I have an old mini-PC (Acer R3610 Revo) that I've repurposed as a general server (docker containers mostly). It has 4GB of physical RAM installed (specs and BIOS show it) but only ~3GB is accessible to Ubuntu. It's running Ubuntu 18.04.
$ lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description: Ubuntu 18.04.6 LTS
Release: 18.04
Codename: bionic
After some reading it looked like it could be due to PAE (or lack there of) but it's running the 64-bit version and to my knowledge that should mean PAE is installed by default.
$ uname -a
Linux master_control 4.15.0-193-generic #204-Ubuntu SMP Fri Aug 26 19:20:21 UTC 2022 x86_64 x86_64 x86_64 GNU/Linux
However when I check the kernel there's no mention of PAE like a lot of the online resources say there should be.
$ uname -r
4.15.0-193-generic
Out of curiosity I attempted to install the pad kernel anyway but it was unable to find the package.
$ sudo apt-get install linux-generic-pae linux-headers-generic-pae
[sudo] password for master:
Reading package lists... Done
Building dependency tree
Reading state information... Done
E: Unable to locate package linux-generic-pae
E: Unable to locate package linux-headers-generic-pae
I did some quick googling and gave up pretty early since I was unsure if that's even the correct route to take under the circumstances.
I thought maybe some of the memory is hardware reserved and went into the BIOS and switched the video memory setting from Auto (was 256MB) to 64MB (the lowest I can go and still get display output). This helped a little taking it from about 2.93GB available in Ubuntu to 3.11GB (according to htop and other output).
$ free -h
total used free shared buff/cache available
Mem: 3.1G 1.3G 368M 3.4M 1.4G 1.7G
Swap: 2.9G 0B 2.9G
There is this discussion that is quite similar but 1GB seems excessive to be used by... I'm not sure what if the APU has been limited to just 64MB 64bit system sees only 3GB of 4GB RAM
Curious if I'm able to free up the last ~1GB or so for Ubuntu or if it's likely hardware reserved or otherwise just not accessible at all.