SyncVals · Trajectory

debug-halt-step-fsm

claude-code claude-opus-4-8 ✗ failed GOOD_FAILURE ↑ 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

GOOD_FAILUREHonest miss, the agent ran correctly but couldn't solve it. Expected for a hard task; the task is sound.

SubtypeImplementation Bugs

EvidenceVerifier output: `TEST reference_equivalence: FAIL (563 mismatches in 2301 samples)`. The held-back reference drives outputs from the current FSM state: `fetch_o <= issue; halted_o <= (st == HOLD);` while transitioning `RUN -> DRAIN -> HOLD`, which implements fetch high through E+1 and halted high at E+2. The agent instead decoded outputs from `next_state`: `case (next_state) S_RUN, S_HALT_PEND, S_STEP_FETCH: next_fetch = 1'b1; ... next_halted = (next_state == S_HALTED);`, causing halt/step outputs to advance one edge too early despite its own limited testbench reporting `ALL PASS`.

Root causeThe task specification is precise and the tests behaviorally check the stated timing against a reference model. The agent made an off-by-one registered-output mistake by assigning outputs for the state being entered rather than the state currently sampled.

RecommendationN/A - task is fine

Trajectory

Tool-by-tool agent trajectory

9 tool calls · 4 tool types · 17 steps

A control block stops and resumes a core over a debug port. Ports: clk, rst, halt_req_i, resume_req_i, step_req_i, dbus_ren_i, dbus_addr_i (2 bits), dbus_rdata_o (8 bits), halted_o, fetch_o. rst is synchronous, active high, clearing all state and outputs to 0. All outputs are registered. While running, fetch_o is high every cycle and halted_o is low. When halt_req_i is sampled high at edge E while running, fetch_o stays high through edge E+1, is low from edge E+2, and halted_o is first high at edge E+2. In any state, with dbus_ren_i high and address A at edge R, dbus_rdata_o presents location A at edge R+1 and holds while dbus_ren_i is low. Address 0 reads 0x5A, address 1 reads 0xC3, address 2 reads 0x0F, address 3 reads 0x96. When step_req_i is sampled high at edge S while halted, fetch_o is high at edge S+1 only, and halted_o is low at edge S+1 and high again at edge S+2. When resume_req_i is sampled high at edge G while halted, halted_o is low and fetch_o high from edge G+1. If halt_req_i and step_req_i are both high at an edge while halted, the core stays halted and fetch_o stays low. Write module TopModule to /app/dut.sv.

I'll design this as a small FSM. Let me first reason carefully about the timing, then write the module. Key observations from the spec (treating "value at edge k" as the registered value sampled at that edge): - **Halt** has a 2-edge latency (sample at E → halted at E+2), so it needs one intermediate "pending" state where `fetch` is still high. - **Step** and **Resume** have 1-edge latency, so they transition directly. - **dbus read** is an independent 1-cycle registered read ("in any state"). - **Reset** forces all outputs to 0.

write · /app/dut.sv

module TopModule (
    input  logic       clk,
    input  logic       rst,
    input  logic       halt_req_i,
    input  logic       resume_req_i,
    input  logic       step_req_i,
    input  logic       dbus_ren_i,
    input  logic [1:0] dbus_addr_i,
    output logic [7:0] dbus_rdata_o,
    output logic       halted_o,
    output logic       fetch_o
);

    // FSM states
    typedef enum logic [2:0] {
        S_RESET,     // outputs 0 (post-reset)
        S_RUN,       // running: fetch=1, halted=0
        S_HALT_PEND, // one extra fetch cycle before halting: fetch=1, halted=0
        S_HALTED,    // halted: fetch=0, halted=1
        S_STEP_FETCH  // single fetch cycle during step: fetch=1, halted=0
    } state_t;

    state_t state, next_state;
    logic   next_fetch, next_halted;

    // Next-state and next-output (Moore) logic
    always_comb begin
        next_state = state;
        case (state)
            S_RESET: begin
                next_state = S_RUN;
            end
            S_RUN: begin
                next_state = halt_req_i ? S_HALT_PEND : S_RUN;
            end
            S_HALT_PEND: begin
                next_state = S_HALTED;
            end
            S_HALTED: begin
                if (halt_req_i)        next_state = S_HALTED;      // halt dominates (covers halt+step)
                else if (step_req_i)   next_state = S_STEP_FETCH;
                else if (resume_req_i) next_state = S_RUN;
                else                   next_state = S_HALTED;
            end
            S_STEP_FETCH: begin
                next_state = S_HALTED;
            end
            default: begin
                next_state = S_RUN;
            end
        endcase

        // Output decode of the state being entered
        case (next_state)
            S_RUN, S_HALT_PEND, S_STEP_FETCH: next_fetch = 1'b1;
            default:                          next_fetch = 1'b0;
        endcase
        next_halted = (next_state == S_HALTED);
    end

    // Debug bus read data (1-cycle registered read, independent of FSM)
    logic [7:0] rd_next;
    always_comb begin
        case (dbus_addr_i)
            2'd0:    rd_next = 8'h5A;
            2'd1:    rd_next = 8'hC3;
            2'd2:    rd_next = 8'h0F;
            default: rd_next = 8'h96;
        endcase
    end

    always_ff @(posedge clk) begin
        if (rst) begin
            state        <= S_RESET;
            fetch_o      <= 1'b0;
            halted_o     <= 1'b0;
            dbus_rdata_o <= 8'h00;
        end else begin
            state    <= next_state;
            fetch_o  <= next_fetch;
            halted_o <= next_halted;

            if (dbus_ren_i)
                dbus_rdata_o <= rd_next;      // present new data next edge
            else
                dbus_rdata_o <= dbus_rdata_o; // hold while ren low
        end
    end

endmodule

Let me verify it compiles cleanly if a simulator is available.

Lint/compile the SystemVerilog module

shell

(command -v verilator >/dev/null && verilator --lint-only -Wall --timing dut.sv 2>&1 | head -40 || echo "verilator not found"); (command -v iverilog >/dev/null && iverilog -g2012 -o /tmp/dut.out dut.sv && echo "iverilog OK" || echo "iverilog not found")

output

verilator not found
dut.sv:34: error: This assignment requires an explicit cast.
1 error(s) during elaboration.
iverilog not found

[stdout]
verilator not found
dut.sv:34: error: This assignment requires an explicit cast.
1 error(s) during elaboration.
iverilog not found

iverilog is strict about enum assignments. Let me switch to plain `localparam` encodings to avoid the casting issue.

/app/dut.sv

− removed

    // FSM states
    typedef enum logic [2:0] {
        S_RESET,     // outputs 0 (post-reset)
        S_RUN,       // running: fetch=1, halted=0
        S_HALT_PEND, // one extra fetch cycle before halting: fetch=1, halted=0
        S_HALTED,    // halted: fetch=0, halted=1
        S_STEP_FETCH  // single fetch cycle during step: fetch=1, halted=0
    } state_t;

    state_t state, next_state;
    logic   next_fetch, next_halted;

+ added

    // FSM states
    localparam logic [2:0] S_RESET      = 3'd0; // outputs 0 (post-reset)
    localparam logic [2:0] S_RUN        = 3'd1; // running: fetch=1, halted=0
    localparam logic [2:0] S_HALT_PEND  = 3'd2; // extra fetch cycle before halting
    localparam logic [2:0] S_HALTED     = 3'd3; // halted: fetch=0, halted=1
    localparam logic [2:0] S_STEP_FETCH = 3'd4; // single fetch cycle during step

    logic [2:0] state, next_state;
    logic       next_fetch, next_halted;

Recompile with iverilog

shell

command -v iverilog >/dev/null && iverilog -g2012 -o /tmp/dut.out dut.sv && echo "iverilog OK" || echo "iverilog not available"

output

iverilog OK

[stdout]
iverilog OK

Compiles cleanly. Let me write a quick testbench to verify the timing behaviors against the spec.

Run timing testbench

shell

cat > /tmp/tb.sv <<'EOF'
module tb;
  logic clk=0, rst;
  logic halt_req_i, resume_req_i, step_req_i, dbus_ren_i;
  logic [1:0] dbus_addr_i;
  logic [7:0] dbus_rdata_o;
  logic halted_o, fetch_o;

  TopModule dut(.*);
  always #5 clk=~clk;

  task step; @(posedge clk); #1; endtask
  integer errors=0;
  task chk(input exp_f, input exp_h, input [127:0] msg);
    if (fetch_o!==exp_f || halted_o!==exp_h) begin
      $display("FAIL %0s: fetch=%b(exp %b) halted=%b(exp %b)", msg, fetch_o, exp_f, halted_o, exp_h);
      errors=errors+1;
    end
  endtask

  initial begin
    rst=1; halt_req_i=0; resume_req_i=0; step_req_i=0; dbus_ren_i=0; dbus_addr_i=0;
    @(posedge clk); #1; chk(0,0,"reset");
    rst=0;
    step; chk(1,0,"run1");
    step; chk(1,0,"run2");
    // HALT at edge E
    halt_req_i=1; @(posedge clk); halt_req_i=0; #1; chk(1,0,"E+1 fetch high"); // value at E+1
    step; chk(0,1,"E+2 halted");         // value at E+2
    step; chk(0,1,"stay halted");
    // STEP at edge S
    step_req_i=1; @(posedge clk); step_req_i=0; #1; chk(1,0,"S+1 fetch pulse");
    step; chk(0,1,"S+2 halted again");
    step; chk(0,1,"still halted");
    // halt+step -> stay halted
    halt_req_i=1; step_req_i=1; @(posedge clk); halt_req_i=0; step_req_i=0; #1; chk(0,1,"halt+step stays halted");
    // RESUME at edge G
    resume_req_i=1; @(posedge clk); resume_req_i=0; #1; chk(1,0,"G+1 running");
    step; chk(1,0,"running after resume");
    // dbus read while running
    dbus_ren_i=1; dbus_addr_i=2'd1; @(posedge clk); dbus_ren_i=0; #1;
    if (dbus_rdata_o!==8'hC3) begin $display("FAIL dbus R+1: %h", dbus_rdata_o); errors=errors+1; end
    step; if (dbus_rdata_o!==8'hC3) begin $display("FAIL dbus hold: %h", dbus_rdata_o); errors=errors+1; end

    if (errors==0) $display("ALL PASS");
    else $display("%0d ERRORS", errors);
    $finish;
  end
endmodule
EOF
iverilog -g2012 -o /tmp/sim /tmp/tb.sv dut.sv && vvp /tmp/sim

output

FAIL dbus R+1: 00
FAIL dbus hold: 00
2 ERRORS
/tmp/tb.sv:46: $finish called at 136 (1s)

[stdout]
FAIL dbus R+1: 00
FAIL dbus hold: 00
2 ERRORS
/tmp/tb.sv:46: $finish called at 136 (1s)

FSM behavior all passes; dbus fails. Let me isolate the dbus read to debug.

Isolated dbus test

shell

cat > /tmp/tb2.sv <<'EOF'
module tb;
  logic clk=0, rst;
  logic halt_req_i=0, resume_req_i=0, step_req_i=0, dbus_ren_i=0;
  logic [1:0] dbus_addr_i=0;
  logic [7:0] dbus_rdata_o;
  logic halted_o, fetch_o;
  TopModule dut(.*);
  always #5 clk=~clk;
  initial begin
    rst=1; @(posedge clk); #1; rst=0;
    @(posedge clk); #1;
    dbus_addr_i=2'd1; dbus_ren_i=1;
    @(posedge clk);            // edge R: ren high, addr=1
    dbus_ren_i=0; #1;
    $display("after R+1 (exp C3): %h", dbus_rdata_o);
    @(posedge clk); #1;
    $display("hold  (exp C3): %h", dbus_rdata_o);
    dbus_addr_i=2'd3; dbus_ren_i=1; @(posedge clk); dbus_ren_i=0; #1;
    $display("addr3 (exp 96): %h", dbus_rdata_o);
    $finish;
  end
endmodule
EOF
iverilog -g2012 -o /tmp/sim2 /tmp/tb2.sv dut.sv && vvp /tmp/sim2

output

after R+1 (exp C3): 00
hold  (exp C3): 00
addr3 (exp 96): 00
/tmp/tb2.sv:20: $finish called at 46 (1s)

[stdout]
after R+1 (exp C3): 00
hold  (exp C3): 00
addr3 (exp 96): 00
/tmp/tb2.sv:20: $finish called at 46 (1s)

/app/dut.sv

contents

60	    logic [7:0] rd_next;
61	    always_comb begin
62	        case (dbus_addr_i)
63	            2'd0:    rd_next = 8'h5A;
64	            2'd1:    rd_next = 8'hC3;
65	            2'd2:    rd_next = 8'h0F;
66	            default: rd_next = 8'h96;
67	        endcase
68	    end
69	
70	    always_ff @(posedge clk) begin
71	        if (rst) begin
72	            state        <= S_RESET;
73	            fetch_o      <= 1'b0;
74	            halted_o     <= 1'b0;
75	            dbus_rdata_o <= 8'h00;
76	        end else begin
77	            state    <= next_state;
78	            fetch_o  <= next_fetch;
79	            halted_o <= next_halted;
80	
81	            if (dbus_ren_i)
82	                dbus_rdata_o <= rd_next;      // present new data next edge
83	            else
84	                dbus_rdata_o <= dbus_rdata_o; // hold while ren low
85	        end
86	    end
87	
88	endmodule
89

Monitor dbus internals

shell

cat > /tmp/tb3.sv <<'EOF'
module tb;
  logic clk=0, rst=1;
  logic halt_req_i=0, resume_req_i=0, step_req_i=0, dbus_ren_i=0;
  logic [1:0] dbus_addr_i=0;
  logic [7:0] dbus_rdata_o;
  logic halted_o, fetch_o;
  TopModule dut(.*);
  always #5 clk=~clk;
  initial $monitor("t=%0t rst=%b ren=%b addr=%b rd_next=%h rdata=%h",
                   $time, rst, dbus_ren_i, dbus_addr_i, dut.rd_next, dbus_rdata_o);
  initial begin
    @(posedge clk); #1; rst=0;
    @(posedge clk); #1;
    dbus_addr_i=2'd1; dbus_ren_i=1;
    @(posedge clk); #1; dbus_ren_i=0;
    @(posedge clk); #1;
    $finish;
  end
endmodule
EOF
iverilog -g2012 -o /tmp/sim3 /tmp/tb3.sv dut.sv && vvp /tmp/sim3

output

t=0 rst=1 ren=0 addr=00 rd_next=5a rdata=xx
t=5 rst=1 ren=0 addr=00 rd_next=5a rdata=00
t=6 rst=0 ren=0 addr=00 rd_next=5a rdata=00
t=16 rst=0 ren=1 addr=01 rd_next=c3 rdata=00
t=25 rst=0 ren=1 addr=01 rd_next=c3 rdata=c3
t=26 rst=0 ren=0 addr=01 rd_next=c3 rdata=c3
/tmp/tb3.sv:17: $finish called at 36 (1s)

[stdout]
t=0 rst=1 ren=0 addr=00 rd_next=5a rdata=xx
t=5 rst=1 ren=0 addr=00 rd_next=5a rdata=00
t=6 rst=0 ren=0 addr=00 rd_next=5a rdata=00
t=16 rst=0 ren=1 addr=01 rd_next=c3 rdata=00
t=25 rst=0 ren=1 addr=01 rd_next=c3 rdata=c3
t=26 rst=0 ren=0 addr=01 rd_next=c3 rdata=c3
/tmp/tb3.sv:17: $finish called at 36 (1s)

The DUT is correct , tb3 shows `rdata` becomes `c3` at the read edge and holds. My earlier testbenches had a classic zero-delay race (deasserting `ren` at the same instant as the clock edge). Let me rerun the full functional test with that fixed.

Full functional test with race fixed

shell

cat > /tmp/tb.sv <<'EOF'
module tb;
  logic clk=0, rst;
  logic halt_req_i=0, resume_req_i=0, step_req_i=0, dbus_ren_i=0;
  logic [1:0] dbus_addr_i=0;
  logic [7:0] dbus_rdata_o; logic halted_o, fetch_o;
  TopModule dut(.*);
  always #5 clk=~clk;
  task step; @(posedge clk); #1; endtask
  integer errors=0;
  task chk(input exp_f, input exp_h, input [255:0] msg);
    if (fetch_o!==exp_f || halted_o!==exp_h) begin
      $display("FAIL %0s: fetch=%b(exp %b) halted=%b(exp %b)",msg,fetch_o,exp_f,halted_o,exp_h);
      errors=errors+1; end
  endtask
  initial begin
    rst=1; @(posedge clk); #1; chk(0,0,"reset"); rst=0;
    step; chk(1,0,"run1"); step; chk(1,0,"run2");
    halt_req_i=1; @(posedge clk); #1; halt_req_i=0; chk(1,0,"E+1 fetch high");
    step; chk(0,1,"E+2 halted"); step; chk(0,1,"stay halted");
    step_req_i=1; @(posedge clk); #1; step_req_i=0; chk(1,0,"S+1 fetch pulse");
    step; chk(0,1,"S+2 halted again"); step; chk(0,1,"still halted");
    halt_req_i=1; step_req_i=1; @(posedge clk); #1; halt_req_i=0; step_req_i=0; chk(0,1,"halt+step->halted");
    resume_req_i=1; @(posedge clk); #1; resume_req_i=0; chk(1,0,"G+1 running");
    step; chk(1,0,"running after resume");
    dbus_addr_i=2'd1; dbus_ren_i=1; @(posedge clk); #1; dbus_ren_i=0;
    if(dbus_rdata_o!==8'hC3) begin $display("FAIL dbus R+1: %h",dbus_rdata_o); errors=errors+1; end
    step; if(dbus_rdata_o!==8'hC3) begin $display("FAIL dbus hold: %h",dbus_rdata_o); errors=errors+1; end
    dbus_addr_i=2'd2; dbus_ren_i=1; @(posedge clk); #1; dbus_ren_i=0;
    if(dbus_rdata_o!==8'h0F) begin $display("FAIL dbus addr2: %h",dbus_rdata_o); errors=errors+1; end
    if(errors==0) $display("ALL PASS"); else $display("%0d ERRORS",errors);
    $finish;
  end
endmodule
EOF
iverilog -g2012 -o /tmp/sim /tmp/tb.sv dut.sv && vvp /tmp/sim

output

ALL PASS
/tmp/tb.sv:31: $finish called at 146 (1s)

[stdout]
ALL PASS
/tmp/tb.sv:31: $finish called at 146 (1s)

All checks pass. The module is written to `/app/dut.sv`. ## Summary `TopModule` implements the debug control block as a Moore FSM with fully registered outputs plus an independent registered debug-bus read. **FSM states:** - `S_RESET` → outputs 0 (entered on synchronous reset), then advances to running - `S_RUN` → `fetch_o=1, halted_o=0` - `S_HALT_PEND` → the one extra fetch cycle after a halt request (`fetch=1`), giving halt its 2-edge latency - `S_HALTED` → `fetch_o=0, halted_o=1` - `S_STEP_FETCH` → the single fetch pulse during a step **Key behaviors verified against the spec:** - **Halt:** request at edge E keeps `fetch_o` high through E+1, low from E+2; `halted_o` first high at E+2 (via the pending state). - **Step:** while halted, request at S gives one `fetch_o` pulse at S+1 only, `halted_o` low at S+1 and high again at S+2. - **Resume:** request at G brings `halted_o` low / `fetch_o` high from G+1. - **Halt+Step conflict:** halt dominates in the halted state → stays halted, `fetch_o` stays low. - **Debug read:** 1-cycle registered read (`ren` at R → data at R+1), holds while `ren` low, with the specified ROM contents (0x5A/0xC3/0x0F/0x96). - **Reset:** synchronous, active-high, clears all outputs to 0. A testbench exercising all five behaviors reports `ALL PASS`. (The two "failures" in my first test run were a zero-delay race in the testbench, not the DUT , confirmed by monitoring the internal signals.)

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 dut_present: PASS (/app/dut.sv exists)
TEST dut_forbidden: PASS (no forbidden constructs)
TEST dut_compiles: PASS (compiled with held-back reference/testbench)
TEST simulation_completed: PASS (Mismatches: 563 in 2301 samples)
TEST reference_equivalence: FAIL (563 mismatches in 2301 samples)
{"score": 0.000000, "results_file": "/tmp/h2t-grader-home.l0iaZe/runtime/cb_verifier_344-5667-28740-18380_344/results.jsonl"}

Reproduce this trial: git checkout 2f94510 && PYTHONPATH=src python3 scripts/build_site.py , then open trial/trial_26b0f36deff141fc. Re-running the agent live requires EVAL_PLATFORM_ENABLE_OAUTH_SMOKE=1 and is non-deterministic.

Trial trial_26b0f36deff141fc · verifier authoritative; classifier explanatory.