Interrupt Servie Routines using an Arduino 2560 Mega

/*
 *   Simple Arduino CW program to emulate a Memory Keyer 
 *   with sound and a flashing LED Has 4 pre-defined memories.
 *   Text is shown on a 0.91" OLED display. We are using an 
 *   Arduino Mega 2560 because we need 4 hardware interrupts
 *   
 *   Paul M Dunphy, VE1DX
 *   
 *   March 2020
 */
 
#include "U8glib.h"
U8GLIB_SSD1306_128X32 u8g(U8G_I2C_OPT_NONE);

char *myMem[] = {
                 "CQ CQ VE1DX TEST",  
                 "DE VE1DX 5NN 05",
                 "TU DE VE1DX TEST",  
                 "AGN ?"
                 };
char mess[100];                

const char* MorseTable[] = {
  NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL,
  NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL,
  NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL,
  NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL,
  // space, !, ", #, $, %, &, '
  NULL, "-.-.--", ".-..-.", ".----.","...-.-", NULL, NULL,NULL,
  // ( ) * + , - . /
  "-.--.", "-.--.-", NULL, ".-.-.", "--..--", "-....-", ".-.-.-", "-..-.",
  // 0 1 2 3 4 5 6 7
  "-----", ".----", "..---", "...--", "....-", ".....", "-....", "--...",
  // 8 9 : ; < = > ?
  "---..", "----.", "---...", "-.-.-.", NULL, "-...-", NULL, "..--..",
  // @ A B C D E F G
  ".--.-.", ".-", "-...", "-.-.", "-..", ".", "..-.", "--.",
  // H I J K L M N O
  "....", "..", ".---", "-.-", ".-..", "--", "-.", "---",
  // P Q R S T U V W
  ".--.", "--.-", ".-.", "...", "-", "..-", "...-", ".--",
  // X Y Z [ \ ] ^ _
  "-..-", "-.--", "--..", NULL, NULL, NULL, NULL, "..--.-",
  // ' a b c d e f g
  NULL, ".-", "-...", "-.-.", "-..", ".", "..-.", "--.",
  // h i j k l m n o
  "....", "..", ".---", "-.-", ".-..", "--", "-.", "---",
  // p q r s t u v w
  ".--.", "--.-", ".-.", "...", "-", "..-", "...-", ".--",
  // x y z { | } ~ DEL
  "-..-", "-.--", "--..", NULL, NULL, NULL, NULL, NULL,
};

int dotLength = 40;
int dashLength = dotLength * 3;
int CW_pin = A0;
int flashPin = 13;
int freq = 650;
int Pin2 = 2;
int Pin3 = 3;
int Pin18 = 18;
int Pin19 = 19;
volatile int mess_number = 0;
volatile int buttonState = 0;

volatile boolean pressed = false;


void clearOLED(){
    u8g.firstPage(); 
    do {
    } while( u8g.nextPage() );
}



void prompt(void) {
  u8g.setFont(u8g_font_unifont);
  u8g.drawStr( 0, 22, "Press 1-4");
}



void showText(void) {
  u8g.setFont(u8g_font_unifont);
  u8g.drawStr( 0, 22, mess);
}



void setup()
  {
    clearOLED();    
    memset(mess,0,strlen(mess));
    pinMode(flashPin, OUTPUT);
    pinMode(CW_pin, OUTPUT);
    pinMode(Pin2, INPUT_PULLUP);
    pinMode(Pin3, INPUT_PULLUP);
    pinMode(Pin18, INPUT_PULLUP);
    pinMode(Pin19, INPUT_PULLUP);
    attachInterrupt(digitalPinToInterrupt(Pin2), ISR_2, RISING);
    attachInterrupt(digitalPinToInterrupt(Pin3), ISR_3, RISING);
    attachInterrupt(digitalPinToInterrupt(Pin18), ISR_18, RISING);
    attachInterrupt(digitalPinToInterrupt(Pin19), ISR_19, RISING);
  }



void loop() 
  {
  int i = 0; 
  char ch;
  
  u8g.firstPage(); 
  do {
       prompt();
     } while( u8g.nextPage() );

  
  if (pressed == true) 
    { 
      strcpy(mess, myMem[mess_number-1]);
      
      u8g.firstPage(); 
      do {
         showText();
         } while( u8g.nextPage() );

      ch = mess[i];
      while (ch != NULL )
        {
          i++;
          makeDashDot(MorseTable[ch]);
          delay(dotLength * 2);
          ch = mess[i];
         }
       pressed = false;
       i = 0; 
     }

  }
  


void makeDashDot(const char * morseCode)
{
  int i = 0;
  while (morseCode[i] != 0)
  {
    if (morseCode[i] == '.') {
      dot();
    } else if (morseCode[i] == '-') {
      dash();
    }
    i++;
  }
}



void dot()
{
  digitalWrite(flashPin, HIGH);
  tone(CW_pin, freq);
  delay(dotLength);
  digitalWrite(flashPin, LOW);
  noTone(CW_pin);
  delay(dotLength);
}



void dash()
{
  digitalWrite(flashPin, HIGH);
  tone(CW_pin, freq);
  delay(dashLength);
  digitalWrite(flashPin, LOW);
  noTone(CW_pin);
  delay(dotLength);
}


void ISR_2()          
{    
   noInterrupts();   
   buttonState = digitalRead(Pin2);                                                             
   mess_number = 1;
   pressed = true;
   interrupts();
}



void ISR_3()          
{       
   noInterrupts();    
   buttonState = digitalRead(Pin3);                                                           
   mess_number = 2;
   pressed = true;
   interrupts();
}



void ISR_18()          
{     
   noInterrupts();  
   buttonState = digitalRead(Pin18);                                                               
   mess_number = 3;
   pressed = true;
   interrupts();   
}



void ISR_19()          
{              
   noInterrupts();       
   buttonState = digitalRead(Pin19);                                                 
   mess_number = 4;
   pressed = true;
   interrupts();
}