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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 918
SEC. 11.3
SYSTEM STRUCTURE
887
locking primitives, like mutexes. When a thread that is waiting for a lock begins
running again, the first thing it does is to retry acquiring the lock.
If only one
thread can hold the lock at a time, all the other threads made runnable might im-
mediately block, incurring lots of unnecessary context switching.
The difference
between dispatcher objects using synchronization vs. notification is a flag in the
dispatcher
header
structure.
As a little aside, mutexes in Windows are called ‘‘mutants’’ in the code be-
cause they were required to implement the OS/2 semantics of not automatically
unlocking themselves when a thread holding one exited, something Cutler consid-
ered bizarre.
The Executive Layer
As shown in Fig. 11-11, below the kernel layer of NTOS there is the executive.
The executive layer is written in C, is mostly architecture independent (the memo-
ry manager being a notable exception), and has been ported with only modest
effort to new processors (MIPS, x86, PowerPC, Alpha, IA64, x64, and ARM).
The
executive contains a number of different components, all of which run using the
control abstractions provided by the kernel layer.
Each component is divided into internal and external data structures and inter-
faces. The internal aspects of each component are hidden and used only within the
component itself, while the external aspects are available to all the other compo-
nents within the executive.
A subset of the external interfaces are exported from
the
ntoskrnl.exe
executable and device drivers can link to them as if the executive
were a library.
Microsoft calls many of the executive components ‘‘managers,’’ be-
cause each is charge of managing some aspect of the operating services, such as
I/O, memory, processes, objects, etc.
As with most operating systems, much of the functionality in the Windows ex-
ecutive is like library code, except that it runs in kernel mode so its data structures
can be shared and protected from access by user-mode code, and so it can access
kernel-mode state, such as the MMU control registers. But otherwise the executive
is simply executing operating system functions on behalf of its caller, and thus runs
in the thread of its called.
When any of the executive functions block waiting to synchronize with other
threads, the user-mode thread is blocked, too.
This makes sense when working on
behalf of a particular user-mode thread, but it can be unfair when doing work relat-
ed to common housekeeping tasks.
To avoid hijacking the current thread when the
executive determines that some housekeeping is needed, a number of kernel-mode
threads are created when the system boots and dedicated to specific tasks, such as
making sure that modified pages get written to disk.
For predictable, low-frequency tasks, there is a thread that runs once a second
and has a laundry list of items to handle.
For less predictable work there is the
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