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 ( –Check and contact me or Bill Hogan 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 Idle Loop From boot Initialization RTI Interrupt System call main() Exception

Operating System Structure Simple Structure: MS-DOS –Written to provide the most functionality in the least space –Applications have direct control of hardware Disadvantages: –Not modular –Inefficient –Low protection or security

General OS Structure Device Drivers Extensions & Add’l device drivers Interrupt handlers File Systems Memory Manager Process Manager Security Module API App Network Support Service Module 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 Device Drivers Extensions & Add’l device drivers Interrupt handlers File Systems Memory Manager Process Manager API App Network Support Boot & init App Object Support 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 Device Drivers Extensions & Add’l device drivers Interrupt handlers File Systems Memory Manager Process Manager Security Module App Network Support 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