源码
stdint.h见【51单片机快速入门指南】1：基础知识和工程创建
中断知识见【51单片机快速入门指南】3：中断系统

将外部中断0和外部中断1均配置为下降沿触发，但外部中断1的优先级最高，外部中断0的优先级最低。

EXTI.c

#include <REGX52.H>
#include "intrins.h"
#include "stdint.h"
#include "EXTI.h"

//定义LED管脚
sbit D1 = P2^0;
sbit D2 = P2^1;

//定义独立按键控制脚
sbit K3 = P3^2;
sbit K4 = P3^3;

void Delay20ms()        //@11.0592MHz
{
    unsigned char i, j;

    i = 36;
    j = 217;
    do
    {
        while (--j);
    } while (--i);
}

//P3.2
void EXTI0_Callback() interrupt 0 //外部中断0中断函数
{
    Delay20ms();
    if(K3 == 0)
    {
        D1 = !D1;
        while(K3 == 0);
    }
}

//P3.3
void EXTI1_Callback() interrupt 2 //外部中断1中断函数
{
    Delay20ms();
    if(K4 == 0)
    {
        D2 = !D2;    
        while(K4 == 0);
    }
}

#if USE_EXTI2_EXTI3

    //P4.3
    void EXTI2_Callback() interrupt 6 //外部中断2中断函数
    {

    }

    //P4.2
    void EXTI3_Callback() interrupt 7 //外部中断3中断函数
    {

    }

#endif

#if EXTI_USE_STC
    sfr    IPH    = 0xb7;
#endif

void EXTI0_Init(uint8_t EXTI_Type, uint8_t Priority)
{
    #if EXTI_USE_STC
        IPH &= ~1;
        IPH |= (2 & Priority) >> 1;
    #endif
    PX0  = (1 & Priority);        //设置中断优先级
    IT0  = EXTI_Type;            //触发方式
    EX0  = 1;                    //打开INT0的中断允许
    EA   = 1;                    //打开总中断
}

void EXTI0_Switch(uint8_t Status)
{
    EX0  = Status;                    
}

void EXTI1_Init(uint8_t EXTI_Type, uint8_t Priority)
{
    #if EXTI_USE_STC
        IPH &= ~4;
        IPH |= (2 & Priority) << 1;
    #endif
    PX1  = (1 & Priority);        //设置中断优先级
    IT1    = EXTI_Type;            //触发方式
    EX1    = 1;                    //打开INT1的中断允许
    EA    = 1;                    //打开总中断
}

void EXTI1_Switch(uint8_t Status)
{
    EX1  = Status;                    
}

#if USE_EXTI2_EXTI3

    sfr XICON = 0xc0;

    sbit PX2 = XICON^3;
    sbit EX2 = XICON^2;
    sbit IE2 = XICON^1;
    sbit IT2 = XICON^0;

    sbit PX3 = XICON^7;
    sbit EX3 = XICON^6;
    sbit IE3 = XICON^5;
    sbit IT3 = XICON^4;

    void EXTI2_Init(uint8_t EXTI_Type, uint8_t Priority)
    {
        #if EXTI_USE_STC
            IPH &= ~0x40;
            IPH |= (2 & Priority) << 5;
        #endif
        PX2  = (1 & Priority);        //设置中断优先级
        IT2    = EXTI_Type;            //触发方式
        EX2    = 1;                    //打开INT1的中断允许
        EA    = 1;                    //打开总中断
    }

    void EXTI2_Switch(uint8_t Status)
    {
        EX2  = Status;                    
    }

    void EXTI3_Init(uint8_t EXTI_Type, uint8_t Priority)
    {
        #if EXTI_USE_STC
            IPH &= ~0x80;
            IPH |= (2 & Priority) << 6;
        #endif
        PX3  = (1 & Priority);        //设置中断优先级
        IT3    = EXTI_Type;            //触发方式
        EX3    = 1;                    //打开INT1的中断允许
        EA    = 1;                    //打开总中断
    }

    void EXTI3_Switch(uint8_t Status)
    {
        EX3  = Status;                    
    }

#endif

EXTI.h

#ifndef EXTI_H_
#define EXTI_H_

#include "stdint.h"

//使用外部中断2（P4.3）和外部中断3（P4.2） 貌似只有LQFP44封装的芯片才有，普通的DIP40芯片没有
#define USE_EXTI2_EXTI3 1

//使用STC单片机的4级优先级
#define EXTI_USE_STC            1

#define EXTI_LOW_LEVEL    0
#define EXTI_FALLING    1

#if EXTI_USE_STC

    //STC单片机的4级优先级
    #define STC_EXTI_Priority_Lowest     0
    #define STC_EXTI_Priority_Lower        1
    #define STC_EXTI_Priority_Higher    2
    #define STC_EXTI_Priority_Highest    3

#endif

//传统51单片机的2级优先级
#define EXTI_Priority_Low            0
#define EXTI_Priority_High            1

void EXTI0_Init(uint8_t EXTI_Type, uint8_t Priority);
void EXTI0_Switch(uint8_t Status);

void EXTI1_Init(uint8_t EXTI_Type, uint8_t Priority);
void EXTI1_Switch(uint8_t Status);

#if USE_EXTI2_EXTI3

    void EXTI2_Init(uint8_t EXTI_Type, uint8_t Priority);
    void EXTI2_Switch(uint8_t Status);

    void EXTI3_Init(uint8_t EXTI_Type, uint8_t Priority);
    void EXTI3_Switch(uint8_t Status);

#endif

#endif

main.c

#include <REGX52.H>
#include "intrins.h"
#include "stdint.h"
#include "EXTI.h"

void main(void)
{    
    EXTI0_Init(EXTI_FALLING, STC_EXTI_Priority_Lowest);
    EXTI1_Init(EXTI_FALLING, STC_EXTI_Priority_Highest);
    while(1)
    {

    }
}

实验现象
按下K3，D0翻转；按下K4，D1翻转。
当按住K3时(处理外部中断0时)，仍可响应外部中断1；

当按住K4时(处理外部中断1时)，不能响应外部中断0。