The layer at the bottom of MS-DOS is BIOS (basic input/output system). This layer of the operating system interfaces directly with the various I/O devices such as printers, keyboards, and monitors. BIOS contains the device drivers that control the flow of data to and from each device except the disk drives. It receives status information about the success or failure of each I/O operation and passes it on to the processor. BIOS takes care of the small differences among I/O units so the user can purchase a printer from any manufacturer without having to write a device driver for it—BIOS makes it perform as it should.