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1.1 A Few Concepts
A HIDD is a Hardware Independant Device Driver - a collection of code that provides an interface to hardware that hides as many details of the hardware as practical. Most applications do not need to know all the details of the device they use, they simply want to get on with using the device.
As we are implementing the Operation System we still want to support the existing software that runs on this platform. This means that we need to also provide an interface to the Exec style devices.
With this in mind, the design goals of the HIDD system are:
To implement the code and interface reuse design goal we have implemented our drivers in an object oriented fashion, providing single inheritance but multiple interfaces (similar to the Java language). The inheritance hierarchy allows us to start with a basic driver, and enhance this by adding more levels of device dependance until we come to a driver for a specific device.
To provide a method of dynamic driver functionality the driver objects are arranged in a tree that shows their connection mechanism. For example a PCI based ethernet card is connected to a PCI bus, so the ethernet card driver is a child of the PCI bus. Similar an IDE harddisk is normally connected to an IDE controller - so the disk will be a child of the IDE controller.
This does not mean that the ethernet card driver is a subclass of the bus driver. Only the dynamic behaviour of the drivers is subject to the connection-oriented approach.
There are a few concepts hidden in all the above. Firstly a knowledge of object-oriented programming is assumed for this document, as is a basic understand of device drivers. This manual is not designed to teach either object oriented programming or device driver programming - both of these come only from experience. I hope that this manual will help however - I am learning as I write it.
Throughout this manual a few terms will be used frequently and often interchangably, hopefully for the few times that I miss these in editing I shall try and clarify them here.
The first two are class and device. The device is the physical implementation of the hardware we are controlling, and the class is the code that is controlling that hardware. You normally only have one class for each device, although some devices may provide multiple functions. In that case you would need to provide multiple classes for the different functions.
The second two are object and driver. The object is an instantiated class and this is the driver for the device. There can be a number of drivers for the same device, but they must all control separate devices. There will be one instantiated driver for every class of device that exists in the system.
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