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Modern Operating Systems by Herbert Bos and Andrew S. Tanenb...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf
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Modern Operating Systems by Herbert Bos and Andrew...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Modern Operating Systems by Herbert...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Page 922
SEC. 11.3
SYSTEM STRUCTURE
891
tells the operating system how long to maintain it (e.g., until the next reboot or
permanently). A publisher atomically updates the state as appropriate.
Subscri-
bers can arrange to run code whenever an instance of state data is modified by a
publisher.
Because the WNF state instances contain a fixed amount of preallocated
data, there is no queuing of data as in message-based IPC—with all the attendant
resource-management problems.
Subscribers are guaranteed only that they can see
the latest version of a state instance.
This state-based approach gives WNF its principal advantage over other IPC
mechanisms: publishers and subscribers are decoupled and can start and stop inde-
pendently of each other.
Publishers need not execute at boot time just to initialize
their state instances, as those can be persisted by the operating system across
reboots. Subscribers generally need not be concerned about past values of state
instances when they start running, as all they should need to know about the state’s
history is encapsulated in the current state.
In scenarios where past state values
cannot be reasonably encapsulated, the current state can provide metadata for man-
aging historical state, say, in a file or in a persisted section object used as a circular
buffer.
WNF is part of the native NT APIs and is not (yet) exposed via Win32 in-
terfaces. But it is extensively used internally by the system to implement Win32
and WinRT APIs.
In Windows NT 4.0, much of the code related to the Win32 graphical interface
was moved into the kernel because the then-current hardware could not provide the
required performance.
This code previously resided in the
csrss.exe
subsystem
process which implemented the Win32 interfaces. The kernel-based GUI code
resides in a special kernel-driver,
win32k.sys
.
This change was expected to im-
prove Win32 performance because the extra user-mode/kernel-mode transitions
and the cost of switching address spaces to implement communication via LPC
was eliminated. But it has not been as successful as expected because the re-
quirements on code running in the kernel are very strict, and the additional over-
head of running in kernel-mode offsets some of the gains from reducing switching
costs.
The Device Drivers
The final part of Fig. 11-11 consists of the
device drivers
.
Device drivers in
Windows are dynamic link libraries which are loaded by the NTOS executive.
Though they are primarily used to implement the drivers for specific hardware,
such as physical devices and I/O buses, the device-driver mechanism is also used
as the general extensibility mechanism for kernel mode.
As described above,
much of the Win32 subsystem is loaded as a driver.
The I/O manager organizes a data flow path for each instance of a device, as
shown in Fig. 11-14. This path is called a
device stack
and consists of private
instances of kernel device objects allocated for the path.
Each device object in the
device stack is linked to a particular driver object, which contains the table of
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