Operating System Structure. Announcements Make sure you are registered for CS 415 First CS 415 project is up –Initial design documents due next Friday,

Operating System Structure

Announcements

• Make sure you are registered for CS 415

• First CS 415 project is up– Initial design documents due next Friday, February 2nd

– Project due following Thursday, February 8th

• Everyone should have access to CMS (http://cms3.csuglab.cornell.edu)– Check and contact me (hweather@cs.cornell.edu) or Bill Hogan

(whh@cs.cornell.edu) today if you do not have access to CMS

• Also, everyone should have CSUGLab account– Contact Bill or I if you do not

Review: Protecting Processes from Each Other

• Problem: Run multiple applications in such a way that they are protected from one another

• Goal: – Keep User Programs from Crashing OS– Keep User Programs from Crashing each other– [Keep Parts of OS from crashing other parts?]

• (Some of the required) Mechanisms:– Dual Mode Operation – Address Translation (base/limit registers, page tables, etc)– Privileged instructions (set timer, I/O, etc)

• Simple Policy:– Programs are not allowed to read/write memory of other Programs or of

Operating System

Review: Dual Mode Operation• Hardware provides at least two modes:

– “Kernel” mode (or “supervisor” or “protected”)

– “User” mode: Normal programs executed • Some instructions/ops prohibited in user mode:

– Example: cannot modify page tables in user mode

• Attempt to modify Exception generated • Transitions from user mode to kernel mode:

– System Calls, Interrupts, Other exceptions

Today’s Lectures

• I/O subsystem and device drivers

• Interrupts and traps

• Protection, system calls and operating mode

• OS structure

• What happens when you boot a computer?

Operating System Structure

• An OS is just another kind of program running on the CPU – a process:– It has main() function that gets called only once (during boot)

– Like any program, it consumes resources (such as memory)

– Can do silly things (like generating an exception), etc.

• But it is a very sophisticated program:– “Entered” from different locations in response to external events

– Does not have a single thread of control• can be invoked simultaneously by two different events

• e.g. sys call & an interrupt

– It is not supposed to terminate

– It can execute any instruction in the machine

How do you start the OS?

• Your computer has a very simple program pre-loaded in a special read-only memory– The Basic Input/Output Subsystem, or BIOS

• When the machine boots, the CPU runs the BIOS • The BIOS, in turn, loads a “small” OS executable

– From hard disk, CD-ROM, or whatever– Then transfers control to a standard start address in this image– The small version of the OS loads and starts the “big” version.

• The two stage mechanism is used so that BIOS won’t need to understand the file system implemented by the “big” OS kernel

• File systems are complex data structures and different kernels implement them in different ways

• The small version of the OS is stored in a small, special-purpose file system that the BIOS does understand

What does the OS do?

• OS runs user programs, if available, else enters idle loop

• In the idle loop:

– OS executes an infinite loop (UNIX)

– OS performs some system management & profiling

– OS halts the processor and enter in low-power mode (notebooks)

– OS computes some function (DEC’s VMS on VAX computed Pi)

• OS wakes up on:

– interrupts from hardware devices

– traps from user programs

– exceptions from user programs

OS Control Flow

Operating System Modules

IdleLoop

From boot

Initialization

RTI

InterruptSystem call

main()

Exception

Operating System Structure

• Simple Structure: MS-DOS– Written to provide the most functionality in the least space

– Applications have directcontrol of hardware

• Disadvantages:– Not modular

– Inefficient

– Low protection or security

General OS Structure

DeviceDrivers

Extensions &Add’l device drivers

Interrupthandlers

File Systems

Memory Manager

ProcessManager

SecurityModule

API

App App

NetworkSupport

ServiceModule

Boot &init

App

Monolithic Structure

Layered Structure

• OS divided into number of layers – bottom layer (layer 0), is the hardware– highest (layer N) is the user interface– each uses functions and services of only lower-level layers

• Advantages:– Simplicity of construction– Ease of debugging– Extensible

• Disadvantages:– Defining the layers– Each layer adds overhead

Layered Structure

DeviceDrivers

Extensions &Add’l device drivers

Interrupthandlers

File Systems

Memory Manager

ProcessManager

API

App App

NetworkSupport

Boot &init

App

ObjectSupport

Machine dependent basic implementations

Hardware Adaptation Layer (HAL)

Microkernel Structure

• Moves as much from kernel into “user” space

• User modules communicate using message passing

• Benefits:– Easier to extend a microkernel

– Easier to port the operating system to new architectures

– More reliable (less code is running in kernel mode)

– More secure

– Example: Mach, QNX

• Detriments:– Performance overhead of user to kernel space communication

– Example: Evolution of Windows NT to Windows XP

Microkernel Structure

DeviceDrivers

Extensions &Add’l device drivers

Interrupthandlers

File Systems

Memory Manager

ProcessManager

SecurityModule

App

NetworkSupport

Boot &init

App

Basic Message Passing Support

Modules• Most modern OSs implement kernel modules

– Uses object-oriented approach

– Each core component is separate

– Each talks to the others over known interfaces

– Each is loadable as needed within the kernel

• Overall, similar to layers but with more flexible

• Examples: Solaris, Linux, MAC OS X

UNIX structure

Windows Structure

Modern UNIX Systems

MAC OS X

Virtual Machines

• Implements an observation that dates to Turing– One computer can “emulate” another computer

– One OS can implement abstraction of a cluster of computers, each running its own OS and applications

• Incredibly useful!– System building

– Protection

• Cons– implementation

• Examples– VMWare, JVM

VMWare Structure

But is it real?

• Can the OS know whether this is a real computer as opposed to a virtual machine?– It can try to perform a protected operation…

but a virtual machine monitor (VMM) could trap those requests and emulate them

– It could measure timing very carefully… but modern hardware runs at variable speeds

• Bottom line: you really can’t tell!

Modern version of this question

• Can the “spyware removal” program tell whether it is running on the real computer, or in a virtual machine environment created just for it?– Basically: no, it can’t!

• Vendors are adding “Trusted Computing Base” (TCB) technologies to help– Hardware that can’t be virtualized– We’ll discuss it later in the course