DRBDOS.SYS

IBMDOS.COM is the filename of the DOS kernel.

The file exists in PC DOS 1.0 to 7.10 and DR DOS 5.0 and higher (except for DR-DOS 7.06), with MS-DOS using MSDOS.SYS (from version 1.10 to 6.22) for the same purpose. DR DOS 3.31 to 3.41 used the equivalent DRBDOS.SYS file instead.

The file is located in the root directory of the drive containing the operating system and gets loaded by the IBMBIO.COM file in the boot process.

By default, the file is located in the root directory of the bootable drive/partition (normally C:\) and has the hidden, read-only, and system file attributes set.

In Digital Research terminology, the kernel component of the operating system is called the BDOS (Basic Disk Operating System), a term originally coined by Gary Kildall in 1975 for CP/M, but which is continued to be used in all other DRI operating systems. Microsoft has used this name as well when referring to the kernel of the MSX-DOS operating system.

...
Wikipedia