For example, a system with separate instruction and data memory banks can allow external devices one memory access to the data bank while the CPU was fetching an instruction from the instruction bank if both accesses are initiated simultaneously. A memory management unit is not essential, for example, the Zilog Z80's "M1" line can be used to distinguish instruction from data access, so while the CPU is reading an instruction from instruction-RAM or ROM, the data RAM is available to other devices without interfering with CPU processing.