Banking troubles

Hi,
I have an Infineon C515C with a PSD854F2 (configurable Flash+PLD from ST).

My banked software runs without a problem using Mode 1, banking register at xdata 02E0h.

However, for IAP (In-Application-Programming) I need the two MSB of the banking register as chip select qualifiers for my external program memory because I want to jump from a bootloader to my main application OR remain in the bootloader and re-program the main flash.

This should be possible by using the modified L51_Bank.A51 described in Keil APNT_130, but whenever the address decoding relies on the correct setting of the mentioned MSBs, my software crashes.

Using the uVision debugger, I think I have found the cause of the problem:
Startup.A51 is correctly executed until

LJMP    ?C_START

The code that is being run between this point and the first C-Code resets my banking register (all bits!)to zero which leads to my software crashing.
This code portion seems to be part of the Keil runtime environment. I have found similar code in Init.A51:

XLoop:          CLR     A
                MOVC    A,@A+DPTR
                INC     DPTR
                XCH     A,R0
                XCH     A,DPL
                XCH     A,R0
                XCH     A,R2
                XCH     A,DPH
                XCH     A,R2
$IF (DS390)
                MOV     DPX,#BYTE0 (?C?XDATASEG)
$ENDIF
                MOVX    @DPTR,A

The last line overwrites xdata at 0x2E0.

My XDATALEN is zero. Init.A51 is not part of my project. This happens even (in the debugger) if I remove everything dealing with the register at 0x2E0. I use C51 V8.05a.

If 0x2E0 is not in the xdata memory map, why does the runtime environment write to it?
Any solutions to this problem?
Regards, Ralph