Solution:

When data arrives at one of four ports, it will generate an interrupt in the interrupt register. The PC software can read this and identify immediately which port generated the interrupt. This saves time and makes programming easier. When a data bit of the interrupt status register is set to 0, the corresponding channel is selected to generate an interrupt. If the bit is 1, then no interrupt is generated. However, this definition is reverse from Windows 95/98/NT. So JP15 just designed for this purpose.