MSX-DOS

MSX-DOS Is a Disk operating system developed by Microsoft for the 8-bit home computer standard MSX, and is a cross between MS-DOS rev 1.0 and CP/M. MSX-DOS and the extended BASIC with floppy disk support were simultaniously developed by Microsoft for the developing home computer standard MSX to add disc capabilities to BASIC, and to give the system a cheaper software medium than Memory Cartridges, and a more powerful storage system than cassette tape. The standard BIOS of an unexpended MSX did not have any floppy disk support, so the additional floppy disk expansion system came with its own BIOS ROM (built-in on the disk controller) called the BDOS, it not only added floppy disk support commands to MSX BASIC but also a booting system, with which it was possible to boot a real disk operating system. In that case BDOS bypassed the BASIC ROMs so that the whole 64K of address space of the Z80 microprocessor inside the MSX computer could be used for the DOS or for other boot-able disks, for example disk based games. At the same time the original BIOS ROM's could still be accessed through a "memory bank switch" mechanism, so the DOS based software could still use BIOS calls to control the hardware and other software mechanisms the main ROM's supplied. Also, because of the BDOS rom basic file access capacity was available even without a command interpreter, by using the BASIC extended commands.
One major difference between MSX-DOS and MS-DOS was that MSX-DOS did not use the "boot sector" on the floppy to boot, but booted using the BDOS ROM routines, and used the media descriptor value from the first byte of FAT to determine file system parameters instead of from the bootsector. Also, because there could be more than one floppy disk controller in two or more cartridge slots, MSX-DOS could boot from several different floppy disk drives. This meant that it was possible to have both a 5¼" floppy disk drive and a 3½" disk drive, and you could boot from either one of them depending on which drive had a boot-able floppy in it.