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OS/Kernel Structure


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Title: OS/Kernel Structure

OS/Kernel Structure
  • Module 01

MS-DOS Layer Structure
Unix is a monolithic system
  • UNIX the original UNIX operating system had
    limited structuring. The UNIX OS consists of two
    separable parts.
  • Systems programs
  • The kernel
  • Consists of everything below the system-call
    interface and above the physical hardware
  • Provides the file system, CPU scheduling, memory
    management, and other operating-system functions
    a large number of functions for one level.

UNIX System Structure
System Structure Layered Approach
  • The operating system is divided into a number of
    layers (levels), each built on top of lower
    layers. The bottom layer (layer 0), is the
    hardware the highest (layer N) is the user
  • With modularity, layers are selected such that
    each uses functions (operations) and services of
    only lower-level layers.
  • Why layering?
  • explicit structure allows identification,
    relationship of complex systems pieces
  • modularization eases maintenance, develop,
    updating of system
  • change of implementation of layers service
    transparent to rest of system
  • e.g., change in gate procedure doesnt affect
    rest of system
  • layering considered harmful?

Layered Structure of the THE OS
  • A layered design was first used in THE operating
  • Its six layers are as follows

layer 5 user programs
layer 4 buffering for input and output
layer 3 Process management
layer 2 memory management
layer 1 CPU scheduling
layer 0 hardware
OS/2 Layer Structure
  • Small operating system core
  • Contains only essential operating systems
  • Many services traditionally included in the
    operating system are now external subsystems
  • device drivers
  • file systems
  • virtual memory manager
  • windowing system
  • security services

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Benefits of a Microkernel Organization
  • Uniform interface on request made by a process
  • All services are provided by means of message
    passing, not system calls.
  • Examples of opening a file
  • Creating threads
  • Extensibility
  • Allows the addition of new services
  • Flexibility
  • New features added
  • Existing features can be subtracted

Benefits of a Microkernel Organization
  • Portability
  • Changes needed to port the system to a new
    processor is changed in the microkernel - not in
    the other services
  • Reliability
  • Modular design
  • Small microkernel can be rigorously tested

Benefits of Microkernel Organization
  • Distributed system support
  • Message are sent without knowing what the target
    machine is
  • Object-oriented operating system
  • Components are objects with clearly defined
    interfaces that can be interconnected to form

Microkernel Design
  • Microkernel must include functions that depend on
    the HW and functions needed to support the
    servers and applications operating in user mode.
  • Low-level memory management
  • mapping each virtual page to a physical page
    frame. Other memory functions like swapping,
    protection, paging, etc is done by VM service.
  • Inter-process communication
  • Using ports. A port is a mailbox associated with
    one receiver and multiple senders.
  • I/O and interrupt management
  • Interrupts are transformed into messages.

  • Examples of ukernels are Mach and Chorus
  • Are the basis for KLM (Kernel Loadable Modules)
  • Linux modules are located in /lib/modules and
    they have had the extension ".ko" since version
  • Sound
  • Device Drivers
  • Network support (Apple Talk, Novel, etc)
  • Language support
  • VM (the virtualization layer)
  • etc

Virtual Machines
  • A virtual machine provides multiprogramming only
    by providing exact virtual copies of the bare
    hardware .
  • A virtual machine provides an interface identical
    to the underlying bare hardware.
  • The operating system creates the illusion of
    multiple processes, each executing on its own
    processor with its own (virtual) memory.
  • Virtual Machine A machine implemented in
    software, not actual hardware. Also known as a
    machine emulator, not as OS simulator.
  • Each virtual machine can run any OS on top of it
  • You can run different OSs, each best suited for
    some task, on the same physical machine
  • Similar to Java approach. JVM is an application
  • Virtual PC for Windows allows you to create
    separate virtual machines on top of your
    Windows desktop, where you can install virtually
    any PC-based operating system including OS/2,
    Linux, Solaris, NetWare or other versions of
    Windows. Each virtual machine emulates a complete
    hardware system from processor to network card
    in a self-contained, isolated software
    environment, enabling the simultaneous operation
    of otherwise incompatible systems.

Virtual Machines (Cont.)
  • The resources of the physical computer are shared
    to create the virtual machines.
  • CPU scheduling can create the appearance that
    users have their own processor.
  • Spooling and a file system can provide virtual
    card readers and virtual line printers.
  • A normal user time-sharing terminal serves as the
    virtual machine operators console.
  • Instruction set emulation
  • Benefits of VM OS
  • A big plus for SW development can run and test
    SW simultaneously on multiple OS on a single
    machine, without rebooting. Testing can take
    place with different system configuration, like
    memory, disk space, etc.
  • IT professionals can safely migrate and deploy
    new operating systems while continuing to run
    and support older or custom legacy applications.
  • Internet VPS
  • Cloud Computing Amazon Elastic CC (or EC2)
  • Testing networked machines with multiple NEs
    (routers, FWs, etc)
  • Testing and parallel and distributed programming
    for cluster nodes
  • Sandboxing play and test in a protected
    environment w/o risking damage to other kernels
  • Server consolidation virtualization allowed a
    single server to replace multiple underutilized
    dedicated servers.
  • Any drawbacks?

System Models
Similar to standalone Hyber-V whereby a thin
layer is installed first
Virtual Machine (Classical Type originated by
IBM VM370 in 1972)
Non-virtual Machine
Virtualization layer or Hypervisor or VMM
Possible that it can span over multiple CPUs
(even in a distributed fashion). Referred as
Platform Virtualization
  • VmWare (from Vmware Inc.),
  • Virtual PC (Connectix, now Mircorsoft, Integrated
    within Windows 7 to run all Windows XP apps)
  • KVM (preferred choice for Red Hat and Ubuntu)
  • Xen (freeware)
  • Virtualbox from Sun Inc.
  • A complete list is available at

VMware architecture Common Type (e.g. vmware)
Modern Virtualization
  • Intel VT and AMD-V processors have hardware
    assisted virtualization to yield better
  • Full vs. Partial Virtualization
  • Much of the underlying hardware is emulated, but
    not all
  • Paravirtualization
  • Guest OS has special calls to VM, called
    hybervisor calls to speed access
  • i.e. it provides an interface to VM
  • Faster as the virtualization layer becomes

Windows Hyper-V (2008 Server)
Hyper-V Architecture (1/2)
  •  A partition is a logical unit of isolation in
    which an OS executes.
  • The virtualization stack runs in the parent
    partition and has direct access to the hardware
  • The parent partition then creates the child
    partitions which host the guest OSs. A parent
    partition creates child partitions using
    the hypercall API.
  • A virtualized partition does not have access to
    the physical processor, nor does it handle its
    real interrupts. Instead, it has a virtual view
    of the processor and runs in Guest Virtual

Hyper-V Architecture (2/2)
  • The hypervisor handles the interrupts to the
    processor, and redirects them to the respective
    partition using a logical Synthetic Interrupt
    Controller (SynIC). Hyper-V can hardware
    accelerate the address translation between
    various Guest Virtual Address-spaces by using
    an IOMMU (I/O Memory Management Unit) which
    operates independent of the memory management
    hardware used by the CPU.
  • Parent partitions run a Virtualization Service
    Provider (VSP), which connects to the VMBus and
    handles device access requests from child
    partitions. Child partition virtual devices
    internally run a Virtualization Service
    Client (VSC), which redirect the request to VSPs
    in the parent partition via the VMBus.

Microsoft Windows
  • Single-user multitasking
  • From Windows 2000 on, Windows development
    developed to exploit modern 32-bit and 64-bit
  • Designed for single users who run multiple
  • Main drivers are
  • Increased memory and speed of microprocessors
  • Support for virtual memory

Windows Architecture
Client/Server Model
  • Windows OS, protected subsystem, and applications
    all use a client/server model
  • Common in distributed systems, but can be used
    internal to a single system
  • Processes communicate via RPC

Windows Objects
  • Windows draws heavily on the concepts of
    object-oriented design.
  • Key Object Oriented concepts used by Windows are
  • Encapsulation
  • Object class and instance

Traditional UNIX Kernel
System V Release 4 (SVR4)
  • Modular Monolithic Kernel
  • Although monolithic, the kernel is structures as
    a collection of modules
  • Loadable modules
  • An object file which can be linked and unlinked
    at run time
  • Characteristics
  • Dynamic Linking
  • Stackable modules
  • For checking dependencies when unloading

Example of loading two modules FAT VFAT
VFAT is a client of FAT
  • next Pointer to the following module. All
    modules are organized into a single linked list.
  • name Pointer to module name.
  • sizeModule size in memory pages.
  • usecountModule usage counter.
  • The counter is incremented when an operation
    involving the modules functions is started and
    decremented when the operation terminates.
  • flagsModule flags.
  • nsyms Number of exported symbols.
  • ndeps Number of referenced modules
  • syms Pointer to this modules symbol table.
  • deps Pointer to list of modules the are
    referenced by this module.
  • refs Pointer to list of modules that use this

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Linux Kernel Components
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