SyncValsverifier → artifact → classifier → verdict
SyncVals · Trajectory

Enable-gated streaming fold stage

codex gpt-5.5 ✓ resolved HARNESS_ERROR ↑ View task
Solved from the instruction alone, tests/ and solution/ were withheld from the agent's workspace and restored only for grading.
Reward = tests/test.sh exit code (0 → resolved); the classification below is post-hoc and cannot change it.
Classification , post-hoc; cannot change the reward
HARNESS_ERRORInfrastructure failure, the agent never ran properly. Not a signal about agent or task.
SubtypeClassification Failed
EvidenceClaude Code classification failed: (classifier unavailable for this trial)
Root causeCould not analyze trial with Claude Code
RecommendationReview trial manually or check authentication
Trajectory
Tool-by-tool agent trajectory
35 tool calls · 2 tool types · 59 steps
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# Enable-gated streaming fold stage ## Context We need the streaming arithmetic stage `fold_stage` for a back-pressured signal path. Upstream logic hands us one signed 12-bit sample at a time, but only on the cycles it raises an accept strobe; in between, the whole stage has to sit still and hold what it has, because the cycles where `en` is low carry nothing. The one thing that makes this stage fiddly is that a single internal step position drives *three* things at once , whether we are loading or combining, what we feed back into the delay line, and when the output goes live , so all of it has to advance together, one notch per accepted sample, or the stream desynchronises. The package and the wrapper are in place; only `rtl/fold_stage.sv` is a stub. Implement it. ## Interface ``` input clk // single clock input rst_n // asynchronous, active low input en // accept strobe input signed [11:0] din // input sample (two's complement) output signed [11:0] dout // output sample (two's complement) output out_valid // output-live flag ``` ## What "accepted" means A sample is **accepted** on a rising edge of `clk` where `en` is high. On an accepted edge every piece of state moves forward exactly one step. On any rising edge where `en` is low, nothing moves: the delay line, the step position, `dout` and `out_valid` all hold their current values unchanged. `din` is meaningless whenever `en` is low. ## Behaviour The stage holds the four most recently inserted values as a shift register; call the oldest of those four `H`. It walks a repeating cycle of eight accepted samples: the first four are **load** steps, the next four are **combine** steps, then it repeats. - On a **load** step, `dout` is `H`, and `din` is inserted as the newest value (the shift drops what used to be `H`). - On a **combine** step, `dout` is the round-half-up half of `H + din`, and the value inserted into the line is `H - din` **saturated** to the signed 12-bit range , `din` itself is *not* inserted on a combine step. Round-half-up means: add one to the sum, then arithmetic-shift-right by one (`(H + din + 1) >>> 1`). It is not round-to-nearest-even, not round-to-nearest, and not truncation toward zero or toward minus infinity. Saturation means: if `H - din` exceeds `+2047` use `+2047`, if it is below `-2048` use `-2048`, otherwise use it unchanged. `out_valid` is **0 on the first four accepted samples after reset and 1 from the fifth accepted sample onward** (count accepted samples, not clock cycles). It is a level, not a pulse: once high it stays high on every accepted sample, and like every other output it holds its value across `en`-low cycles. ## Conventions `dout` and `out_valid` are **registered**: they take their new value on the accepted rising edge and are observable from just after that edge until the next accepted edge. The result of an accepted sample lands on `dout` on that sample's **own** accepted edge , a single registered stage, no extra pipeline delay , and a combine output uses the `din` present on that same edge. `rst_n` is **asynchronous** and **active low**: while it is low the four-deep line is cleared to zero, the step position returns to the first load step, and `dout` and `out_valid` are forced to 0, immediately on the falling edge of `rst_n` rather than waiting for a clock, and reset wins over `en`. The step position holds through `en`-low gaps, so a gap does not change which samples pair up. For an accepted output on edge `E`, with two idle cycles after it: ``` edge E E+1 E+2 E+3 en 1 0 0 1 dout A A A B out_valid 1 1 1 1 ``` ## Submission Write your implementation into `rtl/fold_stage.sv`, keeping the module name, ports and parameters exactly as given. It must elaborate under Icarus Verilog 11 (`iverilog -g2012`); note that on that tool a bit- or part-select of an *expression* (a sum, a shift, a function result) is a syntax error, so do any such bit work inside a `function automatic` on a named value. The grader compiles your stage with a held-back checker and compares `dout` and `out_valid` cycle by cycle against a reference over directed and random stimulus.
I’ll inspect the existing RTL and package/wrapper shape first, then patch only `rtl/fold_stage.sv` and run the available compile/tests if present.
shell
rg --files
output
Chunk ID: f35cd0
Wall time: 0.0000 seconds
Process exited with code 0
Original token count: 20
Output:
fold_core/rtl/fold_top.sv
fold_core/rtl/fold_stage.sv
fold_core/rtl/fold_pkg.sv
shell
git status --short
output
Chunk ID: dd06cc
Wall time: 0.0000 seconds
Process exited with code 127
Original token count: 11
Output:
/bin/bash: line 1: git: command not found
The workspace is very small: just the package, wrapper, and stub under `fold_core/rtl`. `git` is not installed here, so I’ll track changes by reading the files directly.
shell
sed -n '1,220p' fold_core/rtl/fold_stage.sv
output
Chunk ID: 67bfc3
Wall time: 0.0000 seconds
Process exited with code 0
Original token count: 706
Output:
//==============================================================================
// fold_stage.sv
//
// Enable-gated streaming fold stage.
//
// ---------------------------------------------------------------------------
//  >>> IMPLEMENTATION REQUIRED <<<
//
//  The module ports, parameters and the state registers are provided. The
//  datapath, the phase/priming control and the outputs are left as a TODO and
//  currently drive constant zeros, so the stage produces no useful stream and
//  the verifier FAILS.
//
//  Implement the behaviour described in instruction.md. Do NOT change the
//  module name, the port list, or the parameter list.
//
//  iverilog 11 note: a bit/part-select of an EXPRESSION (a sum, a shift, a
//  function-call result) is a syntax error under `iverilog -g2012`. Index only
//  named signals, and do any bit work that needs an intermediate value inside a
//  `function automatic` using named locals.
// ---------------------------------------------------------------------------
//==============================================================================
`timescale 1ns/1ps

module fold_stage #(
    parameter DW    = 12,
    parameter DEPTH = 4,
    parameter NPH   = 8
) (
    input  logic                     clk,
    input  logic                     rst_n,
    input  logic                     en,
    input  logic signed [DW-1:0]     din,
    output logic signed [DW-1:0]     dout,
    output logic                     out_valid
);

  // ---------------------------------------------------------------------------
  // State registers (provided). The head of the feedback line is line[DEPTH-1].
  // ---------------------------------------------------------------------------
  logic signed [DW-1:0] line [0:DEPTH-1];
  logic         [2:0]   phase;
  logic         [2:0]   prime;
  integer               k;

  // ===========================================================================
  // TODO: IMPLEMENT THE STAGE.
  //   * advance every register exactly one step on a rising clk edge with en==1,
  //     and hold all of it when en==0;
  //   * asynchronous active-low reset clears state and outputs;
  //   * drive dout / out_valid per the contract in instruction.md.
  //
  // The placeholder below keeps the module compilable but INCORRECT.
  // ===========================================================================
  always_ff @(posedge clk or negedge rst_n) begin
    if (!rst_n) begin
      for (k = 0; k < DEPTH; k = k + 1) line[k] <= '0;
      phase <= 3'd0;
      prime <= 3'd0;
    end else begin
      // TODO: replace with the real datapath + control.
      phase <= phase;
      prime <= prime;
    end
  end

  // TODO: replace these placeholder outputs with the real ones.
  assign dout      = '0;
  assign out_valid = 1'b0;

endmodule : fold_stage
shell
sed -n '1,220p' fold_core/rtl/fold_top.sv
output
Chunk ID: 8b4fe4
Wall time: 0.0000 seconds
Process exited with code 0
Original token count: 292
Output:
//==============================================================================
// fold_top.sv
//
// Thin integration wrapper. The verification environment drives this wrapper so
// that the instantiation wiring is exercised alongside the stage itself. It
// instantiates fold_stage with the project-default geometry from fold_pkg.
//
// DO NOT change this file.
//==============================================================================
`timescale 1ns/1ps
`include "fold_pkg.sv"

module fold_top (
    input  logic                             clk,
    input  logic                             rst_n,
    input  logic                             en,
    input  logic signed [fold_pkg::DW-1:0]   din,
    output logic signed [fold_pkg::DW-1:0]   dout,
    output logic                             out_valid
);

  // Bare instantiation: the stage carries its own default geometry, so a
  // submission needs no parameter override to elaborate correctly.
  fold_stage u_fold_stage (
      .clk       (clk),
      .rst_n     (rst_n),
      .en        (en),
      .din       (din),
      .dout      (dout),
      .out_valid (out_valid)
  );

endmodule : fold_top
shell
sed -n '1,220p' fold_core/rtl/fold_pkg.sv
output
Chunk ID: ab0260
Wall time: 0.0000 seconds
Process exited with code 0
Original token count: 227
Output:
//==============================================================================
// fold_pkg.sv
//
// Shared geometry for the enable-gated streaming fold stage. The block carries a
// DEPTH-deep signed feedback line, an eight-step phase counter, and a small
// priming counter. Parameters are declared without an explicit data type for
// broad simulator compatibility (Icarus Verilog 11); they default to integer
// constants, which is sufficient here.
//
// DO NOT change this file.
//==============================================================================
`ifndef FOLD_PKG_SV
`define FOLD_PKG_SV

package fold_pkg;

  localparam DW    = 12;   // sample word width (signed two's complement)
  localparam DEPTH = 4;    // feedback line depth (= load/combine window length)
  localparam NPH   = 8;    // phase counter span: 0..3 load window, 4..7 combine

endpackage : fold_pkg

`endif // FOLD_PKG_SV
I found the stub and the wrapper expects the stage defaults directly. I’m going to make `dout` and `out_valid` true registers in the same enable-gated state block, with small helper functions for the rounded average and saturated subtraction so Icarus does not trip over expression selects.
apply_patch
*** Begin Patch
*** Update File: /work/fold_core/rtl/fold_stage.sv
@@
   logic signed [DW-1:0] line [0:DEPTH-1];
   logic         [2:0]   phase;
   logic         [2:0]   prime;
   integer               k;
 
-  // ===========================================================================
-  // TODO: IMPLEMENT THE STAGE.
-  //   * advance every register exactly one step on a rising clk edge with en==1,
-  //     and hold all of it when en==0;
-  //   * asynchronous active-low reset clears state and outputs;
-  //   * drive dout / out_valid per the contract in instruction.md.
-  //
-  // The placeholder below keeps the module compilable but INCORRECT.
-  // ===========================================================================
+  localparam logic signed [DW:0]   SAT_MAX_EXT = {2'b00, {(DW-1){1'b1}}};
+  localparam logic signed [DW:0]   SAT_MIN_EXT = {2'b11, {(DW-1){1'b0}}};
+  localparam logic signed [DW-1:0] SAT_MAX     = {1'b0, {(DW-1){1'b1}}};
+  localparam logic signed [DW-1:0] SAT_MIN     = {1'b1, {(DW-1){1'b0}}};
+
+  function automatic logic signed [DW-1:0] rounded_half_up;
+    input logic signed [DW-1:0] a;
+    input logic signed [DW-1:0] b;
+    logic signed [DW:0] sum;
+    logic signed [DW:0] shifted;
+    begin
+      sum = $signed(a) + $signed(b) + 1;
+      shifted = sum >>> 1;
+      rounded_half_up = shifted[DW-1:0];
+    end
+  endfunction
+
+  function automatic logic signed [DW-1:0] saturated_sub;
+    input logic signed [DW-1:0] a;
+    input logic signed [DW-1:0] b;
+    logic signed [DW:0] diff;
+    begin
+      diff = $signed(a) - $signed(b);
+
+      if (diff > SAT_MAX_EXT) begin
+        saturated_sub = SAT_MAX;
+      end else if (diff < SAT_MIN_EXT) begin
+        saturated_sub = SAT_MIN;
+      end else begin
+        saturated_sub = diff[DW-1:0];
+      end
+    end
+  endfunction
+
   always_ff @(posedge clk or negedge rst_n) begin
     if (!rst_n) begin
       for (k = 0; k < DEPTH; k = k + 1) line[k] <= '0;
       phase <= 3'd0;
       prime <= 3'd0;
+      dout <= '0;
+      out_valid <= 1'b0;
+    end else if (en) begin
+      for (k = DEPTH-1; k > 0; k = k - 1) line[k] <= line[k-1];
+
+      if (phase < DEPTH[2:0]) begin
+        dout <= line[DEPTH-1];
+        line[0] <= din;
+      end else begin
+        dout <= rounded_half_up(line[DEPTH-1], din);
+        line[0] <= saturated_sub(line[DEPTH-1], din);
+      end
+
+      out_valid <= (prime >= DEPTH[2:0]);
+
+      if (phase == (NPH-1)) begin
+        phase <= 3'd0;
+      end else begin
+        phase <= phase + 3'd1;
+      end
+
+      if (prime < DEPTH[2:0]) begin
+        prime <= prime + 3'd1;
+      end
     end else begin
-      // TODO: replace with the real datapath + control.
       phase <= phase;
       prime <= prime;
     end
   end
-
-  // TODO: replace these placeholder outputs with the real ones.
-  assign dout      = '0;
-  assign out_valid = 1'b0;
 
 endmodule : fold_stage
*** End Patch


Exit code: 0
Wall time: 0 seconds
Output:
Success. Updated the following files:
M /work/fold_core/rtl/fold_stage.sv
The implementation is in place. I’m compiling it with Icarus now, and I’ll also do a small local simulation because the enable-hold and fifth-sample valid timing are the easy places to get subtly off by one.
shell
iverilog -g2012 -I fold_core/rtl -o /tmp/fold_compile.vvp fold_core/rtl/fold_pkg.sv fold_core/rtl/fold_stage.sv fold_core/rtl/fold_top.sv
output
Chunk ID: 7a7ff1
Wall time: 0.0000 seconds
Process exited with code 4
Original token count: 58
Output:
fold_core/rtl/fold_stage.sv:52: syntax error
fold_core/rtl/fold_stage.sv:51: error: Syntax error defining function.
fold_core/rtl/fold_stage.sv:64: syntax error
fold_core/rtl/fold_stage.sv:63: error: Syntax error defining function.
shell
sed -n '1,220p' fold_core/rtl/fold_stage.sv
output
Chunk ID: f7979d
Wall time: 0.0000 seconds
Process exited with code 0
Original token count: 952
Output:
//==============================================================================
// fold_stage.sv
//
// Enable-gated streaming fold stage.
//
// ---------------------------------------------------------------------------
//  >>> IMPLEMENTATION REQUIRED <<<
//
//  The module ports, parameters and the state registers are provided. The
//  datapath, the phase/priming control and the outputs are left as a TODO and
//  currently drive constant zeros, so the stage produces no useful stream and
//  the verifier FAILS.
//
//  Implement the behaviour described in instruction.md. Do NOT change the
//  module name, the port list, or the parameter list.
//
//  iverilog 11 note: a bit/part-select of an EXPRESSION (a sum, a shift, a
//  function-call result) is a syntax error under `iverilog -g2012`. Index only
//  named signals, and do any bit work that needs an intermediate value inside a
//  `function automatic` using named locals.
// ---------------------------------------------------------------------------
//==============================================================================
`timescale 1ns/1ps

module fold_stage #(
    parameter DW    = 12,
    parameter DEPTH = 4,
    parameter NPH   = 8
) (
    input  logic                     clk,
    input  logic                     rst_n,
    input  logic                     en,
    input  logic signed [DW-1:0]     din,
    output logic signed [DW-1:0]     dout,
    output logic                     out_valid
);

  // ---------------------------------------------------------------------------
  // State registers (provided). The head of the feedback line is line[DEPTH-1].
  // ---------------------------------------------------------------------------
  logic signed [DW-1:0] line [0:DEPTH-1];
  logic         [2:0]   phase;
  logic         [2:0]   prime;
  integer               k;

  localparam logic signed [DW:0]   SAT_MAX_EXT = {2'b00, {(DW-1){1'b1}}};
  localparam logic signed [DW:0]   SAT_MIN_EXT = {2'b11, {(DW-1){1'b0}}};
  localparam logic signed [DW-1:0] SAT_MAX     = {1'b0, {(DW-1){1'b1}}};
  localparam logic signed [DW-1:0] SAT_MIN     = {1'b1, {(DW-1){1'b0}}};

  function automatic logic signed [DW-1:0] rounded_half_up;
    input logic signed [DW-1:0] a;
    input logic signed [DW-1:0] b;
    logic signed [DW:0] sum;
    logic signed [DW:0] shifted;
    begin
      sum = $signed(a) + $signed(b) + 1;
      shifted = sum >>> 1;
      rounded_half_up = shifted[DW-1:0];
    end
  endfunction

  function automatic logic signed [DW-1:0] saturated_sub;
    input logic signed [DW-1:0] a;
    input logic signed [DW-1:0] b;
    logic signed [DW:0] diff;
    begin
      diff = $signed(a) - $signed(b);

      if (diff > SAT_MAX_EXT) begin
        saturated_sub = SAT_MAX;
      end else if (diff < SAT_MIN_EXT) begin
        saturated_sub = SAT_MIN;
      end else begin
        saturated_sub = diff[DW-1:0];
      end
    end
  endfunction

  always_ff @(posedge clk or negedge rst_n) begin
    if (!rst_n) begin
      for (k = 0; k < DEPTH; k = k + 1) line[k] <= '0;
      phase <= 3'd0;
      prime <= 3'd0;
      dout <= '0;
      out_valid <= 1'b0;
    end else if (en) begin
      for (k = DEPTH-1; k > 0; k = k - 1) line[k] <= line[k-1];

      if (phase < DEPTH[2:0]) begin
        dout <= line[DEPTH-1];
        line[0] <= din;
      end else begin
        dout <= rounded_half_up(line[DEPTH-1], din);
        line[0] <= saturated_sub(line[DEPTH-1], din);
      end

      out_valid <= (prime >= DEPTH[2:0]);

      if (phase == (NPH-1)) begin
        phase <= 3'd0;
      end else begin
        phase <= phase + 3'd1;
      end

      if (prime < DEPTH[2:0]) begin
        prime <= prime + 3'd1;
      end
    end else begin
      phase <= phase;
      prime <= prime;
    end
  end

endmodule : fold_stage
Icarus rejected the newer-style function declarations, not the datapath itself. I’m adjusting those helper functions to the older declaration form that Icarus 11 accepts, and I’ll remove the parameter part-selects while I’m there.
apply_patch
*** Begin Patch
*** Update File: /work/fold_core/rtl/fold_stage.sv
@@
-  function automatic logic signed [DW-1:0] rounded_half_up;
-    input logic signed [DW-1:0] a;
-    input logic signed [DW-1:0] b;
-    logic signed [DW:0] sum;
-    logic signed [DW:0] shifted;
+  function automatic signed [DW-1:0] rounded_half_up;
+    input signed [DW-1:0] a;
+    input signed [DW-1:0] b;
+    reg signed [DW:0] sum;
+    reg signed [DW:0] shifted;
@@
-  function automatic logic signed [DW-1:0] saturated_sub;
-    input logic signed [DW-1:0] a;
-    input logic signed [DW-1:0] b;
-    logic signed [DW:0] diff;
+  function automatic signed [DW-1:0] saturated_sub;
+    input signed [DW-1:0] a;
+    input signed [DW-1:0] b;
+    reg signed [DW:0] diff;
@@
-      if (phase < DEPTH[2:0]) begin
+      if (phase < DEPTH) begin
@@
-      out_valid <= (prime >= DEPTH[2:0]);
+      out_valid <= (prime >= DEPTH);
@@
-      if (prime < DEPTH[2:0]) begin
+      if (prime < DEPTH) begin
*** End Patch


Exit code: 0
Wall time: 0 seconds
Output:
Success. Updated the following files:
M /work/fold_core/rtl/fold_stage.sv
The compatibility patch is applied. I’m rerunning elaboration, then I’ll run a compact self-check with random `en` gaps against a reference model.
shell
iverilog -g2012 -I fold_core/rtl -o /tmp/fold_compile.vvp fold_core/rtl/fold_pkg.sv fold_core/rtl/fold_stage.sv fold_core/rtl/fold_top.sv
output
Chunk ID: d0741d
Wall time: 0.0000 seconds
Process exited with code 0
Original token count: 0
Output:
shell
sed -n '36,125p' fold_core/rtl/fold_stage.sv
output
Chunk ID: b61901
Wall time: 0.0000 seconds
Process exited with code 0
Original token count: 569
Output:
);

  // ---------------------------------------------------------------------------
  // State registers (provided). The head of the feedback line is line[DEPTH-1].
  // ---------------------------------------------------------------------------
  logic signed [DW-1:0] line [0:DEPTH-1];
  logic         [2:0]   phase;
  logic         [2:0]   prime;
  integer               k;

  localparam logic signed [DW:0]   SAT_MAX_EXT = {2'b00, {(DW-1){1'b1}}};
  localparam logic signed [DW:0]   SAT_MIN_EXT = {2'b11, {(DW-1){1'b0}}};
  localparam logic signed [DW-1:0] SAT_MAX     = {1'b0, {(DW-1){1'b1}}};
  localparam logic signed [DW-1:0] SAT_MIN     = {1'b1, {(DW-1){1'b0}}};

  function automatic signed [DW-1:0] rounded_half_up;
    input signed [DW-1:0] a;
    input signed [DW-1:0] b;
    reg signed [DW:0] sum;
    reg signed [DW:0] shifted;
    begin
      sum = $signed(a) + $signed(b) + 1;
      shifted = sum >>> 1;
      rounded_half_up = shifted[DW-1:0];
    end
  endfunction

  function automatic signed [DW-1:0] saturated_sub;
    input signed [DW-1:0] a;
    input signed [DW-1:0] b;
    reg signed [DW:0] diff;
    begin
      diff = $signed(a) - $signed(b);

      if (diff > SAT_MAX_EXT) begin
        saturated_sub = SAT_MAX;
      end else if (diff < SAT_MIN_EXT) begin
        saturated_sub = SAT_MIN;
      end else begin
        saturated_sub = diff[DW-1:0];
      end
    end
  endfunction

  always_ff @(posedge clk or negedge rst_n) begin
    if (!rst_n) begin
      for (k = 0; k < DEPTH; k = k + 1) line[k] <= '0;
      phase <= 3'd0;
      prime <= 3'd0;
      dout <= '0;
      out_valid <= 1'b0;
    end else if (en) begin
      for (k = DEPTH-1; k > 0; k = k - 1) line[k] <= line[k-1];

      if (phase < DEPTH) begin
        dout <= line[DEPTH-1];
        line[0] <= din;
      end else begin
        dout <= rounded_half_up(line[DEPTH-1], din);
        line[0] <= saturated_sub(line[DEPTH-1], din);
      end

      out_valid <= (prime >= DEPTH);

      if (phase == (NPH-1)) begin
        phase <= 3'd0;
      end else begin
        phase <= phase + 3'd1;
      end

      if (prime < DEPTH) begin
        prime <= prime + 3'd1;
      end
    end else begin
      phase <= phase;
      prime <= prime;
    end
  end

endmodule : fold_stage
Elaboration passes now. One arithmetic detail is worth tightening before simulation: I’m explicitly sign-extending operands before add/subtract so the 12-bit inputs cannot overflow inside the expression before landing in the wider temporary.
apply_patch
*** Begin Patch
*** Update File: /work/fold_core/rtl/fold_stage.sv
@@
     input signed [DW-1:0] a;
     input signed [DW-1:0] b;
+    reg signed [DW:0] a_ext;
+    reg signed [DW:0] b_ext;
     reg signed [DW:0] sum;
     reg signed [DW:0] shifted;
     begin
-      sum = $signed(a) + $signed(b) + 1;
+      a_ext = {a[DW-1], a};
+      b_ext = {b[DW-1], b};
+      sum = a_ext + b_ext + 1;
       shifted = sum >>> 1;
       rounded_half_up = shifted[DW-1:0];
     end
   endfunction
@@
     input signed [DW-1:0] a;
     input signed [DW-1:0] b;
+    reg signed [DW:0] a_ext;
+    reg signed [DW:0] b_ext;
     reg signed [DW:0] diff;
     begin
-      diff = $signed(a) - $signed(b);
+      a_ext = {a[DW-1], a};
+      b_ext = {b[DW-1], b};
+      diff = a_ext - b_ext;
 
       if (diff > SAT_MAX_EXT) begin
*** End Patch


Exit code: 0
Wall time: 0 seconds
Output:
Success. Updated the following files:
M /work/fold_core/rtl/fold_stage.sv
The datapath helpers now widen before arithmetic. I’m compiling again and adding a temporary self-check testbench that exercises reset, idle gaps, phase wrapping, rounding, and saturation.
shell
iverilog -g2012 -I fold_core/rtl -o /tmp/fold_compile.vvp fold_core/rtl/fold_pkg.sv fold_core/rtl/fold_stage.sv fold_core/rtl/fold_top.sv
output
Chunk ID: 01a28b
Wall time: 0.0000 seconds
Process exited with code 0
Original token count: 0
Output:
shell
sed -n '45,120p' fold_core/rtl/fold_stage.sv
output
Chunk ID: fb6bb7
Wall time: 0.0000 seconds
Process exited with code 0
Original token count: 519
Output:

  localparam logic signed [DW:0]   SAT_MAX_EXT = {2'b00, {(DW-1){1'b1}}};
  localparam logic signed [DW:0]   SAT_MIN_EXT = {2'b11, {(DW-1){1'b0}}};
  localparam logic signed [DW-1:0] SAT_MAX     = {1'b0, {(DW-1){1'b1}}};
  localparam logic signed [DW-1:0] SAT_MIN     = {1'b1, {(DW-1){1'b0}}};

  function automatic signed [DW-1:0] rounded_half_up;
    input signed [DW-1:0] a;
    input signed [DW-1:0] b;
    reg signed [DW:0] a_ext;
    reg signed [DW:0] b_ext;
    reg signed [DW:0] sum;
    reg signed [DW:0] shifted;
    begin
      a_ext = {a[DW-1], a};
      b_ext = {b[DW-1], b};
      sum = a_ext + b_ext + 1;
      shifted = sum >>> 1;
      rounded_half_up = shifted[DW-1:0];
    end
  endfunction

  function automatic signed [DW-1:0] saturated_sub;
    input signed [DW-1:0] a;
    input signed [DW-1:0] b;
    reg signed [DW:0] a_ext;
    reg signed [DW:0] b_ext;
    reg signed [DW:0] diff;
    begin
      a_ext = {a[DW-1], a};
      b_ext = {b[DW-1], b};
      diff = a_ext - b_ext;

      if (diff > SAT_MAX_EXT) begin
        saturated_sub = SAT_MAX;
      end else if (diff < SAT_MIN_EXT) begin
        saturated_sub = SAT_MIN;
      end else begin
        saturated_sub = diff[DW-1:0];
      end
    end
  endfunction

  always_ff @(posedge clk or negedge rst_n) begin
    if (!rst_n) begin
      for (k = 0; k < DEPTH; k = k + 1) line[k] <= '0;
      phase <= 3'd0;
      prime <= 3'd0;
      dout <= '0;
      out_valid <= 1'b0;
    end else if (en) begin
      for (k = DEPTH-1; k > 0; k = k - 1) line[k] <= line[k-1];

      if (phase < DEPTH) begin
        dout <= line[DEPTH-1];
        line[0] <= din;
      end else begin
        dout <= rounded_half_up(line[DEPTH-1], din);
        line[0] <= saturated_sub(line[DEPTH-1], din);
      end

      out_valid <= (prime >= DEPTH);

      if (phase == (NPH-1)) begin
        phase <= 3'd0;
      end else begin
        phase <= phase + 3'd1;
      end

      if (prime < DEPTH) begin
        prime <= prime + 3'd1;
      end
    end else begin
      phase <= phase;
      prime <= prime;
    end
apply_patch
*** Begin Patch
*** Add File: /tmp/fold_selfcheck.sv
+`timescale 1ns/1ps
+
+module tb;
+  logic clk;
+  logic rst_n;
+  logic en;
+  logic signed [11:0] din;
+  logic signed [11:0] dout;
+  logic out_valid;
+
+  int signed model_line [0:3];
+  int model_phase;
+  int model_prime;
+  int signed model_dout;
+  bit model_valid;
+  int i;
+
+  fold_stage dut (
+      .clk       (clk),
+      .rst_n     (rst_n),
+      .en        (en),
+      .din       (din),
+      .dout      (dout),
+      .out_valid (out_valid)
+  );
+
+  initial clk = 1'b0;
+  always #5 clk = ~clk;
+
+  function automatic int signed sat12;
+    input int signed v;
+    begin
+      if (v > 2047) sat12 = 2047;
+      else if (v < -2048) sat12 = -2048;
+      else sat12 = v;
+    end
+  endfunction
+
+  function automatic int signed s12;
+    input int signed v;
+    begin
+      s12 = sat12(v);
+    end
+  endfunction
+
+  task automatic reset_model;
+    begin
+      for (i = 0; i < 4; i = i + 1) model_line[i] = 0;
+      model_phase = 0;
+      model_prime = 0;
+      model_dout = 0;
+      model_valid = 1'b0;
+    end
+  endtask
+
+  task automatic check_outputs;
+    begin
+      if (dout !== model_dout || out_valid !== model_valid) begin
+        $display("FAIL t=%0t got dout=%0d valid=%0b expected dout=%0d valid=%0b",
+                 $time, dout, out_valid, model_dout, model_valid);
+        $finish;
+      end
+    end
+  endtask
+
+  task automatic step;
+    input bit en_i;
+    input int signed din_i;
+    int signed h;
+    int signed inserted;
+    begin
+      @(negedge clk);
+      en = en_i;
+      din = s12(din_i);
+      @(posedge clk);
+      #1;
+      if (en_i) begin
+        h = model_line[3];
+        if (model_phase < 4) begin
+          model_dout = h;
+          inserted = din;
+        end else begin
+          model_dout = (h + din + 1) >>> 1;
+          inserted = sat12(h - din);
+        end
+
+        model_line[3] = model_line[2];
+        model_line[2] = model_line[1];
+        model_line[1] = model_line[0];
+        model_line[0] = inserted;
+        model_valid = (model_prime >= 4);
+        model_phase = (model_phase == 7) ? 0 : model_phase + 1;
+        if (model_prime < 4) model_prime = model_prime + 1;
+      end
+      check_outputs();
+    end
+  endtask
+
+  initial begin
+    en = 1'b0;
+    din = '0;
+    reset_model();
+    rst_n = 1'b0;
+    #1;
+    check_outputs();
+    repeat (2) @(posedge clk);
+    rst_n = 1'b1;
+
+    step(1, 10);
+    step(0, 999);
+    step(0, -999);
+    step(1, 20);
+    step(1, 30);
+    step(1, 40);
+    step(1, 100);
+    step(1, -101);
+    step(1, 2047);
+    step(1, -2048);
+
+    for (i = 0; i < 300; i = i + 1) begin
+      step(($urandom % 4) != 0, $signed($urandom_range(0, 4095)) - 2048);
+    end
+
+    @(negedge clk);
+    rst_n = 1'b0;
+    reset_model();
+    #1;
+    check_outputs();
+    rst_n = 1'b1;
+    step(0, 123);
+    step(1, -2048);
+    step(1, 2047);
+    step(1, -1);
+    step(1, 0);
+    step(1, 2047);
+    step(1, -2048);
+
+    $display("PASS");
+    $finish;
+  end
+endmodule
*** End Patch


Exit code: 0
Wall time: 0 seconds
Output:
Success. Updated the following files:
A /tmp/fold_selfcheck.sv
The self-check is set up as a temporary file under `/tmp`. I’m running it now against the implemented stage.
shell
iverilog -g2012 -I fold_core/rtl -o /tmp/fold_selfcheck.vvp /tmp/fold_selfcheck.sv fold_core/rtl/fold_stage.sv && vvp /tmp/fold_selfcheck.vvp
output
Chunk ID: b2d930
Wall time: 0.0000 seconds
Process exited with code 5
Original token count: 63
Output:
/tmp/fold_selfcheck.sv:31: syntax error
/tmp/fold_selfcheck.sv:30: error: Syntax error defining function.
/tmp/fold_selfcheck.sv:40: syntax error
/tmp/fold_selfcheck.sv:39: error: Syntax error defining function.
/tmp/fold_selfcheck.sv:67: syntax error
The DUT still compiles; the temporary checker used syntax Icarus also dislikes. I’m downgrading the checker syntax and rerunning, without changing the RTL for that.
shell
nl -ba /tmp/fold_selfcheck.sv | sed -n '24,76p'
output
Chunk ID: 2d643c
Wall time: 0.0000 seconds
Process exited with code 0
Original token count: 373
Output:
    24	      .out_valid (out_valid)
    25	  );
    26	
    27	  initial clk = 1'b0;
    28	  always #5 clk = ~clk;
    29	
    30	  function automatic int signed sat12;
    31	    input int signed v;
    32	    begin
    33	      if (v > 2047) sat12 = 2047;
    34	      else if (v < -2048) sat12 = -2048;
    35	      else sat12 = v;
    36	    end
    37	  endfunction
    38	
    39	  function automatic int signed s12;
    40	    input int signed v;
    41	    begin
    42	      s12 = sat12(v);
    43	    end
    44	  endfunction
    45	
    46	  task automatic reset_model;
    47	    begin
    48	      for (i = 0; i < 4; i = i + 1) model_line[i] = 0;
    49	      model_phase = 0;
    50	      model_prime = 0;
    51	      model_dout = 0;
    52	      model_valid = 1'b0;
    53	    end
    54	  endtask
    55	
    56	  task automatic check_outputs;
    57	    begin
    58	      if (dout !== model_dout || out_valid !== model_valid) begin
    59	        $display("FAIL t=%0t got dout=%0d valid=%0b expected dout=%0d valid=%0b",
    60	                 $time, dout, out_valid, model_dout, model_valid);
    61	        $finish;
    62	      end
    63	    end
    64	  endtask
    65	
    66	  task automatic step;
    67	    input bit en_i;
    68	    input int signed din_i;
    69	    int signed h;
    70	    int signed inserted;
    71	    begin
    72	      @(negedge clk);
    73	      en = en_i;
    74	      din = s12(din_i);
    75	      @(posedge clk);
    76	      #1;
shell
iverilog -g2012 -I fold_core/rtl -o /tmp/fold_compile.vvp fold_core/rtl/fold_pkg.sv fold_core/rtl/fold_stage.sv fold_core/rtl/fold_top.sv
output
Chunk ID: 540751
Wall time: 0.0000 seconds
Process exited with code 0
Original token count: 0
Output:
apply_patch
*** Begin Patch
*** Update File: /tmp/fold_selfcheck.sv
@@
-  int signed model_line [0:3];
-  int model_phase;
-  int model_prime;
-  int signed model_dout;
+  integer model_line [0:3];
+  integer model_phase;
+  integer model_prime;
+  integer model_dout;
@@
-  int i;
+  integer i;
@@
-  function automatic int signed sat12;
-    input int signed v;
+  function automatic integer sat12;
+    input integer v;
@@
-  function automatic int signed s12;
-    input int signed v;
+  function automatic integer s12;
+    input integer v;
@@
   task automatic step;
-    input bit en_i;
-    input int signed din_i;
-    int signed h;
-    int signed inserted;
+    input en_i;
+    input integer din_i;
+    integer h;
+    integer inserted;
*** End Patch


Exit code: 0
Wall time: 0 seconds
Output:
Success. Updated the following files:
M /tmp/fold_selfcheck.sv
The checker syntax is now compatible. I’m rerunning the simulation and will inspect any mismatch if it finds one.
shell
iverilog -g2012 -I fold_core/rtl -o /tmp/fold_selfcheck.vvp /tmp/fold_selfcheck.sv fold_core/rtl/fold_stage.sv && vvp /tmp/fold_selfcheck.vvp
output
Chunk ID: 8fd227
Wall time: 0.0000 seconds
Process exited with code 0
Original token count: 16
Output:
FAIL t=3176000 got dout=-434 valid=1 expected dout=0 valid=0
The temporary checker found a mismatch around the mid-test reset path, so I’m separating whether that is a checker race or an RTL reset issue before making any further RTL edits.
shell
nl -ba /tmp/fold_selfcheck.sv | sed -n '92,132p'
output
Chunk ID: 06ce90
Wall time: 0.0000 seconds
Process exited with code 0
Original token count: 273
Output:
    92	        model_phase = (model_phase == 7) ? 0 : model_phase + 1;
    93	        if (model_prime < 4) model_prime = model_prime + 1;
    94	      end
    95	      check_outputs();
    96	    end
    97	  endtask
    98	
    99	  initial begin
   100	    en = 1'b0;
   101	    din = '0;
   102	    reset_model();
   103	    rst_n = 1'b0;
   104	    #1;
   105	    check_outputs();
   106	    repeat (2) @(posedge clk);
   107	    rst_n = 1'b1;
   108	
   109	    step(1, 10);
   110	    step(0, 999);
   111	    step(0, -999);
   112	    step(1, 20);
   113	    step(1, 30);
   114	    step(1, 40);
   115	    step(1, 100);
   116	    step(1, -101);
   117	    step(1, 2047);
   118	    step(1, -2048);
   119	
   120	    for (i = 0; i < 300; i = i + 1) begin
   121	      step(($urandom % 4) != 0, $signed($urandom_range(0, 4095)) - 2048);
   122	    end
   123	
   124	    @(negedge clk);
   125	    rst_n = 1'b0;
   126	    reset_model();
   127	    #1;
   128	    check_outputs();
   129	    rst_n = 1'b1;
   130	    step(0, 123);
   131	    step(1, -2048);
   132	    step(1, 2047);
shell
iverilog -g2012 -Wall -I fold_core/rtl -o /tmp/fold_compile.vvp fold_core/rtl/fold_pkg.sv fold_core/rtl/fold_stage.sv fold_core/rtl/fold_top.sv
output
Chunk ID: 0867ef
Wall time: 0.0000 seconds
Process exited with code 0
Original token count: 61
Output:
warning: Some design elements have no explicit time unit and/or
       : time precision. This may cause confusing timing results.
       : Affected design elements are:
       :   -- package fold_pkg declared here: fold_core/rtl/fold_pkg.sv:15
apply_patch
*** Begin Patch
*** Update File: /tmp/fold_selfcheck.sv
@@
-    @(negedge clk);
+    @(negedge clk);
+    #1;
     rst_n = 1'b0;
     reset_model();
     #1;
*** End Patch


Exit code: 0
Wall time: 0 seconds
Output:
Success. Updated the following files:
M /tmp/fold_selfcheck.sv
That mismatch was caused by the checker dropping reset exactly on a clock edge, creating a simulator scheduling race in the testbench. I moved the temporary reset assertion off the clock edge and am rerunning the randomized check.
shell
iverilog -g2012 -I fold_core/rtl -o /tmp/fold_selfcheck.vvp /tmp/fold_selfcheck.sv fold_core/rtl/fold_stage.sv && vvp /tmp/fold_selfcheck.vvp
output
Chunk ID: 6a1e6e
Wall time: 0.0000 seconds
Process exited with code 0
Original token count: 16
Output:
FAIL t=3176000 got dout=-434 valid=1 expected dout=0 valid=0
The reset race was not the whole story, so I’m checking the exact reset sequence in the temporary testbench and the generated RTL behavior around it.
shell
nl -ba /tmp/fold_selfcheck.sv | sed -n '99,132p'
output
Chunk ID: 89416a
Wall time: 0.0000 seconds
Process exited with code 0
Original token count: 214
Output:
    99	  initial begin
   100	    en = 1'b0;
   101	    din = '0;
   102	    reset_model();
   103	    rst_n = 1'b0;
   104	    #1;
   105	    check_outputs();
   106	    repeat (2) @(posedge clk);
   107	    rst_n = 1'b1;
   108	
   109	    step(1, 10);
   110	    step(0, 999);
   111	    step(0, -999);
   112	    step(1, 20);
   113	    step(1, 30);
   114	    step(1, 40);
   115	    step(1, 100);
   116	    step(1, -101);
   117	    step(1, 2047);
   118	    step(1, -2048);
   119	
   120	    for (i = 0; i < 300; i = i + 1) begin
   121	      step(($urandom % 4) != 0, $signed($urandom_range(0, 4095)) - 2048);
   122	    end
   123	
   124	    @(negedge clk);
   125	    #1;
   126	    rst_n = 1'b0;
   127	    reset_model();
   128	    #1;
   129	    check_outputs();
   130	    rst_n = 1'b1;
   131	    step(0, 123);
   132	    step(1, -2048);
The DUT compiled cleanly, but I want a minimal reset probe now because the temporary full checker is seeing stale registered outputs immediately after a mid-stream reset assertion. That will tell me whether the issue is in the RTL reset sensitivity or in the checker scaffolding.
apply_patch
*** Begin Patch
*** Add File: /tmp/fold_reset_probe.sv
+`timescale 1ns/1ps
+
+module tb;
+  logic clk;
+  logic rst_n;
+  logic en;
+  logic signed [11:0] din;
+  logic signed [11:0] dout;
+  logic out_valid;
+
+  fold_stage dut (
+      .clk(clk),
+      .rst_n(rst_n),
+      .en(en),
+      .din(din),
+      .dout(dout),
+      .out_valid(out_valid)
+  );
+
+  initial clk = 0;
+  always #5 clk = ~clk;
+
+  initial begin
+    en = 0;
+    din = 0;
+    rst_n = 0;
+    #12;
+    rst_n = 1;
+    @(negedge clk);
+    en = 1;
+    din = 12'sd1;
+    repeat (8) @(negedge clk);
+    $display("before reset t=%0t dout=%0d valid=%0b rst=%0b", $time, dout, out_valid, rst_n);
+    #1;
+    rst_n = 0;
+    #1;
+    $display("after reset t=%0t dout=%0d valid=%0b rst=%0b", $time, dout, out_valid, rst_n);
+    if (dout !== 0 || out_valid !== 0) $display("RESET_FAIL");
+    else $display("RESET_PASS");
+    $finish;
+  end
+endmodule
*** End Patch


Exit code: 0
Wall time: 0 seconds
Output:
Success. Updated the following files:
A /tmp/fold_reset_probe.sv
shell
iverilog -g2012 -I fold_core/rtl -o /tmp/fold_reset_probe.vvp /tmp/fold_reset_probe.sv fold_core/rtl/fold_stage.sv && vvp /tmp/fold_reset_probe.vvp
output
Chunk ID: 6b75f3
Wall time: 0.0000 seconds
Process exited with code 0
Original token count: 24
Output:
before reset t=100000 dout=1 valid=1 rst=1
after reset t=102000 dout=0 valid=0 rst=0
RESET_PASS
The minimal probe confirms the RTL async reset does clear the registered outputs immediately. I’m going back to the full temporary checker; its model/reset bookkeeping is likely the culprit.
apply_patch
*** Begin Patch
*** Update File: /tmp/fold_selfcheck.sv
@@
     #1;
     rst_n = 1'b0;
     reset_model();
     #1;
+    $display("debug reset t=%0t dout=%0d valid=%0b rst=%0b", $time, dout, out_valid, rst_n);
     check_outputs();
*** End Patch


Exit code: 0
Wall time: 0 seconds
Output:
Success. Updated the following files:
M /tmp/fold_selfcheck.sv
shell
iverilog -g2012 -I fold_core/rtl -o /tmp/fold_selfcheck.vvp /tmp/fold_selfcheck.sv fold_core/rtl/fold_stage.sv && vvp /tmp/fold_selfcheck.vvp
output
Chunk ID: fe5351
Wall time: 0.0000 seconds
Process exited with code 0
Original token count: 26
Output:
debug reset t=3122000 dout=0 valid=0 rst=0
FAIL t=3176000 got dout=-434 valid=1 expected dout=0 valid=0
The full checker issue is clear now: after reset deassertion it left `en` high until the next negedge, so the DUT legitimately accepted a sample before the model step began. I’m fixing that checker setup and rerunning.
apply_patch
*** Begin Patch
*** Update File: /tmp/fold_selfcheck.sv
@@
-    $display("debug reset t=%0t dout=%0d valid=%0b rst=%0b", $time, dout, out_valid, rst_n);
     check_outputs();
+    en = 1'b0;
     rst_n = 1'b1;
*** End Patch


Exit code: 0
Wall time: 0 seconds
Output:
Success. Updated the following files:
M /tmp/fold_selfcheck.sv
The checker now keeps `en` low while leaving reset, matching the intended post-reset idle state. I’m running the randomized check again.
shell
iverilog -g2012 -I fold_core/rtl -o /tmp/fold_selfcheck.vvp /tmp/fold_selfcheck.sv fold_core/rtl/fold_stage.sv && vvp /tmp/fold_selfcheck.vvp
output
Chunk ID: 195fa0
Wall time: 0.0000 seconds
Process exited with code 0
Original token count: 2
Output:
PASS
The randomized self-check passes after the testbench fix. I’m doing one final cleanup pass on the RTL comments/hold branch, then I’ll run the compile and self-check one more time.
apply_patch
*** Begin Patch
*** Update File: /work/fold_core/rtl/fold_stage.sv
@@
-//  >>> IMPLEMENTATION REQUIRED <<<
-//
-//  The module ports, parameters and the state registers are provided. The
-//  datapath, the phase/priming control and the outputs are left as a TODO and
-//  currently drive constant zeros, so the stage produces no useful stream and
-//  the verifier FAILS.
-//
-//  Implement the behaviour described in instruction.md. Do NOT change the
-//  module name, the port list, or the parameter list.
-//
 //  iverilog 11 note: a bit/part-select of an EXPRESSION (a sum, a shift, a
 //  function-call result) is a syntax error under `iverilog -g2012`. Index only
 //  named signals, and do any bit work that needs an intermediate value inside a
 //  `function automatic` using named locals.
@@
-    end else begin
-      phase <= phase;
-      prime <= prime;
     end
   end
 
 endmodule : fold_stage
*** End Patch


Exit code: 0
Wall time: 0 seconds
Output:
Success. Updated the following files:
M /work/fold_core/rtl/fold_stage.sv
The RTL now relies on the normal register hold behavior when `en` is low, so there are no redundant self-assignments. Final compile and simulation are running.
shell
iverilog -g2012 -I fold_core/rtl -o /tmp/fold_compile.vvp fold_core/rtl/fold_pkg.sv fold_core/rtl/fold_stage.sv fold_core/rtl/fold_top.sv
output
Chunk ID: f22d75
Wall time: 0.0000 seconds
Process exited with code 0
Original token count: 0
Output:
shell
iverilog -g2012 -I fold_core/rtl -o /tmp/fold_selfcheck.vvp /tmp/fold_selfcheck.sv fold_core/rtl/fold_stage.sv && vvp /tmp/fold_selfcheck.vvp
output
Chunk ID: b54839
Wall time: 0.0000 seconds
Process exited with code 0
Original token count: 2
Output:
PASS
shell
sed -n '1,170p' fold_core/rtl/fold_stage.sv
output
Chunk ID: d897bf
Wall time: 0.0000 seconds
Process exited with code 0
Original token count: 866
Output:
//==============================================================================
// fold_stage.sv
//
// Enable-gated streaming fold stage.
//
// ---------------------------------------------------------------------------
//  iverilog 11 note: a bit/part-select of an EXPRESSION (a sum, a shift, a
//  function-call result) is a syntax error under `iverilog -g2012`. Index only
//  named signals, and do any bit work that needs an intermediate value inside a
//  `function automatic` using named locals.
// ---------------------------------------------------------------------------
//==============================================================================
`timescale 1ns/1ps

module fold_stage #(
    parameter DW    = 12,
    parameter DEPTH = 4,
    parameter NPH   = 8
) (
    input  logic                     clk,
    input  logic                     rst_n,
    input  logic                     en,
    input  logic signed [DW-1:0]     din,
    output logic signed [DW-1:0]     dout,
    output logic                     out_valid
);

  // ---------------------------------------------------------------------------
  // State registers (provided). The head of the feedback line is line[DEPTH-1].
  // ---------------------------------------------------------------------------
  logic signed [DW-1:0] line [0:DEPTH-1];
  logic         [2:0]   phase;
  logic         [2:0]   prime;
  integer               k;

  localparam logic signed [DW:0]   SAT_MAX_EXT = {2'b00, {(DW-1){1'b1}}};
  localparam logic signed [DW:0]   SAT_MIN_EXT = {2'b11, {(DW-1){1'b0}}};
  localparam logic signed [DW-1:0] SAT_MAX     = {1'b0, {(DW-1){1'b1}}};
  localparam logic signed [DW-1:0] SAT_MIN     = {1'b1, {(DW-1){1'b0}}};

  function automatic signed [DW-1:0] rounded_half_up;
    input signed [DW-1:0] a;
    input signed [DW-1:0] b;
    reg signed [DW:0] a_ext;
    reg signed [DW:0] b_ext;
    reg signed [DW:0] sum;
    reg signed [DW:0] shifted;
    begin
      a_ext = {a[DW-1], a};
      b_ext = {b[DW-1], b};
      sum = a_ext + b_ext + 1;
      shifted = sum >>> 1;
      rounded_half_up = shifted[DW-1:0];
    end
  endfunction

  function automatic signed [DW-1:0] saturated_sub;
    input signed [DW-1:0] a;
    input signed [DW-1:0] b;
    reg signed [DW:0] a_ext;
    reg signed [DW:0] b_ext;
    reg signed [DW:0] diff;
    begin
      a_ext = {a[DW-1], a};
      b_ext = {b[DW-1], b};
      diff = a_ext - b_ext;

      if (diff > SAT_MAX_EXT) begin
        saturated_sub = SAT_MAX;
      end else if (diff < SAT_MIN_EXT) begin
        saturated_sub = SAT_MIN;
      end else begin
        saturated_sub = diff[DW-1:0];
      end
    end
  endfunction

  always_ff @(posedge clk or negedge rst_n) begin
    if (!rst_n) begin
      for (k = 0; k < DEPTH; k = k + 1) line[k] <= '0;
      phase <= 3'd0;
      prime <= 3'd0;
      dout <= '0;
      out_valid <= 1'b0;
    end else if (en) begin
      for (k = DEPTH-1; k > 0; k = k - 1) line[k] <= line[k-1];

      if (phase < DEPTH) begin
        dout <= line[DEPTH-1];
        line[0] <= din;
      end else begin
        dout <= rounded_half_up(line[DEPTH-1], din);
        line[0] <= saturated_sub(line[DEPTH-1], din);
      end

      out_valid <= (prime >= DEPTH);

      if (phase == (NPH-1)) begin
        phase <= 3'd0;
      end else begin
        phase <= phase + 3'd1;
      end

      if (prime < DEPTH) begin
        prime <= prime + 3'd1;
      end
    end
  end

endmodule : fold_stage
Implemented [fold_stage.sv](/work/fold_core/rtl/fold_stage.sv) with the enable-gated delay line, phase control, registered outputs, async active-low reset, round-half-up combine output, and saturated feedback subtraction. Verified with: ```sh iverilog -g2012 -I fold_core/rtl -o /tmp/fold_compile.vvp fold_core/rtl/fold_pkg.sv fold_core/rtl/fold_stage.sv fold_core/rtl/fold_top.sv ``` Also ran a temporary randomized self-check with idle gaps, reset, wrapping, rounding, and saturation: `PASS`.
Final diff , what the agent changed

No file changes, the agent left the workspace untouched.

Verifier output , the proof behind the reward
tests/test.sh
test.sh: compiling with iverilog...
----- iverilog output -----
---------------------------
test.sh: running simulation with vvp...
----- simulation output -----
INFO samples=544 dmiss=0 vmiss=0
INFO probe_counts wide=8 replay=4 reset_en=7 freeze=71
SAMPLES=544
ALL TESTS PASSED
-----------------------------
test.sh: PASS - 544 samples, no mismatches

Reproduce this trial: git checkout 2f94510 && PYTHONPATH=src python3 scripts/build_site.py , then open trial/trial_c7b1379ed840442c. Re-running the agent live requires EVAL_PLATFORM_ENABLE_OAUTH_SMOKE=1 and is non-deterministic.

Trial trial_c7b1379ed840442c · verifier authoritative; classifier explanatory.