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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
Showing 926 out of 1137
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 926
SEC. 11.3
SYSTEM STRUCTURE
895
to manage the NT namespace and implement objects using a common facility.
These are directory, symbolic link, and object-type objects.
The uniformity provided by the object manager has various facets. All these
objects use the same mechanism for how they are created, destroyed, and ac-
counted for in the quota system.
They can all be accessed from user-mode proc-
esses using handles.
There is a unified convention for managing pointer references
to objects from within the kernel. Objects can be given names in the NT name-
space (which is managed by the object manager). Dispatcher objects (objects that
begin with the common data structure for signaling events) can use common syn-
chronization and notification interfaces, like
WaitForMultipleObjects
.
There is the
common security system with ACLs enforced on objects opened by name, and ac-
cess checks on each use of a handle.
There are even facilities to help kernel-mode
developers debug problems by tracing the use of objects.
A key to understanding objects is to realize that an (executive) object is just a
data structure in the virtual memory accessible to kernel mode.
These data struc-
tures are commonly used to represent more abstract concepts.
As examples, exec-
utive file objects are created for each instance of a file-system file that has been
opened. Process objects are created to represent each process.
A consequence of the fact that objects are just kernel data structures is that
when the system is rebooted (or crashes) all objects are lost.
When the system
boots, there are no objects present at all, not even the object-type descriptors.
All
object types, and the objects themselves, have to be created dynamically by other
components of the executive layer by calling the interfaces provided by the object
manager.
When objects are created and a name is specified, they can later be refer-
enced through the NT namespace.
So building up the objects as the system boots
also builds the NT namespace.
Objects have a structure, as shown in Fig. 11-15. Each object contains a head-
er with certain information common to all objects of all types.
The fields in this
header include the object’s name, the object directory in which it lives in the NT
namespace, and a pointer to a security descriptor representing the ACL for the ob-
ject.
The memory allocated for objects comes from one of two heaps (or pools) of
memory maintained by the executive layer.
There are (malloc-like) utility func-
tions in the executive that allow kernel-mode components to allocate either page-
able or nonpageable kernel memory.
Nonpageable memory is required for any
data structure or kernel-mode object that might need to be accessed from a CPU
priority level of 2 or more. This includes ISRs and DPCs (but not APCs) and the
thread scheduler itself.
The page-fault handler also requires its data structures to
be allocated from nonpageable kernel memory to avoid recursion.
Most allocations from the kernel heap manager are achieved using per-proc-
essor lookaside lists which contain LIFO lists of allocations the same size.
These
LIFOs are optimized for lock-free operation, improving the performance and
scalability of the system.
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