The standard BIOS of an unexpanded MSX computer had no built-in disk support, but provided hooks for a disk extension, so the additional floppy disk expansion system came with its own BIOS extension ROM (built-in on the disk controller) called the BDOS.
For an unexpanded MSX computer, the standard BIOS did not include built-in disk support but provided hooks for a disk extension. Therefore, the additional floppy disk expansion system included its own BIOS extension ROM (located within the disk controller), which was known as BDOS.