Abstract: A source program is executed on microcontroller core 114 of a processing unit 100. The core 114 is capable of native instructions from a predetermined set of micro-controller specific instructions. In a pre-processing step, for the program statements of the source program a program-specific virtual machine is defined with a corresponding set of virtual machine instructions, such that the expression of the program statements in the sequence of instructions requires less storage space compared to using only native instructions. For the program-specific virtual machine an associated conversion means 132 is defined for converting the program-specific virtual machine instructions into the native instructions of the core 114. The source program statements are expressed in a sequence of instructions comprising instructions of the defined virtual machine. The sequence of instructions is stored in an instruction memory 120. The conversion means 114 is represented in the processing unit 100.