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