History and Concepts of the DOS

The real history of our DOS begins with the early planning for the
IBM Personal Computer and the operating system that had been used with
the generation of personal computers that preceded our Pc.
The PC was planned and designed at a time when most personal com­
puters used an 8-bit microprocessor, and the dominant operating system for
those machines was called CP/M (which stood for Control Pro­
gram/Microcomputer). Even though IBM was designing the PC to be a
much more powerful 16-bit computer, IBM wanted to build on the base of
experience and popUlarity of CP/M machines. Even though the PC was
going to be a quite different critter, and even though 8-bit CP/M programs
couldn't be directly used on the PC, making the PC's operating system
similar to CP/M would make it enormously easier to adapt programs (and
adapt user's experience and skills) to the new machine.
Apparently IBM intended to have an updated, 16-bit version of CP/M,
which became known as CP/M-86, as the PC's primary operating system,
but that didn't work out. See the sidebar CP/M Crash Lands for one version
of the story why. For whatever reason, IBM decided not to center the PC
on a version of CP/M, but instead to have a new operating system created
for the PC by Microsoft-that operating system was DOS.

Even though DOS was a competitor to CP/M for the PC-and a com­
petitor which vanquished its opposition-the design and operation of DOS
was closely based on the facilities that CP/M provided and the ideas behind
them. DOS, as it was initially introduced, had very much the flavor and
style of CP/M for an important and deliberate reason: to make it as convenient as possible for computer users who were familiar with CP/M to learn
to use DOS, and to make it easy for existing 8-bit CP/M programs to be
adapted for the PC.
The influence of CP/M appears from the very first thing we see when
we use DOS, the command prompt A>. DOS shows the design influence of
CP/M in that command prompt and many more things in the way that DOS
works with us, the user, and even more in the way that DOS works with our
While experienced eyes can see the similarities between the style of
DOS and the style of its predecessor CP/M, the most important ways that
CP/M set the style for DOS aren't visible, because they are ideas. Foremost
among them was the scope and use that was intended for DOS from the
beginning. DOS was built with the most primitive concepts of personal
computing in mind. This included the assumption that only one person
would be using the computer and that the one user would only ask for the
computer to do one thing at a time (not, for example, printing out one
document while computing on something else, which would be performing
two tasks at once). DOS was designed to be a single-user system and a
single tasking system following the most simple concept of how a computer
might be used. It was natural that DOS was designed this way since its
roots came from an operating system and a family of 8-bit machines which
weren't suited to undertake anything more ambitious.
Our PC family, however, was born to more ambitious goals, and the
limitations of the CP/M heritage would have severely restricted DOS's
ability to grow with the PC. On the other hand, there was an operating
system called UNIX that was widely admired for its broad features, and
Microsoft, DOS's builder, had strong experience with the UNIX style from
creating XENIX, a variation of UNIX. So, when the time came to make a
major revision to the features and internal structure of DOS, many of the
concepts of UNIXlXENIX were stirred into the DOS recipe. The result was
DOS version 2.0 and all the subsequent versions that we have seen since.