OS Structure & Processes Vivek Pai COS 318 September 19, 2002
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Transcript OS Structure & Processes Vivek Pai COS 318 September 19, 2002
OS Structure & Processes
Vivek Pai
COS 318
September 19, 2002
OS Structure & Processes
1
Foodundgedank
You’re the host/hostess of a banquet hall that has
accidentally been double-booked. It has three
rooms – the large dining room, the cocktail room,
and a small buffet line room.
If each guest is given a card with the room layout
with table numbers, and the guest’s own table
number, how can you solve this problem?
What issues are involved regarding the sizes of the
two groups?
OS Structure & Processes
2
Administrative
First precept happened, more to come
– Friday’s booked – same time, same place
Yong is this project’s TA
Project 1 due 11:59pm Oct 1
Almost everyone has given me details
– Get me what remains
Next reading assignment: ???
OS Structure & Processes
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A Friendly Deception
Hiding information
Giving out different information while
appearing to be the same
Stalling when necessary
Drawback? Maybe lots of extra work
Benefit: makes life easier for user
OS Structure & Processes
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Host/Hostess Problem
Each group < 50%
– Have two sets of cards with different
numbering schemes
Each group slightly above 50%
– Slow down the buffet line & entrance
Each group near 100%
– One group in dining room, other in cocktail
room
OS Structure & Processes
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Users, Programs, Processes
Users have accounts on the system
Users launch programs
– Many users may launch same program
– One user may launch many instances of the
same program
Processes are instances of a program
– The “program” can really be a set of processes
OS Structure & Processes
6
Programs As Process Collections
Netscape (output of “ps x”)
– 7253 p0 S
– 7280 p0 I
0:19.26 /usr/local/lib/netscape/communicator-4.75.bin -irix-s
0:00.15 (dns helper) (communicator-4.7)
gcc (via “gcc –pipe –v”)
–
–
–
–
/usr/libexec/cpp |
/usr/libexec/cc1 |
/usr/libexec/as, followed by
/usr/libexec/elf/ld
OS Structure & Processes
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gcc example
You launch gcc
It first launches cpp, cc1, as
It then launches ld
Each instance is a process, and each
program actually exists separately
– You could launch cc1 manually if you wanted
OS Structure & Processes
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So What Is A Process?
It’s one instance of a “program”
It’s separate from other instances
It can start (“launch”) other processes
It can be launched by them
OS Structure & Processes
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What Does This Program Do?
int myval;
int main(int argc, char *argv[])
{
myval = atoi(argv[1]);
while (1)
printf(“myval is %d, loc 0x%lx\n”,
myval, (long) &myval);
}
OS Structure & Processes
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Here’s The Output
OS Structure & Processes
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Instances Of Programs
The address was always the same
The values were different
– Implies that the programs aren’t seeing each
other
– But they think they’re using the same address
Conclusion: addresses are not absolute
What’s the benefit?
OS Structure & Processes
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So What’s In A Process?
Information about the hierarchy
– What launched it
– What it has launched
Information about resources
– Where is it storing data
– What other resources it’s using
Various kinds of mappings
OS Structure & Processes
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Consider How To Read a File
Compare read( ) and fread( )
read(int d, void *buf, size_t nbytes)
read( ) attempts to read nbytes of data from the object
referenced by the descriptor d into the buffer pointed to by
buf.
fread(void *ptr, size_t size, size_t nmemb, FILE *stream)
The function fread( ) reads nmemb objects, each size bytes
long, from the stream pointed to by stream, storing them at
the location given by ptr.
OS Structure & Processes
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Which is a System Call and Why
read(int d, void *buf, size_t nbytes)
fread(void *ptr, size_t size, size_t nmemb, FILE
*stream)
Both do the same thing, right?
What gets “exposed” in each case
What about a lying programmer
– How hard is it to tell he/she is lying
– What other malicious actions can occur?
OS Structure & Processes
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What State Is Implied?
Consider the following sequence:
read(int d, void *buf, size_t nbytes)
read(int d, void *buf, size_t nbytes)
read(int d, void *buf, size_t nbytes)
What happens if two programs were doing this?
OS Structure & Processes
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Some Insight Into OS
Array of
File Pointers
Process
Actual File
Info
What do we gain from this?
OS Structure & Processes
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Some Insight Into OS
Array of
File Pointers
Process
Actual File
Info
Process
OS Structure & Processes
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Examining Those Other Parameters
read(int d, void *buf, size_t nbytes)
fread(void *ptr, size_t size, size_t nmemb, FILE
*stream)
Where is [buf, buf+nbytes)?
Where is [ptr, ptr+size*nmemb)?
OS Structure & Processes
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Remember This?
Application
Libraries
User space/level
Kernel space/level
Portable OS Layer
Machine-dependent layer
OS Structure & Processes
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Address Space
One (common) approach
– Kernel is high memory
– User is low memory
What restrictions apply?
read(f, buf, nbytes)
OS Structure & Processes
0xffff….
Kernel space
User space
0x0000…
21
Some Definitions
Kernel – “heart” of the operating system
– Like the program file generated by compiling
all of the operating system files
Operating system – usually includes more
– Various libraries
– Support programs
Hard and fast definitions? Rarely
OS Structure & Processes
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More Address Space
Program segments
–
–
–
–
Text
Data
Stack
Heap
Any obvious choices?
OS Structure & Processes
0xffff….
Kernel space
User space
0x0000…
23
One Common Layout
Lots of flexibility
– Allows stack growth
– Allows heap growth
– No predetermined division
0xffff….
Kernel space
Stack
So what happens when you
make a system call?
OS Structure & Processes
Heap
Code & Data
0x0000…
24
Kernel Stack
Kernel contains functions
– Just like regular functions
– Automatic variables, formal parameters, etc
– They need a stack
Can we use the user’s stack?
– Is it possible?
– Is it desirable?
OS Structure & Processes
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Is The Kernel A Process?
Monolithic OS
– Most forms of OS you’ve probably used
Layered systems
– THE system (design)
– Multics (design + hardware support)
Hypervisors/virtual machines
Client/Server (microkernels)
– Mach
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What Other Options Possible?
Remember the host/hostess scenario?
– What would happen if you had a break between
courses
– Can you apply this logic to the kernel?
– What are the tradeoffs?
OS Structure & Processes
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