From the T89C51RC2 datasheet...

Hardware Activation of the
Boot Loader
The default boot loader can also be executed by holding PSEN LOW, EA HIGH, and
ALE HIGH (or not connected) at the falling edge of RESET. This allows an application to
be built that will normally execute the end user’s code but can be manually forced into
default ISP operation.

*****As PSEN has the same structure as P1-P3, the current to force PSEN to 0 as I TL is
defined in the DC parameters.*****


User application should take care to release hardware conditions (PSEN LOW, EA
HIGH) 24 clock cycles after falling edge of reset signal.