Hi Cyrille.

Well, you will need to understand memory decoding concepts first, irrespective of whether you are decoding memory or IO. The concepts apply to both.

Now, in a very layman's terminology and to soup it all up in a tablespoon, Memory decoder acts like a Traffic policeman, signalling which lanes are selected at what times (or under what circumstances on the address bus).

Or, if you are the postman and have to deliver letters, then you must be knowing the house numbers and street numbers. If you want to deliver a letter to Mr. RAM-2, then probably you will have to go to Street 9 amd House 48. Now ofcourse there will be a lot of streets and houses in the whole area, but each individual house has a unique address to identify it with.

In every microcontroller or processor system, you have atleast 3 sets of signals, known as busses. They are 1) DATA Bus, 2) Address bus and 3) Control Bus.

Data Bus carries the data (message of the letter from teh post man example), Address bus carries the address and Control bus carries comon sugnals such as ReaD or WRite.

Now, if you have to deliver a letter (data) to House 48 0n Street 9 (Address 49) then in the microcontroller system the busses will look something like this:

DATA BUS: xxxx <Data packet>
ADDRESS BUS: 0100 1001 (binary representation of 49 hex).
CONTROL BUS: WR active

The WR signal will tell the addressed house to open its gates so the letter (data) can be delivered inside. While a RD signal will read data from the device (collect the letters from).

This is the basic, and i know i might be looking like a bafoon sying this all here, but just incase you didnt know. The rest carries on from here.

Although i am sure your EE-101 course will teach u this in detail, you can refer to about any microcontroller / processor book and they all have a good Address decoding section.

Refer to this for example:

THE INTEL MICROPROCESSORS by Barry B. Brey.

Or any text at hand will do.