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Serial DOS
program HelloSerial;
var
DataFile: Text;
begin
Assign(DataFile,'COM1');
Rewrite(DataFile);
Writeln(DataFile,'Hello World');
Close(DataFile);
end.
All of this works because in DOS (and all version of Windows as well... on this particular
point) has a "reserved" file name called COM1 that is the operating system hooks into the
serial communications ports. While this seems simple, it is deceptively simple. You still
don't have access to being able to control the baud rate or any of the other settings for the
modem. That is a fairly simple thing to add, however, using the knowledge of the UART
discussed in the previous chapter Programming the 8250 UART 2 .
To try something even easier, you don't even need a compiler at all. This takes advantage
of the reserved "device names" in DOS and can be done from the command prompt.
C:\>COPY CON COM1
What you are doing here is taking input from CON (the console or the standard keyboard
you use on your computer) and it "copies" the data to COM1. You can also use variations
of this to do some interesting file transfers, but it has some important limitations. Most
importantly, you don't have access to the UART settings, and this simply uses whatever the
default settings of the UART might be, or what you used last time you changed the settings
to become with a serial terminal program.
4.3 Finding the Port I/O Address for the UART
The next big task that we have to work with is trying to find the base "address" of the Port
I/O so that we can communicate with the UART chip directly (see the part about interface
logic in the Typical RS232-Hardware Configuration 3 module for information what this is
about). For a "typical" PC system, the following are usually the addresses that you need to
work with:
Serial Port Name Base I/O Port Address
IRQ (interrupt) Number
COM1 3F8 4
COM2 2F8 3
COM3 3E8 4
COM4 2E8 3
2 http://en.wikibooks.org/wiki/Serial%20Programming%3A8250%20UART%20Programming
http://en.wikibooks.org/wiki/Serial_Programming%3ATypical_RS232-Hardware_
3
Configuration
56