readdata address for avalon slave interface

The Nios II application executes *on* the Nios II processor (i.e. Nios is the host). That IO macro is part of the board support package so that your Nios II software can access memory locations in the system.

Are you using Nios II or something else to access the slave, I assumed you are using Nios II since this post is in the Nios section of the Altera forum. If you wanted to access the slave without Nios II you would need some other mastering component to issue the reads and writes. For example you could connect the slave to the JTAG to Avalon bridge and access your slave over JTAG using system console.