4-Stage Pipeline — HDLbits.com

Scaling up the pipeline

This is structurally the same idea as the 2-Stage Pipeline problem, generalized to an arbitrary number of stages via a generate-friendly loop over a STAGES parameter (default 4). The point of building it this way — rather than hand-writing four named registers — is to make the scaling relationship between stage count, latency, and throughput completely explicit and parametric, which is exactly how real pipelined datapaths (multipliers, floating-point units, deep arithmetic chains) are structured.

Interface

Identical to the 2-Stage Pipeline problem, plus the STAGES parameter:

Signal	Direction	Description
`STAGES`	parameter	Number of pipeline register stages (this problem uses 4).
`clk`/`rst`	input	Clock and synchronous, active-high reset.
`in_valid`/`in_data`	input	New input, accepted every cycle.
`out_valid`/`out_data`	output	Output, valid exactly `STAGES` cycles after the corresponding input.

Latency vs. throughput, concretely

Design	Latency (cycles)	Throughput (results/cycle)
2-Stage Pipeline	2	1
4-Stage Pipeline	4	1

Doubling the stage count doubles the latency but does not change the throughput at all — a new input is still accepted every single cycle, and a result is still produced every single cycle (just a longer-delayed one). This is precisely why real high-frequency designs add pipeline stages: each additional register boundary shortens the combinational logic between flops (helping timing closure / higher clock frequency), at the cost of more cycles of latency before any single result appears — a cost that’s invisible to sustained throughput as long as the pipeline keeps being fed every cycle.

Correctness constraints

out_valid/out_data must reflect the in_valid/in_data from exactly STAGES cycles earlier, for any STAGES >= 1.
The pipeline must accept new input every cycle with no stalling.
The design must work correctly for STAGES values other than 4 (e.g. 1, 2, 8) without modification — this is what “generic” means for a parametric pipeline.