Definition
Serial Functional Unit
A serial functional unit (or unpipelined functional unit) is a functional unit that can process only one operation at a time. An operation occupies the entire unit for its full latency, and no new operation can enter until the current one completes.
A serial unit has throughput operations per cycle, where is the latency. A second operation issued before the first completes causes a structural hazard.
Example
Integer divider
Integer division is typically implemented as a serial unit. The algorithm (non-restoring, SRT, or iterative shift-and-subtract) takes 8–32 cycles, and the unit cannot accept another division until the current one finishes. The divider is a common bottleneck in integer-heavy workloads.