View source for PDDREF:Introduction

{{PDDREF}}
Device drivers are software modules that act as an interface between OS/2* (or its applications) and physical devices. In DOS, applications are responsible for knowing and managing all the differences between different devices of like types. The device drivers of OS/2 enable applications to focus on the logic of the actual application and treat devices in a device-independent way.

This device-independence is a fundamental benefit of the OS/2 programming environment. For example, drawing a circle on the screen or on a printer is as easy as opening a device context for the correct output device and performing the drawing operations. By placing the drawing operation into the device context, the programmer does not have to be concerned about whether the device is a printer or a display.

==Types of OS/2 Device Drivers==
Three types of device drivers are used in OS/2 Version 3:
* Physical device drivers
* Virtual device drivers
* Presentation drivers

====Physical Device Drivers====
Physical device drivers, in most cases, resolve device-independent input and output (I/O) requests from the operating system and its applications with the device-dependent physical attributes of the device. The physical device drivers shipped with OS/2 Version 3 include support for the following:
* Advanced Power Management
* Asynchronous Communication (RS-232C)
* CD-ROM
* Hard Disk and Diskette
* Keyboard
* Mouse
* Parallel Port Printer
* PCMCIA
* System Clock
* Touch Screen
* Video
This book provides a description of the OS/2 physical device drivers, including:
* Physical device driver structure and architecture
* Design considerations, including performance and nested interrupts
* Interfaces to a physical device driver, including the request packet and generic IOCtl interfaces
* Subsystem interfaces to a physical device driver
* Device Helper (DevHlp) services
* Inter-device-driver communication (device drivers calling other device drivers)

====Virtual Device Drivers====
The virtual device driver is an installable module that virtualizes a particular piece of hardware and associated ROM BIOS in the manner expected by a DOS application. This device driver achieves virtualization by emulating I/O port and device memory operations. Virtual device drivers are 32-bit device drivers that operate at Ring 0. To achieve a certain level of hardware independence, a virtual device driver usually communicates with a physical device driver in order to interact with hardware.

Further information on virtual device drivers, virtual device driver interfaces (including detailed descriptions of the calling conventions), and the system services available to these drivers is found in the OS/2 Virtual Device Driver Reference. 

====Presentation Drivers==== 
The Presentation Manager* I/O interface for output devices is a high-level interface. This interface is similar to the API call interface, which uses the program stack to communicate with, or pass parameters to, the presentation drivers. These drivers are special purpose I/O routines operating with I/O privilege at privilege level 2 (Ring 2) or level 3 (Ring 3). Their main function is to process function calls made by the Presentation Manager interface on behalf of Presentation Manager applications. Hardcopy presentation drivers communicate with OS/2 device drivers through the file system emulation functions. Display presentation drivers interface directly with the hardware.

Presentation drivers are dynamic link library modules that are supplied as files and identified by the extension DRV. When the Presentation Manager interface is initialized, the display presentation driver is loaded and enabled automatically. Other presentation drivers (for example, the hardcopy presentation drivers) are loaded and enabled when an application calls the DevOpenDC function to open the device.

Presentation drivers service requests only from applications running in Presentation Manager sessions in the OS/2 mode. Output data and requests for information are passed to the presentation driver as function calls to the presentation driver interface. The definition of the call interface is given in terms of the codes and data passed to the presentation driver interface through the program stack.

Header and include files are shipped with OS/2 to provide support for building presentation drivers that are written in C or assembler language. These files contain function prototypes, defined values, and data structures used by the various functions. Further information on presentation drivers, presentation driver interfaces (including detailed descriptions of the calling conventions), and the system services available to these drivers is found in the OS/2 Presentation Device Driver Reference.

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