A method of executing loops in a computer system is described. The computer system has a sequence of instructions held in program memory and a prefetch buffer which holds instructions fetched from the memory ready for supply to a decoder of the computer system. If the size of the loop to be executed is such that it can by holly contained within the prefetch buffer, this is detected and a lock is put on the prefetch buffer to retain the loop within it while the loop is executed a requisite number of times. This thus allows power to be saved and reduces the overhead on the memory access buffers. According to another aspect, loops can be “skipped” by holding a value of zero in the loop counter register.