1. A portable apparatus, comprising: at least one processing unit 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, to cause the portable apparatus to perform operations comprising: supporting a normal operation mode during which functionalities of the apparatus are available through an operating system of the apparatus, wherein the operating system comprises a plurality of layers including a kernel and library functions -layer; supporting a limited operation mode during which the apparatus is configured to execute a physical activity algorithm based on physical activity data corresponding to a physical activity session performed by a user of the apparatus, wherein the physical activity algorithm applies a direct low-level hardware access bypassing at least the layers above the kernel and the library functions -layer; and switching between the normal operation mode and the limited operation mode, wherein the at least one processing unit comprises: a low power processing unit configured to run at least the physical activity algorithm during the limited operation mode; and a high power processing unit configured to run the operating system of the apparatus during the normal operation mode, wherein the low power processing unit has a lower power consumption in performing a given task than the high power processing unit, and wherein a switch to the limited operation mode comprises: activating the low power processing unit to run at least the physical activity algorithm; and inactivating the high power processing unit.