to update an old product developped in 1990 with ext 64k PROM 32k RAM ..., we have tried to use
P89C51RD2, AT89C51RD2 and finally P89V51RD2.
Our need is to use ext PROM for code and internal flash for datas.

With P89C51RD2and AT89C51RD2 it is possible to use IAP functions from external code because the EA pin is latched after the reset.
But the erase sectors are too wide for our application. we have a lot of different datas who have to be changed independently.

Soon, we tried with P89V51RD2.
It is very interresting for us because of the 128 byte erase sectors.

But we have discovered that BSEL is EA pin sensitive.
Calling 1FF0H with BSEL=0 and EA=0, program jump to 1FF0H ... in the PROM !

We have write in the ext Prom and in flash (outside the bootblock) the same small routine to call IAP.
with an external latch, we change the EA status during this routine.
It is possible to write in flash memory with this "solution" !!

But now, we are not able to use an emultor any more and we have to change our hardware.
Developpment becomes very difficult.

Does anyone know another solution with this µP, a register not in the datasheet, another microcontroller...