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Shadow random access memory (RAM) is a type of memory stored within a RAM stick that has a duplicate version of the computer’s basic input-output operating system (BIOS) information. Aside from assuring that the BIOS is never erased, shadow RAM has an accelerated access speed so the computer can boot quicker. Some computers use BIOS for reasons other than booting, and this RAM typically helps with that. Many computers with shadow RAM, especially those that do not need it, enable the user to enable or disable the RAM on command. There are few problems associated with using this RAM but, if the computer does not need it, then using it will mean there is slightly less memory available to the computer.
When a computer boots, it goes to the read-only memory (ROM) to access the BIOS information. The problem with this is that ROM tends to be slow, because it is a larger memory repository, so booting from ROM can take a long time. With shadow RAM, the BIOS information is copied to and used from the RAM. RAM is about twice as quick as ROM, so this cuts boot time in half. If the user ever accidentally erases the BIOS, then this RAM also will have the information necessary to start the computer.
Most computers only need the BIOS for booting, but others also use it for normal functions. These systems are mostly obsolete or are certain command line interfaces. The shadow RAM holds a version of the BIOS and the access speed is higher, so this typically speeds the processes that require BIOS.
Some systems depend on shadow RAM and the user cannot change how the computer uses this RAM, but other computers use this as a convenience instead of a necessity. The latter systems typically allow users to disable and enable the RAM at any time. It usually is best to keep it on, unless users are doing intensive work.
While there are many advantages to having shadow RAM on, there also are some advantages to having it off. Part of the RAM is being used to hold the BIOS information, so this sector will be unavailable during processing. If someone is using intensive programs that need the entire RAM to properly function, then this can hold the computer back. Outside this scenario, having shadow RAM turned on generally is not a problem and most users should not notice the decreased RAM availability.
Yeah, it makes a great difference to have or not to have the 128KB in a DDR3 bank of 16GB. The more important question is "how can I load more than 128KB of option roms?"