A write data bus is used to move data from the master to a slave, while a read data bus
is used to move data from a slave to the master where every transfer consists of:
• An address and control cycle.
• One or more cycles for the data. 
 The address cannot be extended and therefore all slaves must sample the address during this time. The data, however, can be extended using the HREADY signal. When LOW this signal causes wait states to be inserted into the transfer and allows extra time for the slave to provide or sample data.