1. A portable apparatus ( 100 ), comprising: at least one processing unit ( 500 , 530 ) and at least one memory including a computer program code, wherein the at least one memory and the computer program code are configured, with the at least one processing unit ( 500 , 530 ), to cause the portable apparatus ( 100 ) to perform operations comprising: supporting a normal operation mode ( 210 ) during which functionalities of the apparatus ( 100 ) are available through an operating system ( 300 ) of the apparatus ( 100 ), wherein the operating system ( 300 ) comprises a plurality of layers including a kernel ( 302 ) and library functions-layer; supporting a limited operation mode ( 212 ) during which the apparatus ( 100 ) is configured to execute a physical activity algorithm ( 220 ) based on physical activity data corresponding to a physical activity session performed by a user ( 110 ) of the apparatus ( 100 ), wherein the physical activity algorithm ( 220 ) applies a direct low-level hardware access bypassing at least the layers above the kernel ( 302 ) and the library functions-layer ( 304 ); and switching between the normal operation mode ( 210 ) and the limited operation mode ( 212 ).