Editing Control unit (section)

== Multicycle control units ==

The simplest computers use a [[multicycle microarchitecture]]. These were the earliest designs. They are still popular in the very smallest computers, such as the embedded systems that operate machinery.

In a  computer, the control unit often steps through the [[instruction cycle]] successively. This consists of fetching the instruction, fetching the operands, decoding the instruction, executing the instruction, and then writing the results back to memory. When the next instruction is placed in the control unit, it changes the behavior of the control unit to complete the instruction correctly. So, the bits of the instruction directly control the control unit, which in turn controls the computer.

The control unit may include a binary counter to tell the control unit's logic what step it should do.

Multicycle control units typically use both the rising and falling edges of their square-wave timing clock. They operate a step of their operation on each edge of the timing clock, so that a four-step operation completes in two clock cycles. This doubles the speed of the computer, given the same logic family.

Many computers have two different types of unexpected events. An [[interrupt]] occurs because some type of input or output needs software attention in order to operate correctly. An [[exception handling|exception]] is caused by the computer's operation. One crucial difference is that the timing of an interrupt cannot be predicted. Another is that some exceptions (e.g. a memory-not-available exception) can be caused by an instruction that needs to be restarted.

Control units can be designed to handle interrupts in one of two typical ways. If a quick response is most important, a control unit is designed to abandon work to handle the interrupt. In this case, the work in process will be restarted after the last completed instruction. If the computer is to be very inexpensive, very simple, very reliable, or to get more work done, the control unit will finish the work in process before handling the interrupt. Finishing the work is inexpensive, because it needs no register to record the last finished instruction. It is simple and reliable because it has the fewest states. It also wastes the least amount of work.

Exceptions can be made to operate like interrupts in very simple computers. If [[virtual memory]] is required, then a memory-not-available exception must retry the failing instruction.

It is common for multicycle computers to use more cycles. Sometimes it takes longer to take a conditional jump, because the program counter has to be reloaded. Sometimes they do multiplication or division instructions by a process, something like binary long multiplication and division. Very small computers might do arithmetic, one or a few bits at a time. Some other computers have very complex instructions that take many steps.