## Virtual Device Driver Reference for OS/2

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### Physical Device Drivers

Standard I/O devices are supported by base physical device drivers that are part of the OS/2 operating system. Additional or replacement physical device drivers can be loaded to extend the control provided by the base device drivers, or to support nonstandard I/O devices. Typical examples of these loadable device drivers are ANSI.SYS and ANSI.DLL, which are loaded by "DEVICE=" statements in the CONFIG.SYS file, and provide additional functions on the interfaces to the screen and keyboard. Physical device drivers are initialized at Ring 3 and operate at Ring 0.

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