normal function call mechanism will
not work since you have to change to theOS environment.
- An SVC or INT call generates an interrupt• An SVC or INT call generates an interrupt.
The OS gets control and executes thedesired function.
Virtual Machines
• A
virtual machine
takes the layered
approach to its logical conclusion. It treatshardware and the operating system kernelas though they were all hardware
- A virtual machine provides an interface
identical
to the underlying bare hardware
The operating s stem creates the ill sion of
- The operating system creates the illusion of
multiple processes, each executing on itsown processor with its own (virtual)memory
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Virtual Machines (Cont.)
- The resources of the physical computer
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are shared to create the virtual machines– CPU scheduling can create the appearance
that users have their own processor
- Spooling and a file system can provide virtual
card readers and virtual line printers
- A normal user time-sharing terminal serves as
the virtual machine operator’s console
Virtual Machines
Non-virtual Machine
Virtual Machine
Advantages/Disadvantages
of Virtual Machines
- The virtual-machine concept provides
complete protection of system resourcessince each virtual machine is isolated fromall other virtual machines. This isolation,however, permits no direct sharing ofresources.
Pros / Cons of Virtual Machines
- A virtual-machine system is a perfect
vehicle for operating-systems research and
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y
development.
- The virtual machine concept is difficult to
implement due to the effort required toprovide an
exact
duplicate to the underlying
machinemachine
- The OS on a virtual machine cannot
change the real page tables. Paging issometimes handled completely by the hostOS.
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