Hello all,

Here's an update :

The display I'm using is a TLX-1741-C3M graphic display. I don't use a alphanumeric LCD.
I found several wrong things :

First of all I think my 89C51RD2 is broken. Some data lines got stuck on 2.5V. I think there is a short circuit in P0.
I replaced the 89C51RD2 with a 89C668.

The CCFL backlight of the display creates some nasty interference. I have made several things using this display and that never was a problem. It's solved now by rerouting the 15cm long flatcable to the display.

Erik : you're right. That is what the datasheet says. It also stated that when adressing above 8k the controller automatically "looks outside". This does not happen.
The bit auxr.1 doens't change a thing. I've tried to disbale ALE (which is bit 0 of AUXR). This works.
Also toggling the pins A15, A11,RD,WR with setb and clr works fine. The pins of P0 can be toggled also when using a 4k& pullup.
So nothing is wrong with my hardware.

When I only use the 256 RAM on the chip to run my programm nothing happens on the portpins of P0 and P2
When placing my variabels in the on-chip Xram P0 and P2 show the normal signals.
However when I try to write to A15 (8000Hex) nothing happens.
The action on P0 and P2 (with auxr.1 = 0) is somewhat strange for me. I thought that when accessing onboard Xram nothing happened on the portpins.
Using AUXR.1=1 before I write to 8000HEX and using AUXR.1=0 after I did that doesn't have any effect.

Am I still missing something?