NIOS II Custom Instruction blocks printf

I´m afraid you forgot to implement the Avalon interface and it´s combinational logig. At least the entity looks somehing like that

module xyz(CLK, RST, READDATA, WRITEDATA, READ, WRITE, ADRESS);
input CLK;
input RST;
input  READDATA;
input READ;
output  WRITEDATA;
input WRITE;
input  ADRESS;

At SOPC Builder you conect these signals as Avalon slave to the signals. I think they have the same name or sounds simular.

You also need some logig to put data on the Avalon inteface if you want to read it from your c-code.

always@ (posedge CLK)
beginif(WRITE == 1'b1)
begincase(ADRESS)
begin
...
end
end
else
begin
if(READ == 1'b1)
begincase(ADRESS)
beginresult_adress: READDATA <= result;
end
end
end
end

In your c-Code you can then write:

result = IORD_32DIRECT(XYZ_BASE,result_adress);

I´m not sure if that´s all. I´m afraid there is something missing but I think the basics get clear. And I hope it is that what you like to do ;).