Bootloader for the 8051 MCU
After reset (either a hard reset or an instruction reset) the program counter returns to 00h. Whatever instruction is located there then gets loaded into the processor. 

This is dependent on what kind of I/O (or system architecture) you have attached to the processor. If you have a ROM attached (with the enable lines configured right) then it will load the instruction from 00h into the processor and start your program or OS on the ROM. 

With the At89s52 you need to program the ROM