Operating%20Systems%20I

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O.S. STRUCTURE
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OS Structure (View)
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OS Structure

• An OS consists of
• all these components
• system service routines
• system programs (privileged and non-privileged)
• The big issue
• How to organise all of these components?
• What are the entities and where do they exist?
• How do these entities co-operate?

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Monolithic Approach

• Basically, how to build a complex system that is
• performing well
• reliable
• extensible
• Traditionally, systems were built as monolithic
  kernels

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Monolithic Systems

• Problems with monolithic kernels
• hard to understand
• hard to modify
• unreliable a bug anywhere causes crashes
• hard to maintain
• Since the beginning, several ways of organising
  OS design and implementation were proposed

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Structuring

• Layering is the most popular approach
• the system is implemented as a set of layers
• each layer is a virtual machine to the layer
  below
• each layer provides a machine that has higher
  level features

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Layering in THE

• Layering was used for the first time in
  Dijkstras THE system

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THE layered OS

• THE is composed of a set of static processes
• Each process performs a sequential computation
• Processes communicate through explicit
  synchronisation statements
• Each process could be tested and verified
  independently
• Each layer sees a logical machine provided by the
  lower layers
• layer 0 Allocation of processor and
  multiprogramming
• layer 1 Allocates space for processes in main
  memory
• layer 2 handles commn between process and I/o
• layer 3 handles commn different processes
• Layer 4 handles the organization of file systems
• Layer 5 handles user programs

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Problems with layering

• The approach is not flexible
• Major changes in one layer will have effect on
  other layers
• Security difficult due to interaction of layers
• Difficult to customize the os

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Microkernel Approach

• The structure organisation currently in vogue is
  microkernel
• Benefits of m-kernel implementation
• uniform interfaces
• object-oriented OS
• distributed system support
• minimise the kernel
• put as much as possible in the user process level

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Microkernel OS

• As a result
• better reliability and portability
• easy to extend and customise
• poor performance (unfortunately)
• Examples
• first micro-kernel was Hydra(CMU, 1970)
• CMU Mach system
• Chorus (Unix-like system)
• in some ways Microsoft Windows NT

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Microkernel System Structure
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Hardware Features OS
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OS and Hardware

• OS is partly dedicated to a specific hardware
  architecture
• Hardware support can greatly simplify (or
  complicate) the OS design
• Example
• PC OSs have been primitive, because of the lack
  in hardware support (e.g. VM)

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OS Hardware

• Some hardware features that can directly support
  OS needs
• protected instructions
• OS protection (kernel/user mode)
• memory protection
• interrupts and exceptions
• timer (clock) operations
• synchronisation (atomic instructions)
• I/O control operations

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Protected Instructions

• Some instructions are typically restricted to the
  OS
• users are not allowed direct access to I/O (disk,
  printer,) (can be done through privileged
  instructions or memory mapping)
• Direct access to I/O devices like disks,
  printers, etc.
• Must control instructions that manipulate memory
  management state (page table pointers, TLB load,
  etc.)
• special mode bit settings (kernel mode)
• halt instructions

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Which of the following instructions (or
instruction sequences) should only be allowed in
kernel mode?

• Disable all interrupts.
• Read the time of day clock.
• Set the time of day clock.
• Change the memory map.
• Write to the hard disk controller register.

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Crossing Protection Boundaries

• To execute a privileged instructions, one must
  call OS procedures
• How does a user-mode call a kernel-mode service?
• There must be system call instructions that
• causes an exception, and kernel handler takes
  place
• passes parameters indicating which system routine
  to run
• saves callers state (PC, mode bit, )
• must allow OS to check callers parameters
• must be able to return to user-mode when finished

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Protection and Crossing
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Memory Protection

• Must be able to protect users programs from each
  other
• must protect OS from user programs
• Simplest scheme to do so is to use base and limit
  registers
• base and limit registers are loaded by the OS
  before starting the execution

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Traps and exceptions

• Traps and exceptions are initiated by the
  application
• Hardware must detect special conditions page
  fault, write to
• a read-only page, overflow, trace trap, odd
  address trap,
• privileged instruction trap...
• Must transfer control to handler within the
  O.S.
• Hardware must save state on fault (PC, etc) so
  that the
• faulting process can be restarted afterwards
• Modern operating systems use VM traps for many
  functions
• debugging, garbage collection

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I/O issues

• how to start an I/O (special instructions or
  memory-mapped I/O)
• I/O completion (interrupts)
• Synchronous I/O After I/O starts, control
  returns to user program only
• upon I/O completion.
• wait instruction idles the CPU until the next
  interrupt
• wait loop (contention for memory access).
• At most one I/O request is outstanding at a
  time, no simultaneous I/O processing.
• Asynchronous I/O After I/O starts, control
  returns to user program
• without waiting for I/O completion.

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I/O Control

• Interrupts are the basis for asynchronous I/O
• devices and CPU perform operations asynchronously
• devices send interrupt signal when done
• a vector table containing the list of kernel
  routine addresses to handle various events
• CPU switches to the address indicated by the
  interrupt signal

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Timer

• How does the OS prevent against runaway user
  programs
• (infinite loops)?
• A timer can be set to generate an interrupt in
  a given time.
• Before it transfers to a user program, the OS
  loads the timer
• with a time to interrupt.
• When the time arrives, the executing program is
  interrupted
• and the OS regains control.
• This ensures that the OS can get the CPU back
  even if a user
• program erroneously or purposely continues to
  execute
• past some allotted time.
• The timer is privileged only the OS can load
  it.