There is an easy way of testing that your little circuit works without the requirements of any programming . We can use debug in DOS instead.

The first step is to get to a DOS prompt , in Windows 98 there is an entry in the Start Menu / Programs for the DOS prompt .

Now once you are there type in debug and press enter , you will be greeted with a minus sign ( - ) at the beginning of the next line that is correct .

First we will switch the LED on , to do this on the line with the - type the following

-o 378 ,1

this means output , thats the o . Then 378 which is the address for the printer port on my machine and then the value to output is 1 . Now lets switch the led off , type the following in

-o 378 ,0

The LED now goes off.

Here is a screen capture of all of this , note we have put in a wrong entry to demonstrate what happens if you have a typo .