How to call tightly couple module during C code for NIOS II

Here's an example, as promised.

In my top level .v file, I have the following:

// Instantiate Nios II/e CPU
  cpu_accel CPU0 (
          .clk_0(CLOCK_CPU),
          .in_port_to_the_data_in0(int_datain),
          .out_port_from_the_data_out0(int_dataout0),
          .out_port_from_the_data_out1(int_dataout1),
          .reset_n(KEY),
    );
    // Instantiate custom accelerated verilog component
    FIR FIR0 (
       .PIO_in0(int_dataout0),
       .PIO_in1(int_dataout1),
        .PIO_out0(int_datain0)
    );

"cpu_accel.v" is my generated SOPC builder system. "FIR" is my custom Verilog module. 'int_datain' and 'int_dataout' are simply defined as wires.

You can use IO_RD and IO_WR. In this case, I simply used pointers:

<snip>
int main(int argc, char* argv) {
    
    // PIO pointers
    int* data_out0 = (int *) DATA_OUT0_BASE;
    int* data_out1 = (int *) DATA_OUT1_BASE;
    volatile int* data_in0 = (int *) DATA_IN0_BASE;
    int res;
<snip>
        *(data_out1) = 1;
        *(data_out0) = 2;
// insert wait statement here, if necessary
        res = *data_in0;                        // copy result to local int
<snip>