IN 8051, to use internal ROM we use EA=1, and after the last address ofijternal ROM , microcontroller automatically goes to external memory for next address.
Now we know in 8051, Internal ROM=4Kb, then it means after 0000H to 0fffH when it reaches to 1000H, it automatically takes code from external ROM, then from which memory location in external location our code starts, from 0000H or 1000H