Summary: OS Concepts

• Processes • layers = abstraction – Resources, Deadlocks • policies and – Pipes mechanisms • Memory Management • user versus supervisor (kernel) mode • Input/Output – different spaces • Files – different levels of • Security executions and privileges • The Shell – system calls (kernel) • Concept Recycling application code library 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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1 Steps in Making a System Call

There are 11 steps in making the system call

read (fd, buffer, nbytes) 27

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 */ } }

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2 System Calls (2)

• Processes have three segments: text, data, stack 29

Some System Calls For Process Management

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3 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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4 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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5 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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6 System Calls (5)

Some Win32 API calls 37

Operating System Structure (1)

Simple structuring model for a monolithic system

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7 Monolithic Structure

OS = collection of procedures ~ rather than modules (no information hiding) Still, structure in methodology: parameters put on stack before TRAP, for example. Three “layers” 1. Main program that invokes -- dispatches -- the requested service procedure 2. Set of service procedures, to execute system calls 3. Set of utility procedures, factoring common needs of service procedures

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Operating System Structure (2)

Structure of the THE operating system

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8 Layered Structure in THE 5. System Operator Process 4. 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 management 0. 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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9 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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10 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 machine resources (e.g., disk portions) – saves one mapping layer (e.g., disk addresses) – again separates multiprogramming / niceties

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11 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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12 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

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