I am informed that some new devices in 8051 family of microcontrollrs (87C51RX+ from iNTEL & Philips) have an
undocumented operating mode in which the CPU operates as "Bondout" IC. This mode is selected by the state of some pins at the time the Reset is disasserted. In this mode the CPU fetches instructions from external program memory by time-multipexing program addresses & data with port P0 & P2 SFRs data. (In normal operation with external program memory, the P0 SFR data are never applied to P0 pins)

Does anyone have more details ?