The datasheet of the 89C51RD2 (Philips) says this :

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The ERAM can be accessed by indirect addressing, with EXTRAM
bit cleared and MOVX instructions. This part of memory is physically
located on-chip, logically occupies the first 7936-bytes of external
data memory.

I use Bascom as a compiler and the only thing I do is write to adres 8000H and 8800H.
The routines work fine on a normal 8051 config with RAM and Eprom
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I tried pull-ups (4k7) on P0 which has nothing connected to it. Now something toggles but the waveform is nothing compared to waveform in the normal 8051 config.
It has a steep edge to gnd but a sort of RC curve up.
Read and write pins are performing normal.
I have also noticed that when I write to 8000H and 8800H that the other adresspins (A9 for example) are also active. one would expecting that they are not active.
It seems that the "action" in the internal ram is also visible on P0 and P2.
That would explain the bad waveforms I see at P0.
It wouldn't explain the fact that I need external pullups. Seems to me that P0 is not switched in bus mode.

I just cannot figure out what is going wrong..