CertiGuide to A+ (Core Hardware)  9  Chapter 0001:  Power Supplies - System Board

Bus Mastering 1 XVIII  Memory Mapping

While many users are uninformed about how a computer works, most of them can at least tell you, "I have XXX megs of RAM." What is not considered is how RAM works. Think of a city say, New York City. This city has millions of residence. Going to the city looking for a certain individual, your odds of success are nil without knowing the person's address.

Computer memory works very similar fashion. To find where something is, you need its address. Unlike a physical city, memory addresses work in hexadecimal format.

For a device such as a modem to work in concert with the PC, it must be connected to a communications port. The communications port must have a place to put the data after the modem converts it from an analog signal to a digital signal that is given to the communications port.

The Operating System must know where the newly arrived data landed if you expect your computer to do anything with the data. This gives us the need for specific memory location so the Operating System knows what to do.

Bus Mastering 1 XVIII  Memory Mapping