I had an old PC with Arch Linux
$ uname -a Linux desktop 4.1.4-1-ARCH #1 SMP PREEMPT Mon Aug 3 21:53:10 UTC 2015 i686 GNU/Linux
with 2GB RAM. I've added 2x1Gb DDR2 DIMMs and now the system has DIMMs in all banks (4x1GB). The new memory is detected in the BIOS:
System Memory Available 4024 MB
And also the output of lshw shows all the slots populated with 1GB DIMM
But the Total Memory in /proc/meminfo shows only 2GB:
$ grep MemTotal /proc/meminfo MemTotal: 2058968 kB
I've tried to edit the kernel params in GRUB and add mem=4G, but the total memory remains 2GB.
Any ideas what else to try?