1

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Summary: OS Concepts

• Processes– Resources, Deadlocks

– Pipes

• Memory Management

• Input/Output

• Files

• Security

• The Shell

• Concept Recycling

• layers = abstraction• policies and

mechanisms• user versus supervisor

(kernel) mode– different spaces– different levels of

executions andprivileges

– system calls (kernel)application codelibrary code

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System Calls

Interface that OS provides, programs use

• set of system calls, specific to each OS

• used to transfer control of (single) CPU– program (process) invokes => context switch

– program suspended while syscall executed

– system call wrapped in library function; e.g., count = read (fd, buffer, nbytes)

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Steps in Making a System Call

There are 11 steps in making the system call read (fd, buffer, nbytes)

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System Calls (1)• A stripped down shell:

while (TRUE) { /* repeat forever */ type_prompt( ); /* display prompt */ read_command (command, parameters) /* input from

terminal */

if (fork() != 0) { /* fork off child process */ /* Parent code */ waitpid( -1, &status, 0);/* wait for child to exit */} else { /* Child code */ execve (command, parameters, 0);/* execute command */ }}

3

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System Calls (2)

• Processes have three segments: text, data, stack

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Some System Calls For Process Management

4

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Some System Calls For File Management

how: O_RDONLY, O_WRONLY, O_RDWR, O_CREAT

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Files and inodes

• every file denoted by unique i-number

• i-number is index into table of i-nodes

• directory: file containing a set of pairs:– (i-number, ASCII name)

• to link is to create a new entry with the i-number of existing file

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Some System Calls For Directory Management

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System Calls (3)

(a) Two directories before linking/usr/jim/memo to ast's directory

(b) The same directories after linking

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System Calls (4)

(a) File system before the mount

(b) File system after the mount

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Some System Calls For Miscellaneous Tasks

chmod 741 means -rwxr--x

kill -9 pid means kill process given by pid

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System Calls (5)

Some Win32 API calls

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Operating System Structure (1)

Simple structuring model for a monolithic system

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Monolithic Structure

OS = collection of procedures ~ rather than modules(no information hiding)

Still, structure in methodology: parameters put onstack before TRAP, for example. Three “layers”

1. Main program that invokes -- dispatches -- therequested service procedure

2. Set of service procedures, to execute systemcalls

3. Set of utility procedures, factoring commonneeds of service procedures

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Operating System Structure (2)

Structure of the THE operating system

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Layered Structure in THE5. System Operator Process4. User processes

• I/O, console, memory, etc., taken care 0-3

3. I/O management - abstracts devices• devices, buffers, streams

2. comm. operator - console• above it, each proc has its own console

1. main memory & swap management0. handle interrupts, switch processes

• multiprogramming, above it seq processes

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Operating System Structure (3)

Structure of VM/370 with CMS

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Virtual Machine VM/370

• Virtual Machine Monitor on bare HW– offers exact copies of the underlying HW– does not add any niceties!

• On top of VM, can have several copies of– batch system– single-user, interactive system CMS

• separates– multiprogramming– extended machine

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Operating System Structure (4)

The client-server model

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Operating System Structure (5)

The client-server model in a distributed system

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Micro-, Exo-Kernels, C-S

Insight: OS services done as user processes– user client process requests a server process

– kernel just supports interprocess comm.

Exokernels - similar to VM, but split machineresources (e.g., disk portions)– saves one mapping layer (e.g., disk addresses)

– again separates multiprogramming / niceties

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OS Structure

• Monolithic Systems

• Layered Systems

• Virtual Machines

• Exo- and Micro-kernels– Client-Server Model

mechanisms versus policies

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Metric Units

The metric prefixes

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Other Units• Time between slipping on a peel and

smacking the pavement = 1 bananosecond• Half a large intestine = 1 semicolon• Time it takes to sail 220 yards at 1 nautical

mile per hour = Knot furlong• 1 millionth of a fish = 1 microfiche• 1000 aches = 1 megahertz• Basic unit of laryngitis = 1 hoarsepower• 1 million-million microphones = 1 megaphone• 1 million bicycles = 2 megacycles• 2.4 statute miles of intravenous tubing at Yale

University Hospital:1 I.V. League