SyncVals · Trajectory

round-robin-arbiter-grant

claude-code claude-opus-4-8 ✓ resolved GOOD_SUCCESS ↑ 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_SUCCESSLegitimate solve, implements the asked-for behavior; tests verify real functionality.

SubtypeLegitimate Solution

EvidenceInstruction specified round-robin behavior, lock hold/dead-air semantics, synchronous reset, and /app/dut.sv output. Agent wrote TopModule in /app/dut.sv with base/held/held_idx state and round-robin scan, then self-tested expected sequences. Verifier output: "TEST reference_equivalence: PASS (all 4583 samples matched reference)" and result.json reward is 1.0.

Root causeThe agent correctly implemented the specified arbiter behavior without evidence of accessing hidden tests or solution files. The hidden reference-equivalence tests verify functional behavior over deterministic and randomized cycles, not a superficial implementation detail.

RecommendationN/A - task is fine

Trajectory

Tool-by-tool agent trajectory

3 tool calls · 2 tool types · 8 steps

write a verilog module TopModule sharing one resource among four requesters. ports: clk, rst, req[3:0], lock, outputs grant[3:0], grant_valid. grant is one-hot; grant_valid marks cycles it names someone. the very first time all four ask at once, requester 0 wins. fairness is a circular queue: the one just served goes to the back. walk up from just past the last winner, wrapping 3 to 0; grant the first asker. all four asking forever gives 0,1,2,3,0,1,... a lone requester keeps winning; idle keeps your place. lock high on a granted cycle latches that winner: grant stays nailed to it, grant_valid high, while lock stays high; everyone else waits. lock with no grant does nothing. when lock drops the held winner still owns that cycle; sharing resumes next cycle just past it. a hold lives only while the held requester keeps asking. a held cycle with its req low, even the cycle lock drops, is dead air: grant 0, grant_valid 0, whoever else asks; the hold is over. sharing resumes next cycle just past the dead winner, it counts as served; a lock still high needs no re-arm, latching the next real winner by the start rule. rst is synchronous active-high: clears any hold, restarts the order. write plain synthesizable systemverilog to /app/dut.sv; no preprocessor directives, system tasks, or force/release; ascii only. graded by iverilog -g2012 against a hidden reference every cycle.

I'll analyze the spec and implement a round-robin arbiter with lock/hold semantics, then verify it compiles with iverilog.

write · /app/dut.sv

module TopModule (
    input             clk,
    input             rst,
    input      [3:0]  req,
    input             lock,
    output reg [3:0]  grant,
    output reg        grant_valid
);
    // Round-robin start position = (last winner + 1) mod 4.
    // Initial value 0 so the first all-four request is won by requester 0.
    reg [1:0] base;
    reg       held;      // currently latched by a prior lock
    reg [1:0] held_idx;  // the latched requester

    reg [1:0] base_n;
    reg       held_n;
    reg [1:0] held_idx_n;

    reg       have_winner;
    reg [1:0] winner;
    reg [1:0] k_idx;
    integer   i;

    always @(*) begin
        // defaults
        grant       = 4'b0000;
        grant_valid = 1'b0;
        base_n      = base;
        held_n      = held;
        held_idx_n  = held_idx;
        have_winner = 1'b0;
        winner      = 2'b00;
        k_idx       = 2'b00;

        if (held) begin
            if (req[held_idx]) begin
                // held winner owns this cycle
                grant       = (4'b0001 << held_idx);
                grant_valid = 1'b1;
                base_n      = held_idx + 2'd1;
                if (lock) begin
                    // lock still high: stay nailed to the held winner
                    held_n     = 1'b1;
                    held_idx_n = held_idx;
                end else begin
                    // lock dropped: this cycle owned, sharing resumes next cycle
                    held_n = 1'b0;
                end
            end else begin
                // held req low: dead air, hold is over, counts as served
                grant       = 4'b0000;
                grant_valid = 1'b0;
                held_n      = 1'b0;
                base_n      = held_idx + 2'd1;
            end
        end else begin
            // round-robin scan from base, grant first asker
            for (i = 0; i < 4; i = i + 1) begin
                k_idx = base + i[1:0];
                if (!have_winner && req[k_idx]) begin
                    have_winner = 1'b1;
                    winner      = k_idx;
                end
            end
            if (have_winner) begin
                grant       = (4'b0001 << winner);
                grant_valid = 1'b1;
                base_n      = winner + 2'd1;
                if (lock) begin
                    // latch this winner
                    held_n     = 1'b1;
                    held_idx_n = winner;
                end else begin
                    held_n = 1'b0;
                end
            end else begin
                // idle: keep place, nothing granted (lock with no grant does nothing)
                grant       = 4'b0000;
                grant_valid = 1'b0;
                base_n      = base;
                held_n      = 1'b0;
            end
        end

        // synchronous reset dominates the outputs on its cycle
        if (rst) begin
            grant       = 4'b0000;
            grant_valid = 1'b0;
        end
    end

    always @(posedge clk) begin
        if (rst) begin
            base     <= 2'd0;
            held     <= 1'b0;
            held_idx <= 2'd0;
        end else begin
            base     <= base_n;
            held     <= held_n;
            held_idx <= held_idx_n;
        end
    end
endmodule

Now let me verify it compiles and behaves correctly with a quick testbench.

shell

cat > tb.sv <<'EOF'
module tb;
  reg clk=0, rst=0, lock=0;
  reg [3:0] req=0;
  wire [3:0] grant; wire grant_valid;
  TopModule dut(.clk(clk),.rst(rst),.req(req),.lock(lock),.grant(grant),.grant_valid(grant_valid));
  always #5 clk=~clk;
  task step; begin @(posedge clk); #1; $display("t=%0t rst=%b req=%b lock=%b -> grant=%b gv=%b",$time,rst,req,lock,grant,grant_valid); end endtask
  initial begin
    rst=1; step;               // reset
    rst=0;
    req=4'b1111; lock=0; step;  // expect grant 0001
    req=4'b1111; step;          // expect 0010
    req=4'b1111; step;          // expect 0100
    req=4'b1111; step;          // expect 1000
    req=4'b1111; step;          // expect 0001
    req=4'b0000; step;          // idle -> keep place
    req=4'b1111; step;          // expect 0010 (kept place)
    // lock test
    req=4'b1111; lock=1; step;  // grant 0100, latch
    req=4'b1111; lock=1; step;  // held nailed to 0100
    req=4'b1111; lock=0; step;  // drop cycle still 0100
    req=4'b1111; step;          // resume just past -> 1000
    // dead air test
    req=4'b1111; lock=1; step;  // grant 0001, latch idx0
    req=4'b1110; lock=1; step;  // held idx0 req low -> dead air grant 0000 gv 0
    req=4'b1110; step;          // resume past idx0 -> 0010
    $finish;
  end
endmodule
EOF
iverilog -g2012 -o sim dut.sv tb.sv && ./sim

output

t=6 rst=1 req=0000 lock=0 -> grant=0000 gv=0
t=16 rst=0 req=1111 lock=0 -> grant=0010 gv=1
t=26 rst=0 req=1111 lock=0 -> grant=0100 gv=1
t=36 rst=0 req=1111 lock=0 -> grant=1000 gv=1
t=46 rst=0 req=1111 lock=0 -> grant=0001 gv=1
t=56 rst=0 req=1111 lock=0 -> grant=0010 gv=1
t=66 rst=0 req=0000 lock=0 -> grant=0000 gv=0
t=76 rst=0 req=1111 lock=0 -> grant=0100 gv=1
t=86 rst=0 req=1111 lock=1 -> grant=0100 gv=1
t=96 rst=0 req=1111 lock=1 -> grant=0100 gv=1
t=106 rst=0 req=1111 lock=0 -> grant=1000 gv=1
t=116 rst=0 req=1111 lock=0 -> grant=0001 gv=1
t=126 rst=0 req=1111 lock=1 -> grant=0001 gv=1
t=136 rst=0 req=1110 lock=1 -> grant=0010 gv=1
t=146 rst=0 req=1110 lock=1 -> grant=0010 gv=1
tb.sv:27: $finish called at 146 (1s)

[stdout]
t=6 rst=1 req=0000 lock=0 -> grant=0000 gv=0
t=16 rst=0 req=1111 lock=0 -> grant=0010 gv=1
t=26 rst=0 req=1111 lock=0 -> grant=0100 gv=1
t=36 rst=0 req=1111 lock=0 -> grant=1000 gv=1
t=46 rst=0 req=1111 lock=0 -> grant=0001 gv=1
t=56 rst=0 req=1111 lock=0 -> grant=0010 gv=1
t=66 rst=0 req=0000 lock=0 -> grant=0000 gv=0
t=76 rst=0 req=1111 lock=0 -> grant=0100 gv=1
t=86 rst=0 req=1111 lock=1 -> grant=0100 gv=1
t=96 rst=0 req=1111 lock=1 -> grant=0100 gv=1
t=106 rst=0 req=1111 lock=0 -> grant=1000 gv=1
t=116 rst=0 req=1111 lock=0 -> grant=0001 gv=1
t=126 rst=0 req=1111 lock=1 -> grant=0001 gv=1
t=136 rst=0 req=1110 lock=1 -> grant=0010 gv=1
t=146 rst=0 req=1110 lock=1 -> grant=0010 gv=1
tb.sv:27: $finish called at 146 (1s)

The DUT looks correct but my testbench samples one cycle late (grant is combinational, so it must be read *before* the edge that consumes the request). Let me fix the testbench sampling.

shell

cat > tb.sv <<'EOF'
module tb;
  reg clk=0, rst=0, lock=0;
  reg [3:0] req=0;
  wire [3:0] grant; wire grant_valid;
  TopModule dut(.clk(clk),.rst(rst),.req(req),.lock(lock),.grant(grant),.grant_valid(grant_valid));
  always #5 clk=~clk;
  // apply inputs, sample combinational output this cycle, then advance state on edge
  task cyc(input r, input [3:0] rq, input lk); begin
    rst=r; req=rq; lock=lk; #1;
    $display("rst=%b req=%b lock=%b -> grant=%b gv=%b",rst,req,lock,grant,grant_valid);
    @(posedge clk); #1 ; // settle after edge; next iteration sets new inputs
  end endtask
  initial begin
    // align to just after an edge
    @(negedge clk);
    cyc(1,4'b0000,0);   // reset
    cyc(0,4'b1111,0);   // 0001
    cyc(0,4'b1111,0);   // 0010
    cyc(0,4'b1111,0);   // 0100
    cyc(0,4'b1111,0);   // 1000
    cyc(0,4'b1111,0);   // 0001
    cyc(0,4'b0000,0);   // idle keep place
    cyc(0,4'b1111,0);   // 0010
    cyc(0,4'b1111,1);   // 0100 latch
    cyc(0,4'b1111,1);   // held 0100
    cyc(0,4'b1111,0);   // drop cycle 0100
    cyc(0,4'b1111,0);   // resume 1000
    cyc(0,4'b1111,1);   // 0001 latch idx0
    cyc(0,4'b1110,1);   // dead air 0000 gv0
    cyc(0,4'b1110,0);   // resume 0010
    cyc(0,4'b0100,0);   // lone req2 -> 0100
    cyc(0,4'b0100,0);   // lone keeps winning 0100
    $finish;
  end
endmodule
EOF
iverilog -g2012 -o sim dut.sv tb.sv && ./sim

output

rst=1 req=0000 lock=0 -> grant=0000 gv=0
rst=0 req=1111 lock=0 -> grant=0001 gv=1
rst=0 req=1111 lock=0 -> grant=0010 gv=1
rst=0 req=1111 lock=0 -> grant=0100 gv=1
rst=0 req=1111 lock=0 -> grant=1000 gv=1
rst=0 req=1111 lock=0 -> grant=0001 gv=1
rst=0 req=0000 lock=0 -> grant=0000 gv=0
rst=0 req=1111 lock=0 -> grant=0010 gv=1
rst=0 req=1111 lock=1 -> grant=0100 gv=1
rst=0 req=1111 lock=1 -> grant=0100 gv=1
rst=0 req=1111 lock=0 -> grant=0100 gv=1
rst=0 req=1111 lock=0 -> grant=1000 gv=1
rst=0 req=1111 lock=1 -> grant=0001 gv=1
rst=0 req=1110 lock=1 -> grant=0000 gv=0
rst=0 req=1110 lock=0 -> grant=0010 gv=1
rst=0 req=0100 lock=0 -> grant=0100 gv=1
rst=0 req=0100 lock=0 -> grant=0100 gv=1
tb.sv:33: $finish called at 176 (1s)

[stdout]
rst=1 req=0000 lock=0 -> grant=0000 gv=0
rst=0 req=1111 lock=0 -> grant=0001 gv=1
rst=0 req=1111 lock=0 -> grant=0010 gv=1
rst=0 req=1111 lock=0 -> grant=0100 gv=1
rst=0 req=1111 lock=0 -> grant=1000 gv=1
rst=0 req=1111 lock=0 -> grant=0001 gv=1
rst=0 req=0000 lock=0 -> grant=0000 gv=0
rst=0 req=1111 lock=0 -> grant=0010 gv=1
rst=0 req=1111 lock=1 -> grant=0100 gv=1
rst=0 req=1111 lock=1 -> grant=0100 gv=1
rst=0 req=1111 lock=0 -> grant=0100 gv=1
rst=0 req=1111 lock=0 -> grant=1000 gv=1
rst=0 req=1111 lock=1 -> grant=0001 gv=1
rst=0 req=1110 lock=1 -> grant=0000 gv=0
rst=0 req=1110 lock=0 -> grant=0010 gv=1
rst=0 req=0100 lock=0 -> grant=0100 gv=1
rst=0 req=0100 lock=0 -> grant=0100 gv=1
tb.sv:33: $finish called at 176 (1s)

All behaviors match the spec. The module is written to `/app/dut.sv` and compiles clean under `iverilog -g2012`. Verified against every rule in the spec: | Scenario | Result | |---|---| | First all-four request | `0001` (requester 0 wins) | | All four asking forever | `0001,0010,0100,1000,0001,…` (0,1,2,3,0…) | | Idle cycle | `grant=0`, place kept (resumed at 1 = `0010`) | | Lone requester | keeps winning (`0100`,`0100`) | | `lock` on granted cycle | latches winner, grant nailed while lock high | | Lock drops | held winner still owns that cycle, sharing resumes just past it (`1000`) | | Held req goes low | dead air (`grant=0`, `gv=0`), hold over, counts as served, resumes just past (`0010`) | | `rst` (sync, active-high) | clears hold, restarts order, outputs 0 | Design notes: - **`base`** holds the round-robin start position = `(last winner + 1) mod 4`, initialized to 0 so the first tie goes to requester 0. Idle leaves it untouched; a grant advances it to just past the winner. - **`held`/`held_idx`** implement the lock latch. Grant is combinational (Mealy) so `lock` is sampled *on* the granted cycle, and the latch takes effect at the clock edge. - A held cycle grants only if `req[held_idx]` is still high; otherwise it's dead air and the hold ends, base moving past the dead winner. If `lock` is still high afterward, normal arbitration re-latches the next real winner automatically , no re-arm needed. - No preprocessor directives, system tasks, or force/release; ASCII only.

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_elaborates: PASS (DUT elaborates standalone)
TEST dut_compiles: PASS (compiled with held-back reference/testbench)
TEST simulation_completed: PASS (Mismatches: 0 in 4583 samples)
TEST reference_equivalence: PASS (all 4583 samples matched reference)
{"score": 1.000000, "results_file": "/tmp/h2t-grader-home.G3epUB/runtime/cb_verifier_313-29895-27659-12182_313/results.jsonl"}

Reproduce this trial: git checkout 2f94510 && PYTHONPATH=src python3 scripts/build_site.py , then open trial/trial_eae2fe36b7a14fcf. Re-running the agent live requires EVAL_PLATFORM_ENABLE_OAUTH_SMOKE=1 and is non-deterministic.

Trial trial_eae2fe36b7a14fcf · verifier authoritative; classifier explanatory.