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Operating System Structures

Vivek Pai

Princeton University

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Gedankenexperiment

What does this program do?static void Loop(void){

static char *startAddr;char local;printf(“diff is %d\n”, startAddr – (&local));startAddr = &local;Loop( );

}int main(int argc, char *argv[ ]){

Loop( );}

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Mechanics

Have you: Subscribed to pu.cs.318? Sent me mail with your details?

– Hey, it was an assignment after all… Sent me a picture of yourself?

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More Mechanics

Syllabus is now “complete”, along with goals for each lecture

Preparatory readings are listed– External readings are not yet included

Projects are being finalized– Regular lab not yet operational

How do we want to handle quizzes?– Scheduled or random?

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Reading Assignment

Sections 1.0-1.3 inclusive Keep up with what’s on home page I’ll try to remember to remind you

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A Typical Computer from a Hardware Point of View

CPU

ChipsetMemory

I/O bus

CPU. . .

Network

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A Typical Computer System

Operating System Software

Programs and data

MemoryCPU

CPU

...

OSApps

Data

Network

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Typical Unix OS Structure

Application

Portable OS Layer

Libraries

Machine-dependent layer

User space/level

Kernel space/level

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Typical Unix OS Structure

Application

Portable OS Layer

Libraries

Machine-dependent layer

Written by programmerCompiled by programmerUses function calls

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Typical Unix OS Structure

Application

Portable OS Layer

Libraries

Machine-dependent layer

Written by elvesProvided pre-compiledDefined in headersInput to linker (compiler)Invoked like functionsMay be “resolved” when

program is loaded

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Typical Unix OS Structure

Application

Portable OS Layer

Libraries

Machine-dependent layer

“Guts” of system callsAll “high-level” code

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Typical Unix OS Structure

Application

Portable OS Layer

Libraries

Machine-dependent layer

BootstrapSystem initializationInterrupt and exception I/O device driverMemory managementKernel/user mode

switchingProcessor management

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Another Look: Unix “Onion”

Applications

OS Service

Device

Driver

Hardware

User and Kernelboundary

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What’s An Application?

Four parts (“segments”) Code/Text – instructions Data – initialized global variables Stack Heap

What’s a stack and heap?

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OS Service Examples

Examples that are not provided at user level– System calls: file open, close, read and write– Control the CPU so that users won’t stuck by

runningwhile ( 1 ) ;

– Protection: • Keep user programs from crashing OS• Keep user programs from crashing each other

Examples that can be provided at user level– Read time of the day– Protected user level stuff

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Processor Management

Goals– Overlap between I/O and

computation– Time sharing– Multiple CPU allocations

Issues– Do not waste CPU

resources– Synchronization and mutual

exclusion– Fairness and deadlock free

CPU I/O CPU

CPU

I/O

CPU

CPUI/O

CPU

CPU

CPU

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Memory Management

Goals– Support programs to run– Allocation and

management– Transfers from and to

secondary storage Issues

– Efficiency & convenience– Fairness– Protection

Tape 100Mx

Disk 10Mx

Memory 200x

L2 10x

Register

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x86 Architecture Registers

General-purpose registers

Segment registers

EFLAGS register EIP (Instruction Pointer register)

EAXEBXECXEDXEBPESIEDIESP

31 0 015

CSDSSSESFSGS

AXBXCXDX

16-bit 32-bit

DISIBP

SP

ALAHBLCLDL

BHCHDH

8 715

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x86 Memory

0

232-1

Byte order is little endian

31 08 716 15

.

.

.

24 23

Byte 4Byte 0

Byte 5Byte 1Byte 2

Byte 6Byte 3Byte 7

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I/O Device Management

Goals– Interactions between

devices and applications– Ability to plug in new

devices

Issues– Efficiency– Fairness– Protection and sharing

User 1 User n. . .

Library support

I/Odevice

I/Odevice. . .

Driver Driver

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Window Systems

All in the kernel (Windows)– Pros: efficient?– Cons: difficult to develop new services

All at user level– Pros: easy to develop new apps– Cons: protection

Split between user and kernel (Unix)– Kernel: display driver and mouse driver– User: the rest

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File System

A typical file system

– Open a file with authentication

– Read/write data in files

– Close a file

Can the services be moved to user level?

User 1 User n. . .

File system services

File File. . .

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Bootstrapping Power up a computer Processor reset

– Set to known state– Jump to ROM code

Load in the boot loader from stable storage

Jump to the boot loader Load the rest of the

operating system Initialize and run

Bootloader

OSsector 1

OSsector 2

OSsector n

.

.

.

Bootloader

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System Boot

Power on (processor waits until Power Good Signal)

Processor jumps on a PC (“Intel Inside”) to address FFFF0h

• 1M= 1,048,576= 220 =FFFFFh+1 • FFFFFh=FFFF0h+16 is the end of the (first 1MB of)

system memory• The original PC using Intel 8088 had 20 address lines :-)

(FFFFFFF0h) is a JMP instruction to the ROM BIOS startup program

Maps to FFFFFFF0h= 232-16

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ROM BIOS startup program (1) POST (Power-On Self-Test)

• If pass then AX:=0; DH:=5 (586: Pentium);• Stop booting if fatal errors, and report

Look for video card and execute built-in ROM BIOS code (normally at C000h)

Look for other devices ROM BIOS code• IDE/ATA disk ROM BIOS at C8000h (=819,200d)

Display startup screen• BIOS information

Execute more tests• memory• system inventory

SCSI disks: must often provide their own BIOS

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ROM BIOS startup program (2)

Look for logical devices– Label them

• Serial ports– COM 1, 2, 3, 4

• Parallel ports– LPT 1, 2, 3

– Assign each an I/O address and IRQ

Detect and configure PnP devices Display configuration information on screen

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ROM BIOS startup program (3)

Search for a drive to BOOT from– Floppy or Hard disk

• Boot at cylinder 0, head 0, sector 1

Load code in boot sector Execute boot loader Boot loader loads program to be booted

• If no OS: "Non-system disk or disk error - Replace and press any key when ready"

Transfer control to loaded program Is it okay to boot at first sector on the floppy or

disk?

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Ways to Develop An Operating System A hardware simulator A virtual machine A good kernel debugger

– When OS crashes, always goes to the debugger

– Debugging over the network