MSX-DOS - MSX-DOS

MSX-DOS

MSX-DOS and the extended BASIC with 3½-inch floppy disk support were simultaneously 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 unexpanded MSX did not have any floppy disk support, so the additional floppy disk expansion system came with its own BIOS extension 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 the FAT to select file system parameter profiles for its FAT12 file system instead of from the BIOS parameter block in 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.