Architecture of 8259

8259 Microprocessor is architected in a unique style. It can program by means of some interrupts conditions by means of level or interrupt level often called edge-triggered interrupt level. Masking is done to individual interrupt bits. As the number of 8259 increases interrupt pins up to 64 can be obtained. There are 3 registers 8259 contains along with one priority resolver(PR). They are as follows −

• Interrupt Request Register(IIR)  − It stores the bits who requests the interrupt.

• Interrupt service register(ISR) − It stores the currently interrupt levels.

• Interrupt Mask Register(IMR)  − Stores the interrupt levels to be masked.

• PriorityResolver(PR)  − Set the priority of interrupts by examining all the three registers and set the interrupt level inISR having the highest priority.

• SP/EN (low active pin) − When its value is 1 it works in master mode and when its value is 0 it works in slave mode.

• Cascade Buffer  − Used for cascading more Programmable Interrupt Controller.

Fig −8259 interfaced along with 8085 processor