File content as of revision 0:044197641ae8:

#include "mbed.h"

#define BTN0_PIN P6_0
#define BTN1_PIN P6_1
#define BTN0_IRQ IRQ5_IRQn
#define BTN1_IRQ IRQ4_IRQn

InterruptIn button0(BTN0_PIN);
InterruptIn button1(BTN1_PIN);
DigitalOut rled(LED_RED);
DigitalOut bled(LED_BLUE);

#define DELAY_CNT 1000000000

void button0_int_handler() {
    rled = 0;
    for (int i = 0; i <= DELAY_CNT; i++) ;
    rled = 1;
}

void button1_int_handler() {
    bled = 0;
    for (int i = 0; i <= DELAY_CNT; i++) ;
    bled = 1;
}

int32_t R_INTC_SetPriority(uint16_t int_id, uint8_t priority);

int main() {
    rled = 1;
    bled = 1;
    R_INTC_SetPriority(BTN0_IRQ, 1);
    button0.fall(&button0_int_handler);
    button1.fall(&button1_int_handler);
    while(1) {
        wait(1);
    }
}

/**
 * http://japan.renesas.com/req/sample_code.do?event=searchProducts&%20expandedProductsId=1&productsId=186374
 */
#include "r_typedefs.h"
#include "dev_drv.h"                /* Device Driver common header */
//#include "devdrv_intc.h"            /* INTC Driver Header */
#include "iodefine.h"
#define INTC_ID_TOTAL           (587)
/******************************************************************************
* Function Name: R_INTC_SetPriority
* Description  : Sets the priority level of the ID specified by the int_id to 
*              : the priority level specified by the priority.
* Arguments    : uint16_t int_id   : Interrupt ID
*              : uint8_t  priority : Interrupt priority level (0 to 31)
* Return Value : DEVDRV_SUCCESS    : Success of INTC interrupt priority level setting
*              : DEVDRV_ERROR      : Failure of INTC interrupt priority level setting
******************************************************************************/
int32_t R_INTC_SetPriority(uint16_t int_id, uint8_t priority)
{
    uint32_t icdipr;
    uint32_t mask;
    volatile uint32_t * addr;

    /* ==== Argument check ==== */
    if ((int_id >= INTC_ID_TOTAL) || priority >= 32)
    {
        return DEVDRV_ERROR;        /* Argument error */
    }

    priority = priority << 3;       /* Priority[7:3] of ICDIPRn is valid bit */

    /* ICDIPRn has 4 sources in the 32 bits                 */
    /* The n can be calculated by int_id / 4                */
    /* The bit field width is 8 bits                        */
    /* The target bit can be calculated by (int_id % 4) * 8 */
    addr = (volatile uint32_t *)&INTC.ICDIPR0;

    icdipr = *(addr + (int_id / 4));    /* Read ICDIPRn */

    mask = (uint32_t)0x000000FFuL;      /* ---- Mask ----      */
    mask = mask << ((int_id % 4) * 8);  /* Shift to target bit */
    icdipr &= ~mask;                    /* Clear priority      */
    mask = (uint32_t)priority;          /* ---- Priority ----  */
    mask = mask << ((int_id % 4) * 8);  /* Shift to target bit */
    icdipr |= mask;                     /* Set priority        */

    *(addr + (int_id / 4)) = icdipr;    /* Write ICDIPRn */

    return DEVDRV_SUCCESS;
}