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Why is usable RAM less than total RAM?

Why RAM and processor's capacity always display less than its actual capacity..Ex 512MB RAM shows 506MB..

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marked as duplicate by random Apr 9 '10 at 9:33

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3 Answers 3

Two answers/reasons:

  1. 512 MB showing 448MB This is because of the Accelerated Graphics Port (AGP) aperture, during system boot. This "memory hole" is a speed hack where the video card is no longer required to copy data into its framebuffer buffer, but can read it straight from system memory. You can change/lower this in the system bios, but only if you don't require performance on video graphics (includes 2D).

  2. Your system doesn't support the memory size (4GB showing 3372MB) Various 32-bit systems, like MS Windows XP/Vista are not capable to address 3,3 Gb and up because of a kernel hard coded limit. For this you require the 64 Bit version of the operating system.

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If you have an integrated grahics chip it's likely that it reserves a certain amount of RAM. This RAM is used as video RAM - your graphics chip needs its own memory.

Full featured Video cards have dedicated video RAM, but integrated chips use the system's ram to save energy and space (= money).

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You are probably using a laptop or a desktop with a graphics card on the motherboard. These use so-called 'shared memory', meaning that a bit of the systems main memory gets allocated for the GPU to use. The OS can't use that bit, so that's why it reports less memory than you would expect.

You can probably even edit the amount of memory your GPU can use in your BIOS.

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1  
Thats what I thought too at first, but 6MB? Probably some special BIOS/hardware registers mapped into the first 64MB or something. But I don't really know. –  StampedeXV Apr 9 '10 at 8:55

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