Interfacing with narrower width of slave

IOWR_32DIRECT() probably maps down to the correct instruction :-)

If it is 'normal' memory, you'll have to support byte enables - otherwise single byte writes won't work, even for a 16bit write you'll see two writes one will have neither byte enable asserted!

For memory mapped io buffers it is usually best to create a 32bit slave that hard-wires d16-31 to zero for reads.