Most Windows CE device drivers consist of a platform dependent driver (PDD)
and a model device driver (MDD). A monolithic driver combines all PDDs
and MDDs into one driver. A layered driver does not combine them.
MDDs have the following characteristics:
- Contain code that is common to all drivers of a given type.
- Call PDD functions to access the hardware.
- Link to the PDD layer and define the device driver service provider
interface (DDSI) functions that the MDD expects to call in that layer.
- Expose device driver interface (DDI) functions to the operating system (OS).
Other parts of the OS can call these functions. Related devices can share the
same DDI. Monolithic drivers also expose the DDI functions.
- Handle interrupt processing.
- Provide for reuse by developers.
- Can link to multiple PDDs.
- Generally require no changes.
If changed, you might have trouble migrating drivers to future versions.
- Contain any interrupt service threads (ISTs).
PDDs have the following characteristics:
The following list shows considerations for choosing between implementing a
layered driver or a monolithic driver:
- A layered driver might only require modification to the PDD.
- A layered driver adds overhead to function calls in a device driver because
the MDD calls into the PDD.
- A monolithic driver improves driver performance because it combines the MDD
and PDD into one layer, which removes the MDD's function calls into the PDD.
- A monolithic driver is more difficult to migrate to future versions of
Windows CE because most device drivers that Windows CE includes are divided into
a PDD and an MDD.
- A monolithic driver can be simpler and more efficient, if the capabilities
of your device are well matched to the tasks that the functions in the MDD layer
perform.
Regardless of whether you implement a layered driver or a monolithic driver,
you can base your implementation on the source code for any of the sample
drivers.
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