Memory - Physical Address Space (Physical Memory)


The physical address space is the total number of uniquely-addressable physical address (memory locations) at a physical level (ie in the ram) and not logical (ie virtual)

This is the total processor’s physical address space and is linear.


There is two memory model where a memory address maps into the physical address space:


  • Intel 64 architecture supports physical address space greater than 64 GBytes;
  • The actual physical address size of IA-32 processors is implementation specific. In 64-bit mode, there is architectural support for 64-bit linear address space. However, processors supporting Intel 64 architecture may implement less than 64-bits (see Section The linear address space is mapped into the processor physical address space through the PAE paging mechanism.

Maximum Size

The maximum size of the physical memory is limited by:

Calculation Example:

CPU word size Address bus size Number of memory locations Memory Storage by location Addressable memory space
32 bit 32-bit <math>2^{32} = 4,294,967,296</math> 1 byte <math>4,294,967,296/1024/1024/1024 = 4</math> GB
64-bit (8 bytes) 64-bit <math>2^{64}</math> 1 byte <math>2^{64}/1024/1024/1024=17,179,869,184</math> Gb. It is a 11-digit number in Gb
8-bit 20-bit (e.g. Intel 8086) <math>2^{20} = 1,048,576</math> 1 byte 1 Mib
36-bit 18-bit <math>2^{18} = 262,144</math> 1 word <math>262,144 . 36 = 1,179,648 bytes = 1152 KB = 1,... MiB</math>

32-bit computers use some workarounds to be able to address more than 4Gb of Ram by adding:

  • extra registers
  • and extra bits into the addressing portion.

This, however, still does not allow for processes to be able to freely address more than 4Gb of RAM.

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