History of DOS

In the early days of computers, there were no disk drives; delay lines, punched cards, paper
tape, magnetic tape, magnetic drums, were used instead. And in  the early days of microcomputers,
paper tape or audio cassette tape (see Kansas City standard) or nothing were used instead. In the
latter case, program and data entry was done at front panel  switches directly into memory or
through a computer terminal / keyboard, sometimes controlled by a ROM BASIC interpreter; when
power was turned off after running the program, the information so entered vanished.
Both hard disks and floppy disk drives require software to manage rapid access to block storage of
sequential and other data. When microcomputers rarely had expensive disk drives of any kind, the
necessity to have software to manage such devices (ie, the 'disk's) carried much status. To have one
or the other was a mark of distinction and prestige, and so was having the Disk sort of an Operating
System. As prices for both disk hardware and operating system software decreased, there were
many such microcomputer systems.
Mature versions of the Commodore, SWTPC, Atari and Apple home computer systems all
featured a disk operating system (actually called 'DOS' in  the case of the Commodore 64 (CBM
DOS), Atari 800 (Atari DOS), and Apple II machines (Apple DOS)), as did (at the other end of the
hardware spectrum, and much earlier) IBM's System/360, 370 and (later) 390 series of mainframes
(e.g., DOS/360:  Disk  Operating  System / 360and DOS/VSE:  Disk  Operating  System /  Virtual
Storage Extended). Most home computer DOS'es were stored on a floppy disk always to be booted
at start-up, with the notable exception of Commodore, whose DOS resided on ROM chips in the
disk drives themselves, available at power-on.
In large machines there were other disk operating systems, such as IBM's VM, DEC's RSTS
/ RT-11 / VMS / TOPS-10 / TWENEX, MIT's ITS / CTSS, Control  Data's assorted NOS variants, Harris's Vulcan, Bell Labs' Unix, and so on. In microcomputers, SWTPC's 6800 and 6809 machines
used TSC's FLEX disk operating system, Radio Shack's TRS-80machines used TRS-DOS, their
Color Computer used OS-9, and most of the Intel 8080 based machines from  IMSAI, MITS
(makers of the legendary Altair 8800), Cromemco, North Star, etc used the CP/M-80 disk operating
system. See list of operating systems.
Usually, a disk operating system was loaded from a disk. Only a very few comparable
DOSes were stored elsewhere than floppy disks; among these exceptions were the British BBC
Micro's optional Disc Filing System, DFS, offered as a kit with a disk controller chip, a ROM chip,
and a handful of logic chips, to be installed inside the computer; and Commodore's CBM DOS,
located in a ROM chip in each disk drive.