A serial computer is a computer typified by bit-serial architecture – i.e., internally operating on one bit or digit for each clock cycle. Machines with serial main storage devices such as acoustic or magnetostrictive delay lines and rotating magnetic devices were usually serial computers. Serial computers require much less hardware than their parallel computing counterpart, but are much slower. There are modern variants of the serial computer available as a soft microprocessor which can serve niche purposes where size of the CPU is the main constraint.