Today, I upgraded the memory on my Macbook Pro (2010) from 4 GB to 8 GB. One of the main reasons for the memory upgrade was to be able to run Mircosoft Windows 7 virtual machine using Oracle VirtualBox. However, I noticed that VirtualBox was not able to see more than 4 GB of memory even after the upgrade while the system reported that there was 8 GB of installed memory.

A quick research showed that Mac OS X 10.6 (Snow Leopard) uses 32-bit kernel by default. This limits the applications to use only 4 GB of memory. In order for the applications to use larger memories, one need to use the 64-bit kernel (provided that you are on a 64-bit platform). There is an Apple support page that describes how to select the desired kernel.

I changed the defaults to use 64-bit kernel.

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$ sudo systemsetup -setkernelbootarchitecture x86_64

This configuration is stored in the file /Library/Preferences/SystemConfiguration/com.apple.Boot.plist

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<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
  <key>Kernel Architecture</key>
  <string>x86_64</string>
  <key>Kernel Flags</key>
  <string></string>
</dict>
</plist>

After making this change and rebooting, VirtualBox was able to see the whole memory.