Hi,


First Conclusion...
The External Code memory(NVRAM) will be mapped above 12k - say 4000H to C000H (ie AT89S53 - 0000H to 2FFFH ). CS of External NVRAM is permanently grounded. and PSEN directly connected to RD of NVRAM.

That`s not good idea to connect /CS of NVRAM to ground permanently. I suggest you to use Power Supervisor which provides /CS pass-through feature. Otherwise you have a chance to lost data of NVRAM at power-up/down time. As alternative, you may use some Dallas NVRAM parts which have PowerSV built-in. But they are very expensive.

In the above mentioned condition, when the code is being executed from the external memory, if there is a instruction which says call a function from location 0100H(Internal memory), will the control go to internal ROM execute it and then again come back to external ROM.

Yes it will do this way with some exceptions because there is one thing you must take attention on. I talk about Lock Bits protection. So:
- in Protection Mode 2 and above it will not be possible to read internal memory by MOVC instruction if program executes from external memory.
- In Protection Mode 4 it will not be possible to run program from external memory in the whole.
So be careful when set these bits.


Second Conclusion...
2> Depending on certain condition(a port pin P1.0 is held low), I start rewriting the same external NVRAM - but this time I will and the PSEN AND RD, the output of the AND gate will be given to RD of NVRAM.

Raj, make things more simple. It is not needed to do /PSEN AND /RD with extra-gate. As your NVRAM will be connected at 4000H to C000H so:
- connect /WR of MCU to /WR of NVRAM;
- /PSEN to /OE of NVRAM.
- do not connect /RD.
To Write NVRAM use MOVX (write to external data memory);
To Read NVRAM use MOVC (read from external code memory).

Regards,
Oleg