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Why Does RAM Have A Few Extra MB?

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November 14, 2012 6:06:38 AM

For example why is 2GB 2048?
At 64GB it would be 65.536GB which is 65GB not 64GB.

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November 14, 2012 7:52:22 AM

Computers think in hexadecimal (or binary), unlike humans that think in decimal. In hex 1K is 1024 bits.
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November 14, 2012 10:37:51 AM

1 byte = 8 bits
1 kilobyte = 1,024 bytes
1 megabyte = 1,024 kilobytes
1 gigabyte = 1,024 megabytes

So although you refer to your RAM as being 8 gigabytes, it is also 8,192 megabytes (8 x 1,024).
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November 14, 2012 11:59:39 AM

The kilo/mega/giga/etc. prefixes are defined as powers of 1000 in the SI system.

The application of SI prefixes to powers-of-1024 is somewhat of an abuse but is tolerated in most contexts as a close-enough approximation.

However, HDD manufacturers and some other companies have been sued for using the proper metric definition at a time where most people thought the stretched definitions were universal in computer storage. That is when HDD labels started including disclosures such as "1MB = 1 000 000 bytes"

The reason why HDDs use the proper metric definition while most solid-state memory uses the 'stretched' definition is because HDD sizes have incremental capacity measured in sectors of either 512 bytes or 4096 bytes while integrated circuits use binary row/column addresses and each extra address bit doubles addressable capacity so chip capacities practically always increase by doubles since it is the most natural way.

To disambiguate this, the Ki, Mi, Gi, etc. prefixes were introduced several years ago as the proper power-of-1024 prefixes so the proper name for a 4GB DIMM should be a 4GiB DIMM.
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November 14, 2012 12:30:46 PM

yeah the proper term for 1024*1024mb would be 1 gibibyte while gigabyte should have refered to 1000*1000. Its all to do with base 2 calculations
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