Systems Architecture, Fifth Edition

1
SCSC 311 Information Systems hardware and
software
(Part I)
2
Index (Part I)

• Overview of operating system
• Single-Tasking OS vs. Multitasking OS
• Resources allocation
• Interrupts

3
Computer System Organization

• Computer System contains one or more CPUs,
  devices (device controllers) connect through
  common bus providing access to shared memory
• Concurrent executing programs competing for CPU
  and other shared resources

Functionality comes with great complexity!
4
How do we tame complexity?

• Every piece of computer hardware is different
• Different CPU Pentium, PowerPC, MIPS
• Different amounts of memory, disk,
• Different types of devices Mice, Keyboards,
  Sensors, Cameras, Fingerprint readers
• Different networking environment Cable, DSL,
  Wireless
• Some questions we want to answer
• Do the programmers need to write a single program
  that performs many independent activities?
• Does every program have to be altered for every
  piece of hardware?
• Does a faulty program crash everything?
• Does every program have access to all hardware?

5
Virtual Machine Abstraction
Application Operating System Hardware
Virtual Machine Interface
Physical Machine Interface

• For Any OS area (e.g. file systems, virtual
  memory, networking, scheduling)
• the hardware interface messy physical reality
• the application interface a nicer abstraction

6
Components of a Computer System
OS implements a virtual machine that is easier
and safer to program and use than the raw
hardware.
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What does an Operating System do?

• OS acts as
• Coordinator
• Manages all resources
• Settles conflicting requests for resources
• Prevent errors and improper use of the computer
• Facilitator
• Provides facilities that everyone needs
• Standard Libraries, Windowing systems
• Make application programming easier, faster, less
  error-prone
• OS consists of
• Kernel
• service layer
• command layer

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• the users interface to OS
• Shell or GUI

• Contains set of functions executed by application
  programs and command layer.
• a request to execute a service layer function is
  called a service call

• Manages resources
• interacts directly with hardware
• service layer functions rely on the kernel to
  interact with hardware

Functions within one layer can be modified
without affecting other layers ? make OS more
maintainable
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Kernel

• Kernel is the one program running at all times on
  the computer.
• Kernel directly interacts with hardware
• Everything else is either system program (ships
  with the OS) or application program

10
OS Management Functions

• OS management functions can be loosely divided
  to
• oriented to hardware resources
• oriented to users and their programs

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Index (Part I)

• Overview of operating system
• Single-Tasking OS vs. Multitasking OS
• Resources allocation
• Interrupts

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Single-Tasking OS vs. Multitasking OS

• Single-Tasking OS
• Involves only two executing programs
• OS reserve resources required when it boots
  up
• One application program has all remaining
  resources
• Single tasking OS are small and efficient
  why?
• Application error or service call are processed
  through interrupts.
• When an interrupt is detected, OS takes control
  of CPU back from application program
• Multitasking OS
• OS and multiple application programs are running
• More flexible to build and use
• since large program can be built from smaller
  independent modules or processes.
• e.g. load / start / stop module without reboot
  in Linux
• e.g. Your PC is receiving emails while youre
  editing a file
• Most modern OS, such as Linux, Mac, MS Windows
• However, multitasking OS require more complicated
  resource allocation scheme.

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Single-Tasking OS

• Examples of single tasking OS
• Very early computers
• Early PCs
• Embedded controllers (elevators, cars, etc)
• Single-tasking OS consists of a library of
  standard services
• Standard device drivers
• Interrupt handlers (next slide)
• Errors and service alls from application program
  are normally processed through interrupts these
  interrupts are generated by software
• Math libraries
• How does single-tasking OS work? (p435)
• In a single-tasking environment, when an
  application program begins executing, the
  operating system grants it control of all unused
  hardware

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Protection Mechanisms in Multitasking OS

• 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?
• Protection mechanisms (more details next)
• Address Translation
• Dual Mode Operation

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Address Translation

• Address Space
• A group of memory addresses usable by some
  processes
• Each program (process) and kernel has different
  address spaces.
• Address Translation
• Translate from Virtual Addresses (emitted by CPU)
  into Physical Addresses (of RAM)
• Mapping often performed in Hardware by Memory
  Management Unit (MMU)

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Prog 1 Virtual Address Space
Prog 2 Virtual Address Space
Translation Map 1
Translation Map 2
Physical Address Space
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Dual Mode Operation

• Hardware provides at least two operation modes
• Kernel mode ( supervisor or protected mode)
• User mode Normal programs executed
• Some instructions/operations are prohibited in
  user mode
• E.g., cannot modify page tables in user mode,
  user program attempts to modify ? an exception is
  generated
• Transitions from user mode to kernel mode
• Through system calls, Interrupts, or exceptions
• Kernel takes control back from user process,
  verifies the system calls, and handles the
  interrupts or exceptions.

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Index (Part I)

• Overview of operating system
• Single-Tasking OS vs. Multitasking OS
• Resources allocation
• Interrupts

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Resources

• Resources passive entities needed by
  processes/threads to do their work
• CPU time, disk space, memory
• Resources may require exclusive access or may be
  sharable
• e.g. Read-only files are typically sharable
• Printers are not sharable during time of
  printing
• One of the major tasks of an operating system is
  to manage resources

20
Resource Allocation

• Resource Allocation in OS
• Keep detailed records of available resources
  know which resources can satisfy which requests
• Schedule resources based on specific allocation
  policies
• Update records to reflect resource commitment and
  release by programs and users

21
Real Resources vs. Virtual Resources

• Real resources
• Physical devices and associated system software
• e.g. HD, RAM, CPU, printer,
• Virtual resources
• Resources that are apparent to a process or user
• Meet or exceed real resources by
• Rapidly shifting resources unused by one program
  to other programs that need them
• Q1 Can you give one example?
• Substituting one type of resource for another
• Q2 Can you give one example?

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Issues with Resource Sharing

• Starvation thread waits indefinitely
• e.g., low-priority thread waiting for resources
  constantly in use by high-priority threads
• Deadlock circular waiting for resources
• e.g., Thread A owns Res 1 and is waiting for Res
  2Thread B owns Res 2 and is waiting for Res 1

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Dining Philosophers Problem

• Five chopsticks and five philosophers
• What if all grab at same time?
• ? Deadlock!
• Q. How to fix deadlock after it occurs?
• One solution make one of them give up a
  chopstick ? Eventually everyone will get chance
  to eat
• Any other proposals?
• How to prevent deadlock?
• One solution never let a philosopher take the
  last chopstick if no hungry philosopher has two
  chopsticks afterwards

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Three Methods for Handling Deadlocks

• Detect and recover allow entering deadlock
  state, detect it and then recover
• Requires deadlock detection algorithm
• Prevent or avoid deadlock ensure that system
  will never enter a deadlock
• Ignore deadlock and pretend that deadlocks never
  occur in the system
• used by most operating systems, including UNIX,
  Window
• The simplest way to deal with deadlock
• What would happen?

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Index (Part I)

• Overview of operating system
• Single-Tasking OS vs. Multitasking OS
• Resources allocation
• Interrupts

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Interrupt Processing (1) (revisit P227-231)

• Peripheral device sends an interrupt, which is an
  electrical signal over control bus
• CPU continuously monitors the bus for interrupt
  signals, and copy them to interrupt register in
  the form of a numeric value -- interrupt code
• The control unit checks interrupt register at the
  end of each execution cycle (steps on the
  next slide)
• If an interrupt code is found, CPU suspends the
  execution of the current process, reset interrupt
  register to zero, and process the interrupt.
• When the interrupt has be processed, the CPU
  resumes executing the suspended process.
• Remark coordinating peripheral device
  communication with interrupts allows the CPU do
  something useful while its waiting for an
  interrupt.
• Q. Can you give one example?

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Interrupt Processing (2) (revisit P230)

• Q Why do we need stack in interrupt processing?
  
• Ans OS needs to be able to restart a suspended
  process at exactly the position is was
  interrupted.
• CPU pushes current register values on stack the
  saved register values are called the machine
  state.
• CPU executes a master interrupt handler
  supervisor
• Supervisor searches interrupt table based on
  interrupt code, and transfers control to a
  corresponding interrupt handler
• When the interrupt handler finishes, CPU pops the
  stack and load them back into the appropriated
  registers ? the suspended process resumes from
  point of interruption