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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 905
874
CASE STUDY 2: WINDOWS 8
CHAP. 11
storage. Modifications to files or directory subtrees can be detected through a noti-
fication mechanism, or by reading the
journal
that NTFS maintains for each vol-
ume.
Each file-system volume is implicitly mounted in the NT namespace, accord-
ing to the name given to the volume, so a file
\ foo \ bar
might be named, for ex-
ample,
\ Device \ HarddiskVolume \ foo \ bar
.
Internal to each NTFS volume, mount
points (called
reparse points
in Windows) and symbolic links are supported to help
organize the individual volumes.
The low-level I/O model in Windows is fundamentally asynchronous.
Once an
I/O operation is begun, the system call can return and allow the thread which initi-
ated the I/O to continue in parallel with the I/O operation.
Windows supports can-
cellation, as well as a number of different mechanisms for threads to synchronize
with I/O operations when they complete. Windows also allows programs to speci-
fy that I/O should be synchronous when a file is opened, and many library func-
tions, such as the C library and many Win32 calls, specify synchronous I/O for
compatibility or to simplify the programming model.
In these cases the executive
will explicitly synchronize with I/O completion before returning to user mode.
Another area for which Win32 provides calls is security.
Every thread is asso-
ciated with a kernel-mode object, called a
token
, which provides information about
the identity and privileges associated with the thread.
Every object can have an
ACL
(
Access Control List
) telling in great detail precisely which users may ac-
cess it and which operations they may perform on it.
This approach provides for
fine-grained security in which specific users can be allowed or denied specific ac-
cess to every object.
The security model is extensible, allowing applications to add
new security rules, such as limiting the hours access is permitted.
The Win32 namespace is different than the native NT namespace described in
the previous section.
Only parts of the NT namespace are visible to Win32 APIs
(though the entire NT namespace can be accessed through a Win32 hack that uses
special prefix strings, like
‘‘ \ \
.’ ’
). In Win32, files are accessed relative to
drive let-
ters
.
The NT directory
\ DosDevices
contains a set of symbolic links from drive
letters to the actual device objects.
For example,
\ DosDevices \ C:
might be a link
to
\ Device \ HarddiskVolume1
.
This directory also contains links for other Win32
devices, such as
COM1:
,
LPT:
, and
NUL:
(for the serial and printer ports and the
all-important null device).
\ DosDevices
is really a symbolic link to
\ ??
which
was chosen for efficiency. Another NT directory,
\ BaseNamedObjects
, is used to
store miscellaneous named kernel-mode objects accessible through the Win32 API.
These include synchronization objects like semaphores, shared memory, timers,
communication ports, and device names.
In addition to low-level system interfaces we have described, the Win32 API
also supports many functions for GUI operations, including all the calls for manag-
ing the graphical interface of the system.
There are calls for creating, destroying,
managing, and using windows, menus, tool bars, status bars, scroll bars, dialog
boxes, icons, and many more items that appear on the screen.
There are calls for
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