4:2 Carry-Save Compressor

Verilog problems / Combinational Design

What you must build

Reduce five weight-1 inputs (four data bits plus a carry-in) down to a sum bit and two weight-2 outputs — the building block that makes fast multiplier reduction trees possible.

Engineers use “4:2 Carry-Save Compressor” as a building block in combinational design. Interviewers ask for the same ports and the same corner cases this judge covers. Completing it in the browser is the same skill as writing synthesizable RTL at work, minus the EDA license.

Port contract

The judge instantiates exactly these ports. Extra ports or a different module name fail to elaborate.

How to approach this kata

This is a hard kata. Sketch the state bits and the illegal overlaps (full/empty, wrap, simultaneous enable) on paper first. A design that “usually works” in your head will fail a directed corner in the hidden tests.

Hidden tests instantiate top_module, drive the ports, and compare every sample against a golden model. They do not grade coding style. They do grade X/Z, off-by-one counters, and ignoring enables. Sign in only when you want the run saved on the leaderboard — the specification below is public.

Starter shape

Copy this skeleton into the editor (or press Reset starter). Fill the body; do not rename the module.

module top_module(
  input  a,
  input  b,
  input  c,
  input  d,
  input  cin,
  output sum,
  output carry,
  output cout
);

  // Your code here — total = a+b+c+d+cin (0..5); sum=total[0], carry=total[1], cout=total[2].

Why this shows up in interviews

4:2 Carry-Save Compressor sits under Combinational Design (combinational, arithmetic). Concept: A 4:2 compressor doesn't need to be thought of gate-by-gate — it just needs to preserve total value: a+b+c+d+cin = sum + 2×(carry+cout) . Count the total number of 1s among the five inputs (0-5); the low bit of that count is sum , and the upper two bits split across carry and cout .

A passing solution is synthesizable intent: no delays in the DUT, no initial blocks inside top_module, and no reference to testbench tasks. Use blocking assignments only in combinational always blocks; use non-blocking for registers clocked by clk.

Related problems

• 2-to-1 Multiplexer — Select between two 1-bit inputs using sel. When sel=0 output a; when sel=1 output b.
• 1-Bit Full Adder — Compute sum and carry-out for a + b + cin.
• 4-to-1 Multiplexer — Select one of four 1-bit inputs using a 2-bit sel. sel=00→in0, 01→in1, 10→in2, 11→in3.
• 4-to-2 Priority Encoder — Output the index of the highest-priority (MSB-most) set bit in a 4-bit input, plus a valid flag when any bit is set.

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