Red Hat Knowledgebase

The default kernel in Red Hat Enterprise Linux 5 for the x86 32 bit architecture only allows a system to use up to 4 GB of memory. If a system has more than 4 GB of memory it needs to use kernel-PAE to see all the memory. This article is not relevant to x86_64 as that architecture can use more than 4 GB by default. Here is the description of kernel-PAE provided by yum:

Description:
This package includes a version of the Linux kernel with support for up to
64GB of high memory. It requires a CPU with Physical Address Extensions (PAE).
The non-PAE kernel can only address up to 4GB of memory.
Install the kernel-PAE package if your machine has more than 4GB of memory. 

 # yum install kernel-PAE

Open /boot/grub/grub.conf with a text editor like vi or gedit. Look at the default= line. It should indicate which kernel is set to be booted by default. Read from the top down. The first title line is the first entry, it would be 0. The second would be 1, the third 2 and so on. Make sure the PAE kernel is set to be booted by default. Once the PAE kernel is installed and the system is rebooted verify the PAE kernel is being used by checking the output of this command:

# uname -r

 Check to see if there is more than 4 GB of memory:

# free -m

Looking at the output in the total column it should be evident that there is more than 4 GB of memory if the system is using the kernel-PAE.