computer:memory:physical_address

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.

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

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 3.3.7.1). The linear address space is mapped into the processor physical address space through the PAE paging mechanism.

The maximum size of the physical memory is limited by:

the width of the address bus
the unit of storage of an address. ie if an address store:
- a byte (most common)
- or a word

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:

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