I just tried this. I have an 89C51RD2 where I have messed up the boot verctor. I have it plugged into a New Micros Inc board where EA is true and code runs from external ROM and RAM. The NMI monitor runs fine, I can load code into RAM is it runs OK. I wrote a small program to test your theory i.e.

org 8000H

BOOTROM EQU 0FC00H

orl 0A2H,#020H
nop
nop
nop
ljmp BOOTROM
end 

I ran it and I get no response to commands from the terminal. So I modified the code as follows:

org 8000H

START EQU 0000H

orl 0A2H,#020H
nop
nop
nop
ljmp START
end 

This changes the boot flag then restarts the monitor. So I dumped 20H bytes at FC00 before running the program and got:

FC00: 00 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F etc

After running the second prog I dumped from FC00 again and got:

FC00: 75 89 02 75 C8 30 E4 F5 CD F5 CC 30 B0 FD 20 B0 etc

Which is clearly different but if I then run from FC00 I get no response from the terminal. I think we are on the right track though.

Ian