Access requests to physical media are sent to "Input/Output Supervisor", a component responsible for scheduling the requests. Each physical media has its own device driver: access to the disk is performed by a "port driver", while access to a SCSI device is handled by a "miniport" driver working atop the SCSI layer. Port and miniport drivers perform I/O operations in 32-bit protected mode, bypassing MS-DOS and BIOS, giving a significant performance improvement. In case there is no native Windows driver for a certain storage device, or if a device is forced to run in compatibility mode, the "Real Mode Mapper" can access it through MS-DOS.