Page 339 - DCAP103_Principle of operating system

Principles of Operating Systems

Notes Another kernel control object is the APC (Asynchronous Procedure Call). APCs are like DPCs
except that they execute in the context of a specific process. When processing a key press, it
does not matter on whose context the DPC runs in because all that is going to happen is that
the key code will be inspected and probably put in a kernel buffer. However, if an interrupt
requires copying a buffer from kernel space to a buffer in some user process address space (e.g.,
as it may on completion of a read from the modem), then the copying procedure needs to run
in the receiver’s context. The receiver’s context is needed so the page table will contain both
the kernel buffer and the user buffer (all processes contain the entire kernel in their address
spaces, as we will see later). For this reason, the kernel distinguishes between DPCs and APCs.
The other kind of kernel objects are dispatcher objects. These include semaphores, mutexes,
events, waitable timers, and other objects that threads can wait on. The reason that these have to
be handled (in part) in the kernel is that they are intimately intertwined with thread scheduling,
which is a kernel task. As a little aside, mutexes are called “mutants” in the code because they
were required to implement the OS/2 semantics of not automatically unlocking themselves
when a thread holding one exiled, something the Windows 2000 designers considered bizarre.
(The OS/2 semantics are relevant because NT was originally conceived of as a replacement for
OS/2, the operating system shipped on IBM’s PC/2.)
11.1.3 Executive

Above the kernel and device drivers is the upper portion of the operating system, called the
executive, shown as the shaded area in Figure 11.1. The executive is written in C, is architecture
independent, and can be ported to new machines with relatively little effort. It consists of
10 components, each of which is just a collection of procedures that work together to accomplish
some goal. There are no hard boundaries between the pieces and different authors describing
the executive might even group the procedures differently into components. It should be noted
that components on the same level can (and do) call each other extensively.

The object manager manages all objects known to the operating system. These include processes,
threads, files, directories, semaphores, I/O devices, timers, and many others. The object manager
allocates a block of virtual memory from kernel address space when an object is created and
returns it to the free list when the object is deal-located. Its job is to keep track of all the objects.
To avoid any confusion, most of the executive components labeled “manager” in Figure 11.1
are not processes or threads, but merely collections of procedures that other threads can execute
when in kernel mode. A few of them, such as the power manager and plug-and-play manager,
really are independent threads though.
The object manager also manages a name space in which newly created objects may be placed
so they can be referred to later. All other components of the executive use objects heavily to do
their work. Objects are so central to the functioning of Windows 2000 that they will be discussed
in detail in the next section.

The I/O manager provides a framework for managing I/O devices and provides generic I/O
services. It provides the rest of the system with device-independent I/O, calling the appropriate
driver to perform physical I/O. It is also home to all the device drivers (indicated by D in
Figure 11.1). The file systems are technically device drivers under control of the I/O manager.
Two different ones are present for the FAT and NTFS file systems, each one independent of
the others and controlling different disk partitions. All the FAT file systems are managed by a
single driver.

The process manager handles processes and threads, including their creation and termination.
It deals with the mechanisms used to manage them, rather than policies about how they are

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