Книга: Real-Time Concepts for Embedded Systems

12.3 The I/O Subsystem 

Each I/O device driver can provide a driver-specific set of I/O application programming interfaces to the applications. This arrangement requires each application to be aware of the nature of the underlying I/O device, including the restrictions imposed by the device. The API set is driver and implementation specific, which makes the applications using this API set difficult to port. To reduce this implementation-dependence, embedded systems often include an I/O subsystem.

The I/O subsystem defines a standard set of functions for I/O operations in order to hide device peculiarities from applications. All I/O device drivers conform to and support this function set because the goal is to provide uniform I/O to applications across a wide spectrum of I/O devices of varying types.

The following steps must take place to accomplish uniform I/O operations at the application-level.

1. The I/O subsystem defines the API set.

2. The device driver implements each function in the set.

3. The device driver exports the set of functions to the I/O subsystem.

4. The device driver does the work necessary to prepare the device for use. In addition, the driver sets up an association between the I/O subsystem API set and the corresponding device-specific I/O calls.

5. The device driver loads the device and makes this driver and device association known to the I/O subsystem. This action enables the I/O subsystem to present the illusion of an abstract or virtual instance of the device to applications.

This section discusses one approach to uniform I/O. This approach is general, and the goal is to offer insight into the I/O subsystem layer and its interaction with the application layer above and the device driver layer below. Another goal is to give the reader an opportunity to observe how the pieces are put together to provide uniform I/O capability in an embedded environment.

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