The 8259 is a specialized I/O port chip. It is never used in the interfacing of I/O devices but is only used for controlling the interrupts in a microcomputer.8259 consists of A0 as the only address input pin. Hence for a microprocessor, only two addresses are possible for the 8259 ports. The two ports can be termed as low port and high port.
The processor selects the low port when A0 = 0
The processor selects the high port when A0 = 1
The processor issues some words termed as command words to these ports so as to configure the 8259 better. There are several command words often classified as initialization command words and operation command words. 8259 has four initialization command words namely ICW1, ICW2, ICW3, and ICW4 and three operation command words namely as OCW1, OCW2, and OCW3. The processor reads the status of 8259 by reading at the two ports termed as low port and the high port. Moreover, there are several status words to be read further.
8259 always makes use of a number of 8-bit registers shown as follows for its working procedure.
Interrupt request register, Interrupt mask register and Interrupt service register
The processor writes and reads the command and status words, or accesses registers using only the high port and the low port. Identification of a command or status word or a register is completely based on A0 value.
Fig:8259 interfaced along with 8085 processor