Memory - Physical Address Space (Physical Memory)

1 - About

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.

3 - Type

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

  • The flat model where the physical address space and linear address space are the same
  • The segmented model where the physical address space is split into segment creating a logical address space for each segment

4 - CPU

  • 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.

5 - 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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