Programming NIOSII interfaced with custom component, using Avalon MM Interface

Your custom component uses 16 bit Avalon data bus or 32 bit Avalon data bus.If it is using 16 bit Avalon data bus then you should write IOWR_16DIRECT instead of IOWR_32DIRECT and same way for IORD function also.

Other then that your C code is correct.

Whenever you use IOWR function,Avalon Master generate Avalon write request and provide address of Slave Component Register on Avalon address bus line and provides data on Avalon data bus.Avalon Slave component uses this write request and address information to update its registers.Same way for IORD function,Avalon Master generates read request and address of the Slave component register.In this case slave component provides data on Avalon data bus.