Each CPU has four heads associated with it. The first is the instruction pointer (IP) which determines the next instruction to be executed by the CPU. After each execution, the IP is automaticall advanced (unless the instruction executed dictates otherwise). The second is the read-head (RH), which determines what instruction is copied from a copy command, and then the write-head (WH) determines the position it is copied to. Finally, the flow-head (FH) is used to mark positions in the genome to move the other heads to. The flow-head is commonly used to mark the beginning of a loop to jump the IP back to from the loop's end.