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DOS device access

DOS operating system can access hardware from application layer through lower layer memory and I/O registers as it runs on real mode. Thus DOS can use application utility provided by vendors for their hardware. But each hardware vendor has their own way to access their hardware thus their utility varies. It does not provide any method to uniformly access any input/output device. Thus there is a concept of device drivers and application APIs have been arrived in Windows.

Device Context Abstraction and Low Layers

Device Context is an abstraction in windows to access any output hardware which needs text and drawing output. Windows Operating system returns a handle to the device context of the device. Device context is a structure in kernel mode of the operating system where it stores the attributes and other properties of the device. Hardware may vary in many aspects but device context is common for all for any particular device type. Hardware vendors supply device dependent device drives or miniport drivers for their hardware which takes care of the lower layer access and manageability of the hardware. The upper layer is the Win32 APIs to access this hardware from application layer. Windows provides a set of APIs to access a type of device through this device context. Lower layer access and manageability may vary from hardware to hardware and from vendor to vendor but methods and steps via windows API are same for accessing same type of device. It creates a uniform access point for device hardware. Thus an application written for printing documents works for all printers even if printer vendors and hardware series are different. The following diagram shows a highest to lowest layer of the printing device stack.

Windows printing device stack

Windows printing device stack

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