After, the ALE signal goes low. From now on the address of the target port to be read will be latched. Then the BUS takes the -IOR signal to a low level. So that the addressed device will take a data byte to the D0-D7 data bus. The microprocessor will read then the data bus and take the -IOR signal to a high again.
ALE goes low again.
The microprocessor send out the data byte to be write. It then asserts the -IOW signal. After the device have time to read the data byte, the uP raises the -IOW signal high again. In the PC memory map we can find two kinds of interrupts, the software interrupts and the hardware interrupts. In this tutorial only the hardware interrupts will be explained.