It is also an 8-bit register. The processor here writes to SLR but cannot read. The content of this register here implies a different meaning for Master8259 and Slave 8259. Through Master 8259 information is carried through the IR inputs to which Slave 8259s are connected. If the SLR of Master 8259 is loaded with the value 00001111, then it signifies that:
The Slave exists on 8259 namely called as IR0, IR1, IR2 and IR3.
No Slave exists on 8259 on this registers IR4, IR5, IR6 and IR7.
A Slave 8259 serves information about the IR input of Master 8259 to which the Slave 8259 is connected. Here, only the LS 3 bits of SLR are to be noted. By issuing the ICW3 command the SLR is written which uses 8259s high port. Moreover, Master 8259 plays a great role through the IR inputs through which the information is carried. All the pins named as Slave and Non-Slave pins play a vital role in the entire process in the entire scheme.
Fig:8259 interfaced along with 8085 processor