Zafar,

In one recently project I had to use external flash rom.
I used an AT89s52 with 8K internal flash and protection bits.
From time to time( when the time was not critical) I jumped from external flash to the internal flash. Therefore, the external flash is not useful without the internal flash.
I asked to a good professional friend to disassemble the external code and try to reverse it.
He didn’t succeed.
Of course I don’t mean it’s a perfect solution but I think is good enough one

Cristian