assign oe = (!RD_l && !CS_l);
assign dio = oe ? diomux : 8'bZZZZ_ZZZZ;

always @(*) begin : diomuxselect
    case ({A, ctrlword})
        CASE0 : diomux = ...;
        CASE1 : diomux = ...;
         ...
        CASEN : diomux = ...'
    endcase
end // diomuxselect