Memory mapping in the 8085 microprocessor
When the 8085 starts up, it will start fetching instructions from address zero.  It is thus necessary that the memory which is located there contain defined contents.  That does not imply, however, that address zero must be mapped permanently to ROM.  One could build a system with e.g. 2K of ROM, 64K of RAM, and a floppy drive, and use an I/O bit to control whether memory reads come from ROM or RAM.  On startup, have all reads come from ROM (writes go to RAM regardless of the control bit), and have the ROM program copy itself to RAM.  Then enable reads from RAM, and load software from the floppy.  Since the bottom portion of the address space would be RAM, software loaded from floppy would be able to set up interrupt vectors for its own purposes--something that would not be possible if ROM were still mapped there.