Let us consider the pining details of the 68HC11 as shown below.

 
Each pin has a defined function, some easy, some complex. A microprocessor designer should understand all the pins and timing waveforms. Let us examine some of the important pins. The address pins A0 ->A15 provide the outside world with details of the address the  p wants to read or write to.  The data pins D7-> D0 provide the outside world with the pathway to receiving or sending data to and from the mp. The third group of pins are called the ' asynchronous' bus control. This provides details of valid address' (Address strobe AS), read/write (R/W) and system clock (E). Some other pins exist for interrupts and other IO devices. We shall consider a simple hardware design using ideal components, but first we shall discuss the read/write cycles of the 68HC11.