Architectural Support for Operating Systems

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Architectural Support for Operating Systems
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Announcements
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Todays material

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

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Computer System Architecture
Synchronizes memory access
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I/O operations

• I/O devices and the CPU can execute concurrently.
• I/O is moving data between device controllers
  buffer
• CPU moves data between controllers 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

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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?

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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)

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Polling I/O handshaking

• To write data to a device
• Host repeatedly reads busy bit in status register
  until clear
• Host sets write bit in command register and
  writes output in data-out register
• Host sets command-ready bit in control register
• Controller notices command-ready bit, sets busy
  bit in status register
• Controller reads command register, notices write
  bit reads data-out register and performs I/O
  (magic)
• Controller clears command-ready bit, clears the
  error bit (to indicate success) and clears the
  busy bit (to indicate its finished)

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Polling I/O

• Whats the problem?
• CPU could spend most its time polling devices,
  while other jobs go undone.
• But devices cant be left to their own devices
  for too long
• Limited buffer at device - could overflow if
  doesnt get CPU service.
• Modern operating systems uses Interrupts to solve
  this dilemma.
• Interrupts Notification from interface that
  device needs servicing
• Hardware sends trigger on bus
• Software uses a system call

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Interrupts

• CPU hardware has a interrupt-request line (a
  wire) it checks after processing each
  instruction.
• On receiving signal Save processing state
• 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.
• Modern OS needs more sophisticated mechanism
• Ability to defer interrupt
• Efficient way to dispatch interrupt handler
• Multilevel interrupts to to distinguish between
  high and low priority interrupts.

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Modern interrupt handling

• Use a specialized Interrupt Controller
• CPUs typically have two interrupt request lines
• Maskable interrupts Can be turned of by CPU
  before critical processing
• Nonmaskable interrupts signifies serious error
  (e.g. unrecoverable memory error)
• Interrupts contain address pointing to interrupt
  vector
• Interrupt vector contains addresses of
  specialized interrupt handlers.
• If more devices than elements in interrupt
  vector do interrupt chaining
• A list of interrupt handlers for a given address
  which must be traversed to determine the
  appropriate one.
• Interrupt controller also implements interrupt
  priorities
• Higher priority interrupts can pre-empt
  processing of lower priority ones

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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..

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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

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

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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

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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

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Protecting Memory

• Protect program from accessing other programs
  data
• Protect the OS from user programs
• Simplest scheme is base and limit registers
• Virtual memory and segmentation are similar

memory
Prog A
Base register
Loaded by OS before starting program
Prog B
Limit register
Prog C
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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, TLB load, etc.
• Setting of special mode bits
• Halt instruction
• Needed for
• Abstraction/ease of use and protection

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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 god, 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?

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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 Mode Mode bit 1
Resume process
User process
System Call
Trap Mode bit 0
Kernel Mode Mode bit 0
Return Mode bit 1
Save Callers state
Execute system call
Restore state
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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)

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Types of System Calls

• Process Control
• end or abort process create or terminate
  process, wait for some time allocate or
  deallocate memory
• File Management
• open or close file read, write or seek
• Device Management
• attach or detach device read, write or seek
• Information Maintenance
• get process information get device attributes,
  set system time
• Communications
• create, open, close socket, get transfer status.

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

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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()
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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.
• Ancient 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)

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Fixed Pie, Infinite Demand

• How to divide pie?
• User? Yeah, right.
• Usually treat all apps same, then monitor and
  re-apportion
• Whats 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.

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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 O/S executable
• From hard disk, CD-ROM, or whatever
• Then transfers control to a standard start
  address in this image
• The small version of the O/S loads and starts the
  big version.
• The two stage mechanism is used so that BIOS
  wont need to understand the file system
  implemented by the big O/S kernel
• File systems are complex data structures and
  different kernels implement them in different
  ways
• The small version of the O/S is stored in a
  small, special-purpose file system that the BIOS
  does understand

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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 (DECs VMS on VAX
  computed Pi)
• OS wakes up on
• interrupts from hardware devices
• traps from user programs
• exceptions from user programs

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OS Control Flow
From boot
main()
System call
Initialization
Interrupt
Exception
Idle Loop
Operating System Modules
RTI
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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 security

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General OS Structure
API
Process Manager
Network Support
Service Module
Monolithic Structure
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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

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Layered Structure
API
Object Support
Network Support
Process Manager
M/C dependent basic implementations
Hardware Adaptation Layer (HAL)
Boot init
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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

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Microkernel Structure
Process Manager
Network Support
Basic Message Passing Support
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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

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UNIX structure
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Windows Structure
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Modern UNIX Systems
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MAC OS X
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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,
• CLR, Xen

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VMWare Structure
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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 cant tell!

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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 (by the
  spyware)?
• Basically no, it cant!
• Vendors are adding Trusted Computing Base (TCB)
  technologies to help
• Hardware that cant be virtualized
• Well discuss it later in the course