Old Code working once properly newly compiled does no longer work properly

So perhaps you should look carefully at your xdata implementation - again, timing would be prime suspect...

Is the LCD connected to the data/address bus...?

So perhaps you should look carefully at your xdata implementation - again, timing would be prime suspect...

Is the LCD connected to the data/address bus...?

Oscillatorfrequency is 12 MHz. Chosen in order to have 1 machine cycle = 1 microsecond.

The LCD is connected to Port P4. DB0 of LCD at P4.0 and DB7 of LCD to P4.7.

I have a second version, where idata for the constant strings idata is changed to code. That wokrs too, but is a bit slower due to the code memory access.

There is sometime something wrong with strings and memory access and the optimization. There have been displays of characters belonging to different strings. But xdata should cause lesser problems due to more xdata than idata.

There is no extern access ot P0 and P2. That can only be done by changing the original layout of the hps teaching system. And that is not an option.

The wait function is not state of the art but works. May be I change that to timing with timer 0.

The LCD is busy while DB7 is zero. The manual says while 1, but that is wrong. Between EN = 1 and EN = 0 the dummy++ causing a delay of 1 microsecond is necessary. Without the delay, the timing is out of order.

#include<reg515c.h>
#define LCD P4

sbit RS   = P3^3;
sbit RW   = P3^4;
sbit EN   = P3^5;
sbit LCD0 = P4^0;
sbit LCD1 = P4^1;
sbit LCD2 = P4^2;
sbit LCD3 = P4^3;
sbit LCD4 = P4^4;
sbit LCD5 = P4^5;
sbit LCD6 = P4^6;
sbit LCD7 = P4^7;

char dummy = 0x00;

void warte (unsigned int zeit) //Zeit wird in Millisekunden übergeben
{                              // bei fosz 12 MHz
   unsigned int i, j;

   for (i=0; i < zeit; i++)
   {
        for (j=0; j<55; j++) {;}
   }

}

void init(bit D, bit N, bit F)
{
  char i;
  // LCD Port für die Datenbits
  // RS, RW, EN Portleitungen für Steuerbits
  //             RS RW D7 D6 D5 D4 D3 D2 D1 D0
  //             0  0  0  1  1  D  N  F  x  x    Function Set
  //             D = 1 Display on, D = Display off
  //             N = 1 5 x 8 Punkte zweizeilig
  //             N = 0, F = 0 5 x 8 Punkte  einzeilig
  //             N = 0, F = 1 5 x 10 Punkte einzeilig

  RS  = 0;       // Steuerleitungen auf Befehle zum Display schreiben
  RW  = 0;
  LCD = 0x30;    // 0x30 Function set
  LCD4 = D;
  LCD3 = N;
  LCD2 = F;
  LCD1 = 0;
  LCD0 = 0;
  for(i = 3; i > 0; i-- )
  {  EN = 1;
     dummy++;
     EN = 0;
     warte(20);   // 20 ms warten
  }
}

void clrscr(void)
{
   // Display mit Leerzeichen überschreiben und Cursor an Anfang Zeile 1, Spalte 1
   RS  = 0;       // Steuerleitungen auf Befehle zum Display schreiben
   RW  = 0;
   LCD = 0x01;
   EN = 1;
   dummy++;
   EN = 0;
   warte(20);   //  20 ms warten
}

void home(void)
{
   // Cursor zurück in Zeile 1 Spalte 1, Displayinhalt nicht gelöscht
   RS  = 0;       // Steuerleitungen auf Befehle zum Display schreiben
   RW  = 0;
   LCD = 0x02;
   EN = 1;
   dummy++;
   EN = 0;
   warte(20);   // 20 ms warten
}

void entry(bit ID, bit S)
{
   // Displayadresse incrementieren
   RS  = 0;       // Steuerleitungen auf Befehle zum Display schreiben
   RW  = 0;
   LCD = 0x04;
   LCD1 = ID;    // ID = 1 Inkrementieren, 0 Dekrementieren
   LCD0 = S;     // S = 1 Display Shift, ID = 1 Linksshift, ID = 0 Rechtsshift
   EN = 1;
   dummy++;
   EN = 0;
   warte(20);   // 20 ms warten
}

void display_on_off(bit D, bit C, bit B)
{
   // Cursor zurück in Zeiel 1 Spalte 1, Displayinhalt nicht gelöscht
   RS  = 0;       // Steuerleitungen auf Befehle zum Display schreiben
   RW  = 0;
   LCD = 0x08;
   LCD2 = D;    // D = 1 Dislay on, D = 0 Display off
   LCD1 = C;    // C = 1 Cursor on, C = 0 Cursor off
   LCD0 = B;    // B = 1 Blink  on, B = 0 blink off
   EN = 1;
   dummy++;
   EN = 0;
   warte(20);   // 20 ms warten
}

void datschreib(char *z)
{
        char i;
        RW = 0;
        RS = 1;
        for(i = 0; z[i] != '\0'; i++)
        {
            LCD = z[i];
            EN = 1;
            dummy++;
            EN = 0;
            dummy++;
            LCD = 0xFF;
            do
            {
               RW = 1;
               RS = 0;
               EN = 1;
               dummy++;
               EN = 0;
               dummy++;
            }while(LCD7 == 0);

            RW = 0;
            RS = 1;
        }
}

void lcdadr(char a7)
{  // Zeile 1 Anfangsadresse 00H = 0  dezimal
   // Zeile 2 Anfangsadresse 40H = 64 dezimal
   // Adresse 7 Bit => 128 Speicherplätze
   RS = 0;
   RW = 0;
   LCD = a7 | 128;
   EN = 1;
   dummy++;
   EN = 0;
   LCD = 0xFF;
   do
   {
      RW = 1;
      RS = 0;
      EN = 1;
      dummy++;
      EN = 0;
      dummy++;
   }while(LCD7 == 0);
}