Zones

RAM is not one huge chunk of memory, nor is it a stream of memory split into any size.

RAM in Linux is split up into zones, followed by Pages#.

These definitions reflect how memory is managed on x86 architecture machines. They can differ widely on other architectures, such as ARM.

Direct Memory Access (DMA)

Lowest 16 MB range of your memory. That is, bytes at index \(0\) to \(16,777,216\) (\(= 2^{20}\) bytes)

Direct Memory Access 32 (DMA32)

There was a time when 32-bit was the new cool higher limit, compared to todays 64-bit.

32-bit allowed to reference memory up to 32 bits in key. This zone spans all the way up to \(4\) GB (\(= 4,294,967,296 = 2^{32}\) bytes)

Normal

  • 32-bit machines: RAM in range \(16\) MB to \(896\) MB

  • 64-bit machines: RAM in range \(4\) GB and beyond (or actually, to \(16\) EB, \(2^{64}\) bytes)

HighMem

Only a concept on 32-bit Linux machines. It would range from \(896\) MB and beyond, even past \(4\) GB if correctly set up.

References

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