DE10-NANO - How to use the ADC with FPGA and NIOSII

Hello, I just started learning about FPGA, NIOSII and QuartusII.

I have the DE10-NANO Dev. Board and I want to learn how to use the ADC and FPGA (NIOS II) to read some analog data.

I've programmed the FPGA with the ADC demonstration provided by Terasic.

Here is the code that converts the data from CH 0 as it comes in the demo:

#include <stdio.h>#include <io.h>
#include <unistd.h>
#include "system.h"
void main(void){
    int ch = 0;
    const int nReadNum = 10; // max 1024
    int i, Value, nIndex=0;
    printf("ADC Demo\r\n");
    while(1){
        ch = IORD(SW_BASE, 0x00) & 0x07;
        printf("======================= %d, ch=%d\r\n", nIndex++, ch);
        // set measure number for ADC convert
        IOWR(ADC_LTC2308_BASE, 0x01, nReadNum);
        // start measure
        IOWR(ADC_LTC2308_BASE, 0x00, (ch << 1) | 0x00);
        IOWR(ADC_LTC2308_BASE, 0x00, (ch << 1) | 0x01);
        IOWR(ADC_LTC2308_BASE, 0x00, (ch << 1) | 0x00);
        usleep(1);
        // wait measure done
        while ((IORD(ADC_LTC2308_BASE,0x00) & 0x01) == 0x00);
        // read adc value
        for(i=0;i<nReadNum;i++){
            Value = IORD(ADC_LTC2308_BASE, 0x01);
            printf("CH%d=%.3fV (0x%04x)\r\n", ch, (float)Value/1000.0, Value);
        }
        usleep(200*1000);
    } // while
}

My problem is that I don't know where am I supposed to search to get the basics of how it works. I don't understand where the channel is selected here, or where the ADC is configured. I understood that the IORD and IOWR use the starting address and the offset to get to the address I need but where should I find how are all these addresses arranged and what are the parameters needed for each case.

I'll be glad if you could help me understand how all this work and even better give me some tips about how and where should I been looking for this info.

Thank you very much in advance.