OS History--DOS and Windows
CS 321 2007 Lecture, Dr. Lawlor
Operating systems for personal computers are extremely important, but quite new, and something of a historical accident.
Back in the late 1970's, many hobbyists were building strange personal computers (PCs), often with homemade wooden cases like the Apple 1.
DOS is born from CP/M
At the time, the operating system of choice for PCs was either "none"
(write bare-metal programs in assembly language) or "CP/M", a
commercial OS from a tiny (almost one-man) company called Digital
As the 1980's began, IBM was preparing a very quick "skunkworks"
project to capitalize on the hobbyist interest in small personal
computers. IBM approached Digital Research to have them port CP/M
onto their new 8086 machine, but IBM did not receive a warm
welcome--reportedly Gary Kildall, the lead programmer and CEO, went
golfing instead of meeting with the IBM representative. So IBM
shopped around, and found a willing entrepneur, Bill Gates, who sold
IBM an operating system he didn't actually have yet.
Bill Gates, at the time a star programmer but college dropout, found a
little one-man company working on a project called "QDOS" (reportedly,
"Quick and Dirty Operating System"). QDOS was similar to the
popular CP/M, but Bill felt he could easily port the small OS to IBM's
8086 PC, so Bill bought the rights to QDOS so he could build an
operating system to sell to IBM. Bill called his new OS "DOS", or
"Disk Operating System".
IBM purchased the rights to distribute DOS from Bill gates, and sold
his OS as "PC-DOS" along with every IBM PC. The IBM PC was a
huge, extraordinary commercial success, bought in large numbers by
hobbyists, corporations, scientists, and all and sundry. Hence
PC-DOS instantly became a very popular product.
Curiously, Bill Gates did not sell all the rights to DOS to IBM--Bill Gates, and his tiny 70's startup company Microsoft,
kept the right to sell their own separate copy of DOS, "MS-DOS".
This wouldn't have mattered if everybody bought their IBM PC's from
IBM, but the IBM PC's BIOS was cloned almost immediately,
and "IBM Compatible PCs" began entering the market. So if you
built an IBM Compatible PC, you needed an OS, and IBM wasn't going to
sell it to the clone manufacturers, but Microsoft would!
Hence via a series of rather unlikely historical accidents, and some
extremely shrewd business decisions, Microsoft became the de-facto
world provider of operating systems.
All through the 1980's, several companies sold DOS-compatible operating systems:
- Microsoft sold "MS-DOS".
- IBM sold "PC-DOS", which was pretty much an IBM-branded version of Microsoft's DOS.
- Digital Research sold an MS-DOS compatible version of CP/M called "DR-DOS" (Digital Research's DOS).
Software Architecture of DOS
When an IBM PC starts up, the BIOS loads the "master boot record" (or
"boot sector") from the first 512 bytes of the disk. This
consists of 16-bit x86 PC code. From this "boot sector code", you
can make interrupt calls (exactly like system calls) to call the "Basic
Input/Output System" or BIOS, whose code is stored in ROM. Every
PC's BIOS, even today, is software-compatible with the original IBM
The BIOS provides a very primitive programming interface:
It is possible to write programs that call only BIOS routines, but it's ugly.
- Read and write disk blocks.
- Get keyboard and joystick input.
- Send bytes to the printer or screen.
On top of the BIOS, for programmers DOS mostly provides a filesystem
interface, with directories and files. For users, DOS provides a
command line interface.
At startup, the DOS boot sector loads up "MSDOS.SYS" and "IO.SYS",
which stay in memory and provides programmer services. For
programmers, DOS mostly provides a filesystem interface, with
directories and files. This "terminate and stay resident" (TSR)
software could be controlled by the "config.sys" file.
For users, DOS provides a command line interface, via a little program
called "COMMAND.COM". This program reads command-line commands
such as "DIR" from the user, and then executes them. COMMAND.COM
has a small number of such commands built in, but can also read in the
code for new commands from files called "executables". DOS comes
with a bunch of built-in executables. Once COMMAND.COM starts up, it reads "AUTOEXEC.BAT", a "batch file" of ordinary DOS command lines.
Unlike with modern operating systems, a DOS executable does NOT run in
a process, alongside DOS and any other running programs. Instead,
a DOS executable basically takes over the entire machine, and can
perform direct hardware access if desired. When the program
exits, the MSDOS.SYS code (still in memory) reloads COMMAND.COM.
A DOS executable can directly access:
This means you generally have to reboot when a DOS program hangs or
crashes. DOS loads almost instantly, so this is no big deal.
- Physical memory (address 0 is just another address)
- Hardware devices (for example, using the "INB" assembly instruction)
- OS data structures (for example, the keyboard interrupt)
DOS has no virtual memory, only one user can run the computer at a
time, one program can be loaded at a time, and graphics handling or any
specialized hardware handling is entirely up to the application--DOS
only provided text-mode interface functions.
The Rise of Windows
As the 1980's rolled around to the 1990's, the shortcomings of DOS were
becoming more and more evident. Microsoft worked and worked on
"Microsoft Windows", a program to add a graphical user interface (GUI)
on top of the DOS command line.
The first even moderately successful version of Windows was Windows
3.1. It ran directly on top of DOS, in the sense that you had to
first install DOS, then install Windows. Once you booted to a DOS
prompt, you could then type "win" to start up Windows. When you
exited Windows 3.1, you got back to a DOS prompt.
While Windows 3.1 was running, it provided many OS-like features far
beyond DOS. It had a decent graphical user interface interface,
supporting overlapping windows and a mouse. It allowed multiple
programs to run at the same time (multitasking). It did not
provide any particular hardware protection, however, and it was common
for one crashing program to bring down the entire machine. In
response to an unimportant program causing an illegal instruction,
Windows 3.1 prints "A program has encountered an error. Please
close the program, close all other applications, and shut down
windows." The idea is that if a program crashed, it probably
screwed up lots of other stuff in Windows too, so you should try to
save your work and reboot as soon as possible.
Like DOS, all of Windows 3.1 was written and ran in 16-bit mode, even
though all processors starting with the 386 could run 32-bit code.
The first consumer version of Windows with protected memory was Windows
95. Windows 95 could run 16-bit or 32-bit programs well, could
(mostly) protect the machine from runaway and crashing programs,
although malicious programs could still easily circumvent Windows'
limited hardware protection.
Windows 95 also had a dramatically revamped user interface, with the
"Start" menu, taskbar, right-clicking, and a nicer beveled 3D look.
Windows 98 and Millenium Edition both followed in this same vein,
adding little beyond new hardware drivers to the original Windows
95. All were written in a mix of 16-bit (DOS style) and 32-bit
code. Even Windows Millenium could be exited back to a DOS
prompt, and restarted with the "win" command exactly like Windows 3.1.
Even prior to the release of Windows 95, Microsoft had been working on a new version of Windows called Windows NT,
built using modern operating system principles--with a fully 32-bit
kernel, multiple users, multiple programs, and real protected
Windows NT 4.0 was released alongside Windows 95, with a very similar
GUI interface. Windows NT 4.0 actually ran on non-x86 hardware,
including the PowerPC, MIPS, and DEC Alpha chips, although this
capability was dropped in later commercial versions.
Windows XP, 2000, and Vista are actually the modern versions of Windows NT.