CMSC421
Chapter 22
Operating System Concepts
Windows XP
Windows XP
 Windows XP operating system is a 32/64-
bit preemptive multitasking operating
system for AMD K6/K7, Intel IA32/IA64 and
later microprocessors. It is the successor
to Windows NT/2000 and the replacement
for Windows 95/98.
History
 Many features are from NT/2000.
 Uses a client-server architecture (like
Mach) to implement multiple operating
system personalities, such as Win32 and
POSIX.
 Comes in two versions, Personal and
Professional.
Design Principles
Security
 Reliability
 Windows and POSIX Application Compatibility
 High Performance
 Extensibility
 Portability
 International Support

Security
 Adherence to the design standards to
receive U.S. Government C-2 security
designation.
 Extensive code review and testing, plus use
of sophisticated automatic analysis tools
to investigate potential defects that might
represent security vulnerabilities.
Reliability
 Used extensive manual and automatic code
review to identify over 63,000 lines in the
source files that might contain issues not
detected by testing.
 Extends driver verification to catch more
subtle bugs, improves the facilities for
catching programming errors in user-level
code,.
Reliability -- II
 Subjects third party applications, drivers
and devices to a rigorous certification
process.
 New facilities for monitoring the health of
the PC including downloading fixes for
problems before they are encountered by
the users.
Windows/ POSIX Application
Compatibility
Application compatibility is difficult to achieve
because each application checks for a particular
version of Windows, may have dependence on the
quirks of the implementation of APIs.
 XP introduces a compatibility layer that falls
between applications and the Win32 API. This
layer makes XP look “bug-for-bug” compatible
with previous versions of Windows.

Windows/ POSIX Application
Compatibility -- II
 Maintains support for running many 16-bit
applications using a “thunking layer” that
translates 16-bit API calls into equivalent
32-bit calls.
 64-bit version provides a thunking layer
that translates 32-bit API calls into native
64-bit calls.
Windows/ POSIX Application
Compatibility -- III
 POSIX support is a POSIX subsystem called
Interix.
 Most available UNIX-compatible software
compiles and runs under Interix without
modification.
High Performance
 Desktop systems are largely constrained
by I/O performance.
 Servers, and large multi-threaded and
multi-processor environments are often
constrained by CPU. Locking and cacheline management are key to scalability.
High Performance Techniques
 Asynchronous I/o
 Optimized protocols for networks
 Kernel-based graphics
 Sophisticated caching of file-system data.
 Memory management
 Synchronization algorithms
 Reducing code-path length in critical
functions.
High Performance Techniques -II
 Using better algorithms and per-processor
data structures.
 Implementing more scalable locking
protocols.
 Except while executing in the kernel
dispatcher, threads in the subsystems of
Windows XP can be preempted by higherpriority threads.
Extensibility
 Used a layered architecture.
 Uses environmental subsystems to
emulate different operating systems (DOS,
Windows, POSIX).
 Uses loadable drivers in the I/O system.
 Uses RPCs for distributed processing.
Portability
 Can be moved from one hardware
architecture to another with relatively few
changes.
 Majority of the system is written in C and
C++.
 Processor-dependent code is isolated in
DLL called hardware-abstraction layer
(HAL).
Portability -- II
 Only shipped on IA32 compatible platforms,
it was also tested on DEC Alpha.
 “Maintaining portability is a matter of use it
or loss it.”
International Support
 Uses National Language Support API.
 Uses UNICODE as the native language
character code.
 ASCII is converted to UNICODE.
 System text strings are kept in resource
files that can be replaced to localize the
system for different languages.
 Multiple locales can be used concurrently.
System Components
 Hardware-Abstraction Layer
 Kernel
 Executive
Logon
process
Security
subsystem
OS/2
applications
OS/2
subsystem
Win16
applications
Win32
applications
Win16
VDM
MSDOS
applications
MSDOS
VDM
POSIX
applications
POSIX
subsystem
Authentication
package
Security account
manager database
I/O manager
File system
Cache
manager
Win32
subsystem
executive
Object
manager
Security
reference
monitor
Process
manager
Plug and
play
manager
Device
drivers
Network
drivers
kernel
Hardware abstraction layer
Hardware
Windows XP block diagram
Virtual
memory
manager
Local
procedure
call
facility
Window
manager
Graphic
device
drivers
Hardware Abstraction Layer
 HAL is the layer of software that hides
hardware differences from upper levels of
the operating system.
 HAL exports a virtual-machine interface
that is used by the kernel dispatcher, the
executive, and the device drivers.
 Only a single version of each device driver
is required.
Kernel
 Kernel Dispatcher
 Threads and Scheduling
 Implementation of Synchronization
Primitives
 Software Interrupt: Async Proc Calls
 Software Interrupt: Deferred Proc Calls
 Exceptions and Interrupts
Kernel -- II
Is object oriented
 An object type is a system defined data type with
a set of attributes and a set of methods.
 An object is an instance of an object type.
 The kernel performs its job by using a set of
kernel objects whose attributes shore the kernel
data, and whose methods perform the kernel
activities.

Kernel Dispatcher
 Provides the foundation for the executive
and subsystems
 Most is never paged out
 Can not be preempted
Kernel Dispatcher
Responsibilities
 Thread scheduling
 Implementation of synch primitives
 Timer management
 Software interrupts
 Exception dispatching
Threads and Scheduling
 Uses processes and threads for executable
code.
 Process has a virtual memory address
space and information to initialize each
thread.
 Each thread is an executable unit
dispatched by the kernel.
Thread states
 Ready
 Standby
 running
 waiting
 transition
 terminated
Priority scheme
 32-level scheme
 Variable class (0 - 15)
 Real-time class (16 - 31)
 Uses a queue for each priority
Implementation of
Synchronization Primitives
Dispatcher objects control dispatching and
synchronization
 Event objects record an event occurrence
 Mutant provides mutex
 Semaphore object acts as a counter or a gate
 Thread object is the entity that is scheduled
 Timer objects keep track of time

Asynchronous Procedure Call
Break into an executing thread and call
procedure.
 APCs are used to begin execution of a new thread,
terminate processes, and deliver notification that
an asynch I/O has completed.
 Queued to a specific thread
 Allow the system to execute both system and user
code within a process’ context

Deferred Procedure Call
 DPCs are used to postpone interrupt
processing.
 Process timer expirations
 Preempt thread execution at end of
scheduling quantum
Exceptions and Interrupts
 Architecture-independent exceptions
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memory-access violation
integer and floating point divide by zero
overflow or underflow
illegal instruction
data misalignment
privileged instruction
page read error, etc
Exceptions
 Trap handlers deal with simple exceptions
 More elaborate handling is performed by
the kernel’s exception dispatcher.
 If no handler is found, a system error
occurs and results in a “blue screen of
death”
 Exception handing is more complex for
user-mode processes
Exceptions -- II
 The interrupt dispatcher in the kernel
handles interrupts by calling an Interrupt
Service Routine (ISR) supplied by a device
driver or a kernel trap handler routine.
 For portability, the interrupt dispatcher
maps hardware interrupts into a standard
set.
Executive

Provides a set of services that all environmental
subsystems use:
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Object manager
Virtual memory manager
Process manager
Local procedure call facility
I/O manager
Security reference monitor
Plug-and-play and security managers
registry
booting
Object Manager
 A generic set of interfaces for managing
the kernel-mode entities that is
manipulated by user-mode programs.
 Examples:
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semaphores
mutexes
events
processes
threads
Handles
 Each process has an object table that track
the objects used by the process
 User-mode code accesses these objects
using an opaque value called a handle that
is returned by many APIs.
 Object handles can be created by
duplicating an existing handle, either from
either the same or different process.
Virtual Functions
 Objects are manipulated by a set of virtual
functions with implementations provided
by each object type:
• query - gets object’s name
• parse - search for an object given the object’s
name
• security - makes security checks on all object
operations.
Virtual Memory Manager
 The design of the VM Manager assumes
that the underlying hardware supports
virtual-to-physical mapping, a paging
mechanism, transparent cache coherence
on multiprocessor systems and allows
multiple page-table entries to map to the
same physical page frame.
Virtual Memory Manager -- II
On IA32 processors each process has a 4GB
virtual address space.
 The upper 2GB are mostly identical for all
processes, and are used by Windows XP in kernel
mode to access the operating system code and
data structures.
 Key areas that are not identical are page table
self-map, hyperspace and session space.

Page Table Self-map
 The hardware uses a process’s page tables
using physical page-frame numbers.
 The VM manager maps the page tables into
a single 4GB region in the process’ address
space so they are accessed through virtual
addresses.
Hyperspace
 Hyperspace maps the current process’
working set information into the kernelmode address.
Session Space
 Session space is used to share the win32
and other session-specific drivers between
all the processes in the same terminalserver session rather than with all the
processes in the system.
Process Manager
 Provides services for creating, deleting,
and using processes, threads, and jobs.
 It has no knowledge about parent-child
relationships; left to environmental
subsystems.
 Processes contain one or more threads.
 Processes can be collected together into
job objects.
Process Creation
 Once a new process is created, the initial
thread is created and an APC (Async
Procedure Call) is delivered to the thread to
prompt the start of execution at the usermode image loader (ntdll.DLL).
 Also supports a UNIX fork( ) style of process
creation.
Local Procedure Call Facility
 XP uses a client-server model.
 Environmental subsystems are servers that
implement particular operating system
personalities.
 XP uses LPCs to pass requests and results
(messages) between client and server
processes within a single machine.
I/O Manager
 Responsible for file-systems, device
drivers, and network drivers.
 Works with VM to provide memory mapped
I/O operations.
 Is fundamentally asynchronous.
 Device drivers are written according to the
Windows Driver Model (WDM)
specification.
WDM
 Because of the richness of the WDM, it can
be an excessive amount of work to write a
full WDM device driver for each new
hardware device.
Cache Manager
 Centralized facility.
 Works with file-system and VM mangers.
 Caching in XP is based on files rather than
raw blocks.
 Size of the cache changes dynamically.
Cache
 Cache is divided into blocks of 256KB.
 Each block can hold a memory-mapped
region (view) of a file.
 Each cache block is described by a virtualaddress control block (VACB) that stores
the virtual address and file offset of the
view.
Security Reference Monitor
 Centralized management of system entities
in the object manager enables the use of a
uniform mechanism to perform run-time
access validation and audit checks for
every user-accessible entity in the system.
Security Tokens
 The SRM is responsible for manipulating
the privileges in security tokens.
 Special privileges are required to perform
back or restore operations, overcome
certain checks as an administrator, debug
processes, etc.
 Restricted tokens used to restrict the
damage of untrusted code.
Logging
 The SRM is logging security audit events.
A
C2 rating requires the ability to detect and
log all unauthorized attempts to access
system resources.
Plug-and-Play Managers
 Recognizes and adapts to changes in the
hardware configuration.
 For PnP to work, both the device and driver
must support the PnP standard.
 Must be able to change the interrupts
dynamically.
 Must support dynamically configurable
resources.
Registry
 Configuration information is stored in an
internal database -- the registry.
 Entries (hives) are for system information,
default user preferences, software
installation, and security.
 Used to boot the system.
Booting
 Sequence
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power on
BIOS begins executing
boot loader (NTLDR)
determines boot device
loads Hardware-Abstraction Library
loads kernel and system hive
loads drivers
Booting -- II
• Begins kernel execution
• Creates 2 threads
– system process for all internal worker threads
– user-mode SMSS (similar to UNIX init process)
• finishes initialization
• creates WINLOGON
• creates CSRSS (Win32 subsystem)
Environmental Subsystem
 Is user-mode processes layered over the
native windows XP exec servers to run
programs developed for other Oss (16-bit
Windows, DOS, and POSIX).
 Win32 calls the VM manager to load the
executable code and return what type the
application is.
Environmental Subsystem - II
 Uses LPC subsystem to provide OS services
to client processes.
 Since each subsystem is run as a separate
user-mode process, a crash in one has no
effect on other processes. The exception is
in Win32, which provides all keyboard,
mouse, and graphical display capabilities - If it fails, the system requires a reboot.
MS-DOS Environment
 Virtual DOS Machine emulates Intel 486
instructions, emulates MS-DOS ROM DIOS
and INT 21h software interrupt services and
has virtual device drivers for the screen,
keyboard, and communication ports.
 Some MSDOS programs fail because they
require access to the disk hardware
directly.
16-Bit Windows Environment
 VDM has Windows on Windows (WOW32)
for Windows 3.1 software and emulates the
kernel routines.
 Applications that rely on the internal
structure of the 16-bit window manager
may not work.
 Only one Win16 application can run at a
time.
32-Bit Windows Env. On IA64
 Requires a “thunking” layer to translate 32-
bit Win32 calles into the corresponding 64bit calls.
 The implementation of 32-bit and 64-bit
windows are essential the same, IA64
provides direction execution of IA32
instructions.-- higher level of compatibility.