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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.
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:
- Loads the rest of itself into memory. For this to work, IBMBIO.COM and its directory entry must be located at fixed physical positions on the disk and stored in consecutive sectors, conditions of which the SYS utility must take care of.
- Loads the DOS kernel. The kernel is stored in IBMDOS.COM.
- Initializes each default device driver in turn (console, disk, serial port, et cetera). At this point, the default devices are available.
- Calls the DOS kernel's initialization routine.
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's 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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