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Modern Operating Systems by Herbert Bos and Andrew...
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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 980
SEC. 11.7
Driver Framework
) for writing drivers as services that execute in the kernel, but
with many of the details of WDM made automagical.
Since underneath it is the
WDM that provides the driver model, that is what we will focus on in this section.
Devices in Windows are represented by device objects.
Device objects are also
used to represent hardware, such as buses, as well as software abstractions like file
systems, network protocol engines, and kernel extensions, such as antivirus filter
drivers. All these are organized by producing what Windows calls a
device stack
as previously shown in Fig. 11-14.
I/O operations are initiated by the I/O manager calling an executive API
with pointers to the top device object and to the IRP representing the
I/O request.
This routine finds the driver object associated with the device object.
The operation types that are specified in the IRP generally correspond to the I/O
manager system calls described above, such as
, and
Figure 11-36 shows the relationships for a single level of the device stack.
each of these operations a driver must specify an entry point.
takes the
operation type out of the IRP, uses the device object at the current level of the de-
vice stack to find the driver object, and indexes into the driver dispatch table with
the operation type to find the corresponding entry point into the driver. The driver
is then called and passed the device object and the IRP.
Device object
Instance data
Next device object
Driver object
Driver object
Dispatch table
Loaded device driver
Driver code
Figure 11-36.
A single level in a device stack.
Once a driver has finished processing the request represented by the IRP, it has
three options.
It can call
again, passing the IRP and the next device
object in the device stack.
It can declare the I/O request to be completed and re-
turn to its caller.
Or it can queue the IRP internally and return to its caller, having
declared that the I/O request is still pending.
This latter case results in an asyn-
chronous I/O operation, at least if all the drivers above in the stack agree and also
return to their callers.

Page 981
CHAP. 11
I/O Request Packets
Figure 11-37 shows the major fields in the IRP.
The bottom of the IRP is a dy-
namically sized array containing fields that can be used by each driver for the de-
vice stack handling the request.
fields also allow a driver to specify
the routine to call when completing an I/O request.
During completion each level
of the device stack is visited in reverse order, and the completion routine assigned
by each driver is called in turn.
At each level the driver can continue to complete
the request or decide there is still more work to do and leave the request pending,
suspending the I/O completion for the time being.
Kernel buffer address
IRP Driver-Stack Data
Completion/cancel info
Thread’s IRP chain link
Memory descr list head
User buffer address
Buffer pointers
Next IRP
APC block
& comm.
Operation code
Figure 11-37.
The major fields of an I/O Request Packet.
When allocating an IRP, the I/O manager has to know how deep the particular
device stack is so that it can allocate a sufficiently large IRP.
It keeps track of the
stack depth in a field in each device object as the device stack is formed.
Note that
there is no formal definition of what the next device object is in any stack. That
information is held in private data structures belonging to the previous driver on
the stack.
In fact, the stack does not really have to be a stack at all.
At any layer a
driver is free to allocate new IRPs, continue to use the original IRP, send an I/O op-
eration to a different device stack, or even switch to a system worker thread to con-
tinue execution.
The IRP contains flags, an operation code for indexing into the driver dispatch
table, buffer pointers for possibly both kernel and user buffers, and a list of
Memory Descriptor Lists
) which are used to describe the physical pages repres-
ented by the buffers, that is, for DMA operations.
There are fields used for cancel-
lation and completion operations.
The fields in the IRP that are used to queue the

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