OS Fall’02

Introduction

Operating Systems Fall 2002

OS Fall’02

What is Operating System? It is a program! It is the first piece of software to run

after boot It coordinates the execution of all

other softwareUser programs

It provides various common services needed by users and applications

OS Fall’02

Today’s plan Operating system functionality Hardware support for the operating

system Course overview, bibliography,

administrative questions

OS Fall’02

The Operating System controls the machine

User

Application

Operating System

Hardware

OS Kernel

Hardware

gcc gdb

emacs

vi date

grep

xterm

netscape

diff

OS Fall’02

A better picture

Hardware

OperatingSystem

Privileged instructions

Systemcalls

Machineinstructions

Application

One Hardware

One Operating System

Many applications

OS Fall’02

A typical scenario

1. OS executes and schedules an application to run

2. Application runs• CPU executes app’s instructions• OS is not involved

3. The system clock interrupts the CPU• Clock interrupt handler is executed• The handler is the OS function

OS Fall’02

A typical scenario (continued)

4. In handler: OS chooses another application to run

Context switch

5. The chosen app. runs directly on the hardware

6. The app. performs a system call to read from a file

OS Fall’02

A typical scenario (continued)7. The sys. call causes a trap into the

OS OS sets up the things for I/O and puts

the application to sleep OS schedules another application to run

8. The third application runs

Note: At any given time only one program is running:

OS or a user application

OS Fall’02

The running application state diagram

Applicationcoderuns

OS runs

Sleep

ReadyTo run

Interrupt/System call

Wait forI/O completion

I/Ocompleted

Schedule

Resume executionof the app. code

OS Fall’02

A question The operating system gets an input,

performs a computation, produces an output, and quits

Yes or no?The answer: No

The operating system is a reactive program

OS Fall’02

The OS is a reactive program It is idly waiting for events When an event happens, the OS

reactsIt handles the event Schedules another application to run

The event handling must take as little time as possible

Event typesInterrupts and system calls

OS Fall’02

The OS performs Resource Management

Resources for user programsCPU, main memory, disk space

OS internal resourcesDisk space for paging memory (swap space)Entries in system tables Process table, open file table

Statically allocated

OS Fall’02

CPU management How to share one CPU among many

processes Time slicing:

Each process is run for a short while and then preempted

Scheduling:The decision about which application to run next

OS Fall’02

Memory management Programs need main memory frames to

store their code, data and stack The total amount of memory used by

currently running programs usually exceed the available main memory

Solution: pagingTemporarily unused pages are stored on disk (swapped out)When they are needed again, they are brought back into the memory (swapped in)

OS Fall’02

The OS supports abstractions Creates an illusion that each

application got the whole machine to run on

In reality: an application can be preempted, wait for I/O, have its pages being swapped out, etc…

A tree-like file system organizationDisk controllers can only write/read blocks

OS Fall’02

Hardware support for OS Support for executing certain

instructions in a protected mode Support for interrupts Support for handling interrupts Support for system calls Support for other services

OS Fall’02

CPU execution modes CPU has (at least) 2 execution modes:

User modeKernel mode Supervisor mode, privileged mode, monitor

mode, system mode

The execution mode is indicated by a bit in the processor status word (PSW)

Some CPU instructions are available only when executing in the kernel mode

OS Fall’02

Kernel Mode OS kernel is a collection of functions

responsible for the most basic servicesOS kernel executes in the kernel mode

Privileged instructions:To load/store special CPU registersTo map memory pages to the address space of a specific processInstructions to set the interrupt priority levelInstructions to activate I/O devices

OS Fall’02

Protecting Kernel mode Is it possible for the user program to

cause the CPU to enter kernel mode? Yes: This must be possible (system call) The problem: how to prevent the user

program from executing privileged instructions?

Solution: change the program counter (PC) to point to the OS code upon switch

OS Fall’02

Handling interrupts Interrupts cause the CPU to enter

kernel mode The address of the kernel function to

execute is loaded from the interrupt vector

The interrupt vector address and the interrupt numbering is a part of the hardware specification

Handlers are registered during the boot

OS Fall’02

Interrupt types (I) Asynchronous interrupts are

generated by external devices at unpredictable times

Clock interrupt is the basis for time slicing: Update the system time, preempt/schedule

processes

I/O device interrupt Informs the OS about completion of a

requested I/O

OS Fall’02

Interrupt types (II) Internal (synchronous) interrupts are

generated synchronously by CPU as a result of an exceptional condition

An error condition: the application is trying to perform an illegal operation: E.g., division by 0, issuing a privileged instr.,

… The handler typically kills the application

A temporary problem: E.g., the requested page is not in the memory Handling: bring the page into the memory

OS Fall’02

System calls Used to request a service from the OS

A collection of the system calls is the OS APIPackaged as a library

Typical system callsOpen/read/write/close the fileGet the current timeCreate a new processRequest more memory

OS Fall’02

Handling system calls An application executes a special trap

(syscall) instructionCauses the CPU to enter kernel mode and set PC to a special system entry point (gate routine)

The gate routine address is typically stored in a predefined interrupt vector entry

Intel architecture: int[80]A single entry serves all system calls (why?)

OS Fall’02

An exampleopen(“/tmp/foo”): store the system call number and the parameters in a predefined kernel memory location; trap(); retrieve the response from a predefined kernel memory location; return the response to the calling application;

trap():PC:=&int[80]; // transfer control to the gate routine

Gate routine:switch(sys_call_num) {

case OPEN: …}

OS Fall’02

Other hardware support Translating virtual address into a

physical addressAssist in supporting the virtual memory abstraction

Support for “used” bits for memory pages

Helps to determine which pages can be swapped out when needed

OS Fall’02

What we are going to study Performance evaluation (brief) Process handling

Process concept, scheduling, concurrency control

Memory managementPaging, virtual memory

File system

OS Fall’02

Advanced topics Distributed systems

Communication, networking, middleware

Other possible topics:Reliable distributed systemsReal-time systems Parallel systemsModern storage architectures

OS Fall’02

Bibliography Notes by Dror Feitelson

Will be published weekly

Operating System Concepts, by A. Silberschatz, P. Galvin, G. Gagne

Operating Systems Internals and Design Principles, by W. Stallings

See the notes for more references

OS Fall’02

Next: Performance evaluation