Editing Microarchitecture (section)

=== Multicycle microarchitecture ===
Historically, the earliest computers were multicycle designs. The smallest, least-expensive computers often still use this technique. Multicycle architectures often use the least total number of logic elements and reasonable amounts of power. They can be designed to have deterministic timing and high reliability. In particular, they have no pipeline to stall when taking conditional branches or interrupts.  However, other microarchitectures often perform more instructions per unit time, using the same logic family.  When discussing "improved performance," an improvement is often relative to a multicycle design.

In a multicycle computer, the computer does the four steps in sequence, over several cycles of the clock. Some designs can perform the sequence in two clock cycles by completing successive stages on alternate clock edges, possibly with longer operations occurring outside the main cycle. For example, stage one on the rising edge of the first cycle, stage two on the falling edge of the first cycle, etc.

In the control logic, the combination of cycle counter, cycle state (high or low) and the bits of the instruction decode register determine exactly what each part of the computer should be doing. To design the control logic, one can create a table of bits describing the control signals to each part of the computer in each cycle of each instruction. Then, this logic table can be tested in a software simulation running test code.  If the logic table is placed in a memory and used to actually run a real computer, it is called a [[microprogram]]. In some computer designs, the logic table is optimized into the form of combinational logic made from logic gates, usually using a computer program that optimizes logic. Early computers used ad-hoc logic design for control until [[Maurice Wilkes]] invented this tabular approach and called it microprogramming.<ref name="microprogram">{{Cite journal | last1 = Wilkes | first1 = M. V. | title = The Growth of Interest in Microprogramming: A Literature Survey | doi = 10.1145/356551.356553 | journal = ACM Computing Surveys | volume = 1 | issue = 3 | pages = 139–145 | year = 1969 | s2cid = 10673679 | doi-access = free }}</ref>