Timeline of DOS operating systems

DOS releases have been in the forms of: IBM combined SYSINIT with its customized ROM-BIOS interface code to create the BIOS extensions file IBMBIO.COM, the DOS-BIOS which deals with input/output handling, or device handling, and added a few external commands of their own: COMP, DISKCOMP, DISKCOPY, and MODE (configure printer) to finish their product.

Two copies of the File Allocation Table occupy the two sectors which follow the boot record.

[86] In addition to Microsoft's new commands in MS-DOS 2.0 (above), IBM adds more including FDISK, the fixed disk[F] setup program, used to write the master boot record which supports up to four partitions on hard drives.

Only one DOS partition is allowed, the others are intended for other operating systems such as CP/M-86, UCSD p-System and Xenix.

Space in the user data area of the disk is allocated in clusters which are fixed at 8 sectors each.

With DOS the only partition, the combined overhead is 50 sectors[H] leaving 10,592,256 bytes[I] for user data.

[212][360] Excluding maintenance releases, this is the last version of Windows that could run on 8088 and 8086-based XT-class PCs (in real mode).