It's through the powerful concept of pointers, and you can read about it in the tutorials offered in the software section of the technology education sites links . We are concerned here with another use of address lines. The PC actually has two memory systems. One deals with variables and constants as noted above. The other addresses what are called ports .
Ports provide access to outside-world devices.
The printer, serial communications, disk drives and sound cards, among others, all use ports for monitoring and control. A typical PC uses A0 through A9 as the bases for addressing ports, with A9 always high. The board used in the examples is capable of using all of these addresses in 64 byte blocks. Subdivisions within each block allow access to several sub-systems on the board. All of them will be covered in detail in the hardware and experiments sections.