The arithmetic/logic unit is the component of the CPU where data is held temporarily and where calculations take place. The control unit controls and interprets the execution of instructions. Most of these actions are retrievals of instructions from memory followed by movements of data or addresses from one part of the CPU to another. The control unit determines the particular instruction to be executed by reading the contents of a program counter (PC), sometimes called an instruction pointer, which is a part of the control unit. The program counter contains the address of the current instruction or the next instruction to be executed. Normally, instructions are executed sequentially. The sequence of instructions is modified by executing instructions that change the contents of the program counter. A memory management unit within the control unit supervises the fetching of instructions and data from memory. The I/O interface is also part of the control unit. In some CPUs, these two functions are combined into a single bus interface unit.