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The Magic Of Shadow Ram
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To start from,shadowing is a word many of us have come across while tweaking with BIOS settings.In this article I will explain what actually is shadowing and what effect it has on our system.
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| Importance of ROM |
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We all know that ROM's are of great help to a computer.They support the non-volatile state of the system.For example the BIOS ROM is the place where the POST code,the BIOS settings and the infamous ROUTINES(interrupt) reside.BIOS performs the POST during boot up.Now from here all the operating system's whose code I have read uses one or another BIOS routine.For example the LINUX uses the INT 13h to load the boot code.So it is pretty evident that codes residing in BIOS is of atmost importance.But the saddest part is that ROM's have lower speeds when compared to RAM's.Here is where the need for shadowing arises.
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| What is it really ? |
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Shadowing,as the word meaning denotes presence of a thing having some duplicate form.The so called shadow will not have all the properties of the original.This is also case of SHADOW RAM.First without shadowing ROM is accessed via memory mapping.Memory mapping is that from the memory space of processor some area is reserved for ROM.For example suppose (very wildly) we map our Systems memory from 0- 5Kb for ROM access.So when we give the processor command to access the area at 1Kb the system would automatily access the ROM code.So is the case anywhere between 0 and 5Kb.Also the RAM over this area would be wasted.In shadowing the code in the ROM would be copied to the RAM.If we consider the above case,the code of ROM area would be copied to RAM at 0 - 5Kb.So when access to 1Kb is made actual access is made to the RAM not to the ROM.But the code executed would be same as it is the copy of what is in ROM.
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| Advantages of Shadowing |
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The increase in speed is obvious.The code when executed from ROM takes longer time as the speed of ROM access is greater.When the code has been shadowed to RAM the speed of execution naturally increases as access of RAM code takes lesser time.Also RAM,in absence of shadowing may have been wasted is now utilized to increase efficiency.
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