Memory shadowing

Memory shadowing

• Generally the execution of a program or a configuration from a Read only Memory(ROM)is very slow(120  to 200ns) compared to the execution from a random access memory(40 to 70ns)
• From the timing parameters,it is obvious that RAM access is about three times as fast as ROM access
• In computer systems and video systems,there will be a configuration holding ROM called Basic input output configuration ROM  or simpy BIOS
• In personal computer system, BIOS stores the hardware configuration information like the address assigned for various serial parts and other non-plug 'n' play devoces,etc
• Usually it is read and the system is configured accordingly to it during system boot up and it is time consumin
• Now,the manufacturers included a RAM  behind the logical layer of BIOS at its same address as a shdow to the BIOS ​,and the following steps happen: 

1. During the boot up, BIOS is copied to the shadowed RAM 
2. RAM is write protected
3. BIOS reading is disabled
4. Why both RAM and ROM are needed for holding the same data

• The answer is: RAM is volatile and it cannot hold the configuration data which is copied from the BIOS when the power supply is switched off
• Only a ROM can hold it permanently.But for high system performance,it should be accessed from a RAM instead of accessing from a ROM