The BIOS is specific to the individual computer system and is provided by the manufacturer of the system. It contains the default resident hardware-dependent drivers for the following devices:
Console display and keyboard (CON)
Line printer (PRN)
Auxiliary device (AUX)
Date and time (CLOCK$)
Boot disk device (block device)
The MS-DOS kernel communicates with these device drivers through I/O request packets; the drivers then translate these requests into the proper commands for the various hardware controllers. In many MS-DOS systems, including the IBM PC, the most primitive parts of the hardware drivers are located in read-only memory (ROM) so that they can be used by stand-alone applications, diagnostics, and the system startup program.
The terms resident and installable are used to distinguish between the drivers built into the BIOS and the drivers installed during system initialization by DEVICE commands in the CONFIG.SYS file. (Installable drivers will be discussed in more detail later in this chapter and in Chapter 14.)
The BIOS is read into random-access memory (RAM) during system initialization as part of a file named IO.SYS. (In PC-DOS, the file is called IBMBIO.COM.) This file is marked with the special attributes hidden and system.