The size or length of an instruction varies widely, from as little as 4-bits in some microcontrollers to many as multiples of a bytes in some very long instruction word (VLIW) systems. Most modern processors used in personal computers, mainframes, and supercomputers have instruction sizes between 16 and 64 bits. In some architectures, especially Reduced instruction set computers, instructions are of fixed length, typically corresponding with that architecture's word size.