When running in a 64-bit environment, ArcGIS for Desktop, ArcGIS Engine, and ArcReader run as a 32-bit application. See Does Esri support 64-bit processors with ArcGIS 10.1 products?
When running in a 64-bit environment, ArcGIS for Desktop, ArcGIS Engine, and ArcReader run as a 32-bit application. See Does Esri support 64-bit processors with ArcGIS 10.1 products?