8259A is a programable interupt controller. The
8259A block diagram have 8 units.
They are:
1. Control Logic
2. Rwad/Write Logic
3. Data bus Bufer
4. Casled Bufer
5. The interrupt Mask Register(IMR)
6. The Interrupt Request Register (IRR)
7. The In Service Register (ISR)
8. The Priority Resolver
