## Physical Device Driver Reference

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### Initialization of Previous-Level DOS Device Drivers

A previous-level DOS character device driver can be initialized to run in a single DOS session or in all DOS sessions. A previous-level device driver can be installed for a single DOS session by using DOS settings. DOS device drivers can also be installed for all DOS sessions by specifying the configuration command DEVICE=.

Previous-level device drivers are loaded and initialized in a DOS session. The rules for replacing these drivers are the same. The replacement is guided by the name and attributes of the physical device driver. The functions that can be performed at initialization are more restrictive than for DOS 5.0. No INT 21h functions can be performed from the physical device driver initialization code.