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4 Serial DOS
4.1 Introduction
It is now time to build on everything that has been established so far. While it is unlikely
that you are going to be using MS-DOS for a major application, it is a good operating
system to demonstrate a number of ideas related to software access of the 8250 UART and
driver development. Compared to modern operating systems like Linux, OS-X, or Windows,
MS-DOS can hardly be called an operating system at all. All it really offers is basic access
to the hard drive and a few minor utilities. That really doesn't matter so much for what
we are dealing with here, and it is a good chance to see how we can directly manipulate
the UART to get the full functionality of all aspects of the computer. The tools I'm using
are all available for free (as in beer) and can be used in emulator software (like VMware or
Bochs) to try these ideas out as well. Emulation of serial devices is generally a weak point
for these programs, so it may work easier if you work from a floppy boot of DOS, or on an
older computer that is otherwise destined for the trash can because it is obsolete.
For Pascal, you can look here:
• Turbo Pascal http://bdn.borland.com/article/0,1410,20803,00.html version 5.5 -
This is the software I'm actually using for these examples, and the compiler that most
older documentation on the web will also support (generally).
• Free Pascal http://www.freepascal.org/ - *note* this is a 32-bit version, although
there is a port for DOS development. Unlike Turbo Pascal, it also has ongoing development
and is more valuable for serious projects running in DOS.
For MS-DOS substitution (if you don't happen to have MS-DOS 6.22 somewhere):
• FreeDOS http://www.freedos.org/ Project - Now that Microsoft has abandoned development
of DOS, this is pretty much the only OS left that is pure command line driven
and following the DOS architecture.
4.2 Hello World, Serial Data Version
In the introduction 1 , I mentioned that it was very difficult to write computer software that
implements RS-232 serial communications. A very short program shows that at least a
basic program really isn't that hard at all. In fact, just three more lines than a typical
"Hello World" program.
1
http://en.wikibooks.org/wiki/Programming%3ASerial%20Data%20Communications%23Intended%
20Audience
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