DOS BIOS

IBMBIO.COM is the filename of the system initialization code and builtin device drivers in many DOS operating systems, and as such part of PC DOS and DR DOS 5.0 and higher (with the exception of DR-DOS 7.06). It serves the same purpose as IO.SYS in MS-DOS, or DRBIOS.SYS in DR DOS 3.31 to 3.41.

The filename is referred to in the boot sector by the boot loader.

In the PC bootup sequence, the first sector of the boot disk is loaded into memory and executed. If this is the DOS boot sector, it loads the first three sectors of IBMBIO.COM into memory and transfers control to it. IBMBIO.COM then:

Under DR-DOS, the first step is skipped, since a DR-DOS boot sector mounts the FAT filesystem, locates the IBMBIO.COM (or DRBIOS.SYS) file in the root directory and loads it into memory by itself. It is not necessary for the IBMBIO.COM file to reside at a fixed physical position or be stored in consecutive sectors. Instead, it can be simply copied to the disk (without SYS), given a DR-DOS boot sector already resides on the disk.

Microsoft sometimes calls this component the I/O system, but it is generally known as DOS BIOS (the DOS-related part of the Basic Input/Output System). The term was originally coined by Gary Kildall in 1975 for CP/M, but is also used to describe a similar component or layer in other operating systems by Digital Research, IBM, Microsoft and many others.

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