NI Phar Lap ETS (sometimes abbreviated as PharLap), is a 32-bit operating system and can therefore only address 4 GB of RAM. Of this 4 GB, the full amount will not be available to OS, as some is reserved for other components (e.g. video card, device enumeration, other motherboard requirements). The expected amount is a little less than 3 GB of RAM available to the real-time OS.
Consider changing to NI Linux Real-Time
if you need to use more than 4GB of RAM on NI PXI controllers. NI Linux Real-Time is a 64-bit operating system hence, the maximum memory supported by the controller with NI Linux Real-Time is limited by the controller's hardware. Kindly refer to the user manual or datasheet of the controller to know what is the maximum memory supported by your controller.
NI will remove support for Phar Lap for cRIO in the NI 2020 Software Release and for PXI in the NI 2022 Software Release. For more information, please see the Phar Lap RT OS EOL Road Map