Hi Kunal,

you are right. The switching routine must be resident in an address area, where the state of th EA pin was ignored.
Thus my bootloader checks, if this routine was already programmed into the Flash at 0F000h and if not it copies this routine to this locataion.
Then it checks, if a valid program header and a valid checksum was found on the Flash. If both checked successful, then it call the routine at 0F000h. This routine switch EA=0 and jump to 0000h of the Flash.

There are some other helping routines above 0F000h, e.g. to read from Flash or call the Flash if executed from the internal code space and vice versa.
And the adresses 0F000h, 0F003h until 0F0FFh are reserved for such API calls and all today unused adresses filled with "SETB C", "CLR A" and "RET"


Furthermore the lock bits can not be set to be able to change the EA pin after reset.

A similiar way can also be used on the Dallas devices on using the ROM size feature (no need to change the EA pin externally).



Peter