RAM memory addressing

How much RAM memory can a Linux server address (RHEL, CentOS, Fedora), and what determines this :
1. Processor
2. Motherboard
3. OS
And the lowes values of the three determines the limit 🙁

32-bit machine :
The Linux kernel includes full PAE mode (Physical Address Extension) support starting with version 2.3.23 enabling access of up to 64 GB of memory on 32-bit machines. A PAE-enabled Linux kernel requires that the CPU also support PAE. The Linux kernel supports PAE as a build option and major distributions provide a PAE kernel either as the default or as an option. As of 2009, some common Linux distributions have started to use a PAE-enabled kernel as the distribution-specific default. As of 2012, common Linux distributions have stopped distributing non-PAE kernels, thus making PAE hardware mandatory – examples being Red Hat Enterprise Linux / CentOS from version 6.0, Ubuntu (and derivatives like Linux Mint) from 12.10. Fedora and Debian still distribute both PAE and non-PAE kernels.
If you do not have a PAE kernel:
The maximum physical RAM for the kernel plus all processes is BIOS dependent, typically limited to about 3.25GB.
Swap space can be used to bring the effective usable memory well above 4GB.
Each process is limited to 3GB virtual, and the kernel is limited to 1GB virtual, so total memory use over 4GB is only meaningful for multiple processes.
The 32bit memory mapping system supports either 4GB of address space without PAE or 64GB with PAE. But that does not necessarily tell you how much Linux supports with/without PAE.
If you have a motherboard that supports a maximum of 4GB RAM, it does not matter if you use 32 Bit, 32 Bit PAE or 64 Bit since you will have a hardware limit of 4GB.
64-bit machine :
2^64, so definitely, no problem there 😉
And at the end, recommendations :
If you have 512 MB of RAM or less it is highly recommended to use 32 Bit
If you have less than 4 GB of RAM but more than 512 MB it is recommended to use 32 Bit (But you can still use 64 Bit)
If you have 4 GB of RAM it is recommended to use 32 Bit (But you can still use 64 Bit)
If you have between 4 GB or RAM and 64 GB of RAM you can use either 32 Bit or 64 Bit, but it is recommended to use 64 Bit.
If you have more than 64 GB of RAM you need to use 64 Bit

See this link.

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