OS2BIO.COM
| Company type | System file |
|---|---|
| Industry | Computer software |
| Products | DOS system startup routines and device drivers |
IBMBIO.COM functions as a crucial system file within numerous DOS operating systems, embedding essential system startup routines along with integrated device drivers. Its responsibilities include loading the DOS kernel (IBMDOS.COM), initializing optional components such as disk compression or security modules, presenting boot menus, interpreting configuration files like CONFIG.SYS, and initiating the command interpreter (for example, COMMAND.COM).
This file is an integral element of IBM's PC DOS across all editions and appears in DR DOS 5.0 and later versions—except for DR-DOS 7.06, where the equivalent is named IO.SYS. It fulfills a role analogous to IO.SYS in MS-DOS or DRBIOS.SYS found in earlier iterations of DR-DOS. Notably, IBMBIO.COM was briefly incorporated in IBM's release of OS/2 1.0 to maintain compatibility.
Typically residing in the root directory of the bootable FAT-formatted partition (commonly C:\), IBMBIO.COM is assigned the system, hidden, and read-only file attributes. In DR-DOS environments, there is an option to secure it with a password. Within PC DOS, the system attribute designates it as immovable on disk—a limitation not enforced by DR-DOS. The hidden attribute prevents inadvertent execution from the command line, which would otherwise lead to system failure. Starting with DR-DOS version 7.02, IBMBIO.COM incorporates a small COM-style stub that outputs version details if executed outside of the standard boot sequence.
During the initial stages of PC startup, control is passed first to the volume boot record (VBR). In PC DOS versions prior to 3.3, the VBR directly loads both system files but mandates their placement at fixed, consecutive disk sectors—a condition enforced by the SYS utility.
Beginning with PC DOS 3.3, the VBR loads only the initial three sectors of IBMBIO.COM, which contain an expanded boot loader responsible for:
- Loading the remaining portion of IBMBIO.COM into memory (before PC DOS 5.0, system files still required fixed disk locations; from version 5.0 onward only these first three sectors must be contiguous).
- Loading the DOS kernel located in IBMDOS.COM.
- Initializing built-in device drivers such as console, disk, and serial port handlers.
- Invoking the kernel’s initialization procedure.
In DR-DOS 5.0 and subsequent releases, the VBR itself can mount FAT file systems and locate IBMBIO.COM anywhere within the root directory, removing any constraints on fixed file positioning. The names of IBMBIO.COM and IBMDOS.COM are stored within the boot sector and IBMBIO.COM respectively, enabling flexible file placement without requiring the system attribute. As a result, system files may be copied directly without using SYS provided a DR-DOS boot sector is installed.
Microsoft designates this component as the I/O system; however, it is more widely recognized as DOS BIOS—the segment of Basic Input/Output System associated with DOS operations. The term BIOS was originally introduced by Gary Kildall in 1975 for CP/M BIOS and has since been applied to comparable subsystems across various operating systems developed by Digital Research, IBM, Microsoft, and others.
Certain manufacturers refer to this part as the RAM BIOS within operating systems like DOS or CP/M to differentiate it from a computer’s built-in ROM BIOS.