hi,

I guess I cant use the whole 32K external code of the NVRAM if I am using a AT89S53.

Why not? Assume you did next connections:

MCU                NVRAM
P0       <-->      D0...D7
P0 ---> latch ---> A0...A7
ALE ----^
P2       --->    A8...A14 (P2.7 is not connected)
/WR      --->      /WR
/PSEN    --->      /OE
GND  ---> SV --->  /CS

This way NVRAM covers all the memory space and so

- your code memory space will look like:

0x0000...0x2FFF - internal flash;
0x3000...0x7FFF - NVRAM contents 0x3000...0x7FFF;
0x8000...0xFFFF - NVRAM contents 0x0000...0x7FFF.

- your XDATA memory:

0x0000...0x7FFF - NVRAM contents 0x0000...0x7FFF;
0x8000...0xFFFF - NVRAM contents 0x0000...0x7FFF.

If you use P2.7 (A15) by next way:

P2.7 ---> inverter ---> SV ---> /CS

then it comes to:
- CODE:

0x0000...0x2FFF - internal flash;
0x3000...0x7FFF - gap;
0x8000...0xFFFF - NVRAM contents 0x0000...0x7FFF.

- XDATA:

0x0000...0x7FFF - gap;
0x8000...0xFFFF - NVRAM contents 0x0000...0x7FFF.

"gap" means that read values are not defined, write has no effect.

As you may see by both ways, the contents of NVRAM is full-size allowed. The only thing you need to take care is the absolute addressing when you access to NVRAM. It is because by first scheme, NVRAM presents in two frames (address ranges).

Okay, now about
How can I control the CS???

For example, by P2.7(A15) as I suggested in second scheme. Or do not control it (first one).

Have you tried using an external memory as both data memory and code memory???

No, I have never used external program memory; my projects use external XDATA memory only (big flash).

Regards,
Oleg