Architectural Support for Operating Systems

Announcements Most office hours are finalized Assignments (if any) up every Tuesday –Due one week later –Main TA to contact about assignments: Nazrul Alam Also contact Levent Kartaltepe CS 4411 sections started yesterday –Oliver Kennedy is instructor –intro to the course and goals for the term –Overview of project and details on the first part of the project –C for Java programmers syntax and similarities common pitfalls C software engineering Optional architecture review session –Sometime later this week. Date, time, and location, TBD.

Announcements Make sure you are registered for CS 4410 and/or 4411 First CS 4411 project is up –Initial design documents due next Monday, September 8th –Project due following Wednesday, September 17 th First CS 4410 homework will be available later tonight –Due next Tuesday, September 9 th 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: History of OS Why Study? –To understand how user needs and hardware constraints influenced (and will influence) operating systems Several Distinct Phases: –Hardware Expensive, Humans Cheap Eniac, … Multics –Hardware Cheaper, Humans Expensive PCs, Workstations, Rise of GUIs –Hardware Really Cheap, Humans Really Expensive Ubiquitous devices, Widespread networking Rapid Change in Hardware Leads to changing OS –Batch  Multiprogramming  Timeshare  Graphical UI  Ubiquitous Devices  Cyberspace/Metaverse/?? –Gradual Migration of Features into Smaller Machines Situation today is much like the late 60s –Small OS: 100K lines/Large: 10M lines (5M browser!) – people-years

Review: Migration of OS Concepts and Features

Review: Implementation Issues (How is the OS implemented?) Policy vs. Mechanism –Policy: What do you want to do? –Mechanism: How are you going to do it? –Should be separated, since policies change

Goals for Today and Next Lecture I/O subsystem and device drivers Interrupts and traps Protection, system calls and operating mode OS structure What happens when you boot a computer?

Computer System Architecture Synchronizes memory access

I/O operations I/O devices and the CPU can execute concurrently. I/O is moving data between device & controller’s buffer –CPU moves data between controller’s buffer & main memory Each device controller is in charge of certain device type. –May be more than one device per controller SCSI can manage up to 7 devices –Each device controller has local buffer, special registers A device driver for every device controller –Knows details of the controller –Presents a uniform interface to the rest of OS

Accessing I/O Devices Memory Mapped I/O –I/O devices appear as regular memory to CPU –Regular loads/stores used for accessing device –This is more commonly used Programmed I/O –Also called “channel” I/O –CPU has separate bus for I/O devices –Special instructions are required Which is better?

Polling I/O Each device controller typically has: –Data-in register (for host to receive input from device) –Data-out (for host to send output to device) –Status register (read by host to determine device state) –Control register (written by host to invoke command)

Polling I/O handshaking: To write data to a device: 1.Host repeatedly reads busy bit in status register until clear 2.Host sets write bit in command register and writes output in data-out register 3.Host sets command-ready bit in control register 4.Controller notices command-ready bit, sets busy bit in status register 5.Controller reads command register, notices write bit: reads data-out register and performs I/O (magic) 6.Controller clears command-ready bit, clears the error bit (to indicate success) and clears the busy bit (to indicate it’s finished)

Polling I/O What’s the problem? –CPU could spend most its time polling devices, while other jobs go undone. But devices can’t be left to their own devices for too long –Limited buffer at device - could overflow if doesn’t get CPU service. Modern operating systems use Interrupts to solve this dilemma. CPU Memory Device DataAddr R/W Clock Interrupt Interrupt Priority

Interrupts CPU hardware has a interrupt-request line (a wire) it checks after processing each instruction. On receiving signal save processing state –Save current context –Jump to interrupt handler routine at fixed address in memory –Interrupt handler: Determine cause of interrupt. Do required processing. Restore state. Execute return from interrupt instruction. Device controller raises interrupt, CPU catches it, interrupt handler dispatches and clears it. Most operating systems are interrupt-driven –Hardware: sends trigger on bus –Software: uses a system call

Even interrupts are sometimes to slow: Device driver loads controller registers appropriately Controller examines registers, executes I/O Controller signals I/O completion to device driver –Using interrupts High overhead for moving bulk data (i.e. disk I/O): –One interrupt per byte..

Direct Memory Access (DMA) Transfer data directly between device and memory –No CPU intervention Device controller transfers blocks of data Interrupts when block transfer completed –As compared to when byte is completed Very useful for high-speed I/O devices

Example I/O cache CPU (*N) Thread of execution Memory Instructions and Data Instruction execution cycle Data movement Keyboard Device Driver and Keyboard Controller I/O Request Perform I/O Read Data Interrupt Data Disk Device Driver and Disk Controller DMA

Interrupt Timeline

Modern interrupt handling Modern OS needs more sophisticated mechanism: –Ability to defer interrupt –Efficient way to dispatch interrupt handler –Multilevel interrupts to distinguish between high and low priority Higher priority interrupts can pre-empt processing of lower priority ones –Use a specialized Interrupt Controller CPUs typically have two interrupt request lines: –Maskable interrupts: Can be turned off before critical processing –Nonmaskable interrupts: cannot be turned off Signifies serious error (e.g. unrecoverable memory error) Interrupts contain address pointing to interrupt vector –Interrupt vector contains addresses of interrupt handlers. –If more devices than elements in interrupt vector, then chain: A list of interrupt handlers for a given address which must be traversed to determine the appropriate one.

Traps and Exceptions Software generated interrupt –Exception: user program acts silly Caused by an error (div by 0, or memory access violation) Just a performance optimization –Trap: user program requires OS service Caused by system calls Handled similar to hardware interrupts: –Stops executing the process –Calls handler subroutine –Restores state after servicing the trap

Goals for Today and Next Lecture I/O subsystem and device drivers Interrupts and traps Protection, system calls and operating mode OS structure What happens when you boot a computer?

Why Protection? Application programs could: –Start scribbling into memory –Get into infinite loops Other users could be: –Gluttonous –Evil –Or just too numerous Correct operation of system should be guaranteed  A protection mechanism

Preventing Runaway Programs Also how to prevent against infinite loops – Set a timer to generate an interrupt in a given time – Before transferring to user, OS loads timer with time to interrupt – Operating system decrements counter until it reaches 0 – The program is then interrupted and OS regains control. Ensures OS gets control of CPU – When erroneous programs get into infinite loop – Programs purposely continue to execute past time limit Setting this timer value is a privileged operation – Can only be done by OS

Protecting Memory Protect program from accessing other program’s data Protect the OS from user programs Simplest scheme is base and limit registers: Virtual memory and segmentation are similar memory Prog A Prog B Prog C Base register Limit register Loaded by OS before starting program

Virtual Memory / Segmentation Here we have a table of base and limit values Application references some address x –CPU breaks this into a page number and an offset (or a segment number and offset) (More common) Pages have fixed size, usually 512 bytes (Now rare) Segments: variable size up to some maximum –Looks up the page table entry (PTE) for this reference –PTE tells the processor where to find the data in memory (by adding the offset to the base address in the PTE) –PTE can also specify other things such as protection status of page, which “end” it starts at if the page is only partially filled, etc.

Protected Instructions Also called privileged instructions. Some examples: Direct user access to some hardware resources – Direct access to I/O devices like disks, printers, etc. Instructions that manipulate memory management state – page table pointers, translation look-aside buffer (TLB) load, etc. Setting of special mode bits Halt instruction Needed for: Abstraction/ease of use and protection

Dual-Mode Operation Allows OS to protect itself and other system components –User mode and kernel mode OS runs in kernel mode, user programs in user mode –OS is supreme being, the applications are peasants –Privileged instructions only executable in kernel mode Mode bit provided by hardware –Can distinguish if system is running user code or kernel code –System call changes mode to kernel –Return from call using RTI resets it to user How do user programs do something privileged?

Crossing Protection Boundaries User calls OS procedure for “privileged” operations Calling a kernel mode service from user mode program: –Using System Calls –System Calls switches execution to kernel mode User processSystem Call Trap Mode bit = 0 Save Caller’s stateExecute system callRestore state Return Mode bit = 1 Resume process User Mode Mode bit = 1 Kernel Mode Mode bit = 0

System Calls Programming interface to services provided by the OS Typically written in a high-level language (C or C++) Mostly accessed by programs using APIs Three most common APIs: –Win32 API for Windows –POSIX API for POSIX-based systems (UNIX, Linux, Mac OS X) –Java API for the Java virtual machine (JVM) Why use APIs rather than system calls? –Easier to use

Why APIs? System call sequence to copy contents of one file to another Standard API

Reducing System Call Overhead Problem: The user-kernel mode distinction poses a performance barrier Crossing this hardware barrier is costly. System calls take 10x-1000x more time than a procedure call Solution: Perform some system functionality in user mode Libraries (DLLs) can reduce number of system calls, –by caching results (getpid) or –buffering ops (open/read/write vs. fopen/fread/ fwrite).

Real System Have Holes OSes protect some things, ignore others –Most will blow up when you run: –Usual response: freeze –To unfreeze, reboot –If not, also try touching memory Duality: Solve problems technically and socially –Technical: have process/memory quotas –Social: yell at idiots that crash machines –Similar to security: encryption and laws int main() { while (1) { fork(); }

Fixed Pie, Infinite Demands How to make the pie go further? –Resource usage is bursty! So give to others when idle. –Eg. When waiting for a webpage! Give CPU to idle process years old idea: instead of one classroom per student, restaurant per customer, etc. BUT, more utilization  more complexity. –How to manage? (1 road per car vs. freeway) –Abstraction (different lanes), Synchronization (traffic lights), increase capacity (build more roads) But more utilization  more contention. –What to do when illusion breaks? –Refuse service (busy signal), give up (VM swapping), backoff and retry (Ethernet), break (freeway)

Fixed Pie, Infinite Demand How to divide pie? –User? Yeah, right. –Usually treat all apps same, then monitor and re-apportion What’s the best piece to take away? –Oses are the last pure bastion of fascism –Use system feedback rather than blind fairness How to handle pigs? –Quotas (leland), ejection (swapping), buy more stuff (microsoft products), break (ethernet, most real systems), laws (freeway) –A real problem: hard to distinguish responsible busy programs from selfish, stupid pigs.

Summary: 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

Summary: 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

Summary: OS Structure Monolithic –Advantages: performance –Disadvantages: difficult to extend, debug, secure, and make reliable Layered –Advantages: simplicity of construction, debugging, and extensible –Disadvantages: defining layers, performance overhead Micro-kernel –Advantages: easy to extend, port. More reliable and secure. –Disadvantage: performance overhead Modular –Advantages: monolithic performance with layered flexibility –Disadvantages: modules can still crash system Virtual Machines –Advantages: protection/isolation, great systems building tool –Disadvantage: difficult to implement