The operation of such a computer, once a program and some data have been loaded into RAM, is as follows. The first instruction is transferred from RAM into the control unit and interpreted by the hardware circuitry. For instance, suppose that the instruction is a string of bits that is the code for LOAD 10. This instruction loads the contents of memory location 10 into the ALU. The next instruction, say ADD 15, is fetched. The control unit then loads the contents of memory location 15 into the ALU and adds it to the number already there. Finally, the instruction STORE 20 would store the sum in location 20. At this level the operation of a computer is not much different from that of a pocket calculator. In general, of course, programs are not just lengthy sequences of LOAD, STORE, and arithmetic operations. Most importantly, computer languages include conditional instructions, essentially rules that say, "If memory location satisfies condition , do instruction number next, otherwise do instruction ." This allows the course of a program to be determined by the results of previous operations--a critically important ability.