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

EvidenceVerifier passed: `reference_equivalence PASS ... all 4583 samples matched reference`. Tests compile `/app/dut.sv` with held-back `test.sv` and `ref.sv`, check forbidden constructs, then compare `{grant, grant_valid}` every sampled cycle. Agent wrote a real round-robin arbiter with state such as `ptr`, `hold_active`, `hold_who`, updates like `next_ptr = win + 2'd1`, and lock/dead-air handling; no evidence it read hidden tests or solution.

Root causeThe agent correctly implemented the behavior specified in `instruction.md`, including circular fairness, synchronous reset, lock latching, lock-drop ownership, and dead-air hold release. The verifier exercised behavioral equivalence against the reference over deterministic and randomized stimulus and found zero mismatches.

RecommendationN/A - task is fine

Trajectory

Tool-by-tool agent trajectory

13 tool calls · 3 tool types · 23 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 write the arbiter. Let me trace the key behaviors first, then implement. Key model: combinational grant driven by registered state (scan pointer `ptr`, and a `hold` for locked winners). Reset gates outputs to 0 and restarts the order.

write · /app/dut.sv

module TopModule (
    input  logic       clk,
    input  logic       rst,
    input  logic [3:0] req,
    input  logic       lock,
    output logic [3:0] grant,
    output logic       grant_valid
);

    // Registered state
    logic [1:0] ptr;          // scan starts here (just past last winner)
    logic       hold_active;  // a locked winner is being held
    logic [1:0] hold_who;     // which requester is held

    // Next-state
    logic [1:0] next_ptr;
    logic       next_hold_active;
    logic [1:0] next_hold_who;

    // Combinational outputs (pre reset-gate)
    logic [3:0] grant_c;
    logic       grant_valid_c;

    // Scan results
    logic       found;
    logic [1:0] win;
    logic [1:0] idx;

    always_comb begin
        // circular scan from ptr: first asker wins
        found = 1'b0;
        win   = 2'b00;
        for (int i = 0; i < 4; i++) begin
            idx = ptr + i[1:0];          // 2-bit add wraps mod 4
            if (!found && req[idx]) begin
                found = 1'b1;
                win   = idx;
            end
        end

        // defaults: hold place
        grant_c          = 4'b0000;
        grant_valid_c    = 1'b0;
        next_ptr         = ptr;
        next_hold_active = hold_active;
        next_hold_who    = hold_who;

        if (hold_active) begin
            if (req[hold_who]) begin
                // held winner still asks -> owns this cycle
                grant_c       = (4'b0001 << hold_who);
                grant_valid_c = 1'b1;
                if (lock) begin
                    // hold persists while lock stays high
                    next_hold_active = 1'b1;
                end else begin
                    // lock dropped: this cycle owned, resume next cycle just past it
                    next_hold_active = 1'b0;
                    next_ptr         = hold_who + 2'd1;
                end
            end else begin
                // held req low: dead air, hold over, counts as served
                grant_c          = 4'b0000;
                grant_valid_c    = 1'b0;
                next_hold_active = 1'b0;
                next_ptr         = hold_who + 2'd1;
            end
        end else begin
            if (found) begin
                grant_c       = (4'b0001 << win);
                grant_valid_c = 1'b1;
                next_ptr      = win + 2'd1;
                if (lock) begin
                    // latch this winner (granted cycle + lock)
                    next_hold_active = 1'b1;
                    next_hold_who    = win;
                end else begin
                    next_hold_active = 1'b0;
                end
            end
            // else: idle, keep place (defaults)
        end
    end

    always_ff @(posedge clk) begin
        if (rst) begin
            ptr         <= 2'd0;   // requester 0 wins the first shared cycle
            hold_active <= 1'b0;   // clears any hold
            hold_who    <= 2'd0;
        end else begin
            ptr         <= next_ptr;
            hold_active <= next_hold_active;
            hold_who    <= next_hold_who;
        end
    end

    assign grant       = rst ? 4'b0000 : grant_c;
    assign grant_valid = rst ? 1'b0    : grant_valid_c;

endmodule

Let me verify it compiles under `iverilog -g2012` and sanity-check a few cycles with a quick self-checking bench.

shell

cat > tb_smoke.sv <<'EOF'
module tb;
  logic clk=0, rst, lock;
  logic [3:0] req;
  logic [3:0] grant;
  logic grant_valid;
  TopModule dut(.clk,.rst,.req,.lock,.grant,.grant_valid);
  always #5 clk=~clk;
  task step; @(posedge clk); #1; endtask
  initial begin
    rst=1; req=4'b0000; lock=0; step; step;
    rst=0;
    // all four ask forever, no lock -> 0,1,2,3,0
    req=4'b1111;
    repeat(6) begin $display("grant=%b gv=%b",grant,grant_valid); step; end
    // lone requester keeps winning
    req=4'b0100;
    repeat(3) begin $display("lone grant=%b gv=%b",grant,grant_valid); step; end
    // idle keeps place
    req=4'b0000; step; $display("idle gv=%b",grant_valid);
    req=4'b1111; $display("resume grant=%b",grant); step;
    // lock hold test: grant winner then lock high
    rst=1; step; rst=0;
    req=4'b0010; lock=1; $display("hold t0 grant=%b",grant); step;
    $display("hold t1 grant=%b (others high)",grant); req=4'b1111; step;
    $display("hold t2 grant=%b",grant); step;
    lock=0; $display("drop grant=%b",grant); step;
    $display("after drop grant=%b",grant); step;
    // dead air: lock high, held req drops
    rst=1; step; rst=0; req=4'b0001; lock=1; step; // latch 0
    req=4'b0100; $display("dead grant=%b gv=%b",grant,grant_valid); step;
    $display("resume grant=%b",grant);
    $finish;
  end
endmodule
EOF
iverilog -g2012 -o sim dut.sv tb_smoke.sv && vvp sim

output

dut.sv:29: sorry: constant selects in always_* processes are not currently supported (all bits will be included).
grant=0000 gv=0
grant=0010 gv=1
grant=0100 gv=1
grant=1000 gv=1
grant=0001 gv=1
grant=0010 gv=1
lone grant=0100 gv=1
lone grant=0100 gv=1
lone grant=0100 gv=1
idle gv=0
resume grant=0000
hold t0 grant=0000
hold t1 grant=0010 (others high)
hold t2 grant=0010
drop grant=0010
after drop grant=0100
dead grant=0001 gv=1
resume grant=0100
tb_smoke.sv:32: $finish called at 216 (1s)

[stdout]
dut.sv:29: sorry: constant selects in always_* processes are not currently supported (all bits will be included).
grant=0000 gv=0
grant=0010 gv=1
grant=0100 gv=1
grant=1000 gv=1
grant=0001 gv=1
grant=0010 gv=1
lone grant=0100 gv=1
lone grant=0100 gv=1
lone grant=0100 gv=1
idle gv=0
resume grant=0000
hold t0 grant=0000
hold t1 grant=0010 (others high)
hold t2 grant=0010
drop grant=0010
after drop grant=0100
dead grant=0001 gv=1
resume grant=0100
tb_smoke.sv:32: $finish called at 216 (1s)

The logic is right but iverilog hits a limitation on the part-select `i[1:0]` ("all bits will be included"), which corrupts the scan. Let me remove the part-select , truncation into the 2-bit `idx` already gives mod-4 wrap.

/app/dut.sv

− removed

        for (int i = 0; i < 4; i++) begin
            idx = ptr + i[1:0];          // 2-bit add wraps mod 4

+ added

        for (int i = 0; i < 4; i++) begin
            idx = ptr + i[2:0];          // 2-bit idx truncation wraps mod 4

/app/dut.sv

− removed

        for (int i = 0; i < 4; i++) begin
            idx = ptr + i[2:0];          // 2-bit idx truncation wraps mod 4

+ added

        for (int i = 0; i < 4; i++) begin
            idx = ptr + i[1:0];          // 2-bit idx truncation wraps mod 4

/app/dut.sv

− removed

        found = 1'b0;
        win   = 2'b00;
        for (int i = 0; i < 4; i++) begin
            idx = ptr + i[1:0];          // 2-bit idx truncation wraps mod 4
            if (!found && req[idx]) begin
                found = 1'b1;
                win   = idx;
            end
        end

+ added

        found = 1'b0;
        win   = 2'b00;
        for (int i = 0; i < 4; i = i + 1) begin
            idx = ptr + off[i];          // 2-bit idx truncation wraps mod 4
            if (!found && req[idx]) begin
                found = 1'b1;
                win   = idx;
            end
        end

/app/dut.sv

− removed

        found = 1'b0;
        win   = 2'b00;
        for (int i = 0; i < 4; i = i + 1) begin
            idx = ptr + off[i];          // 2-bit idx truncation wraps mod 4
            if (!found && req[idx]) begin
                found = 1'b1;
                win   = idx;
            end
        end

+ added

        found = 1'b0;
        win   = 2'b00;
        for (int i = 0; i < 4; i = i + 1) begin
            i2  = i[1:0];                // truncate loop var to 2 bits
            idx = ptr + i2;             // 2-bit add wraps mod 4
            if (!found && req[idx]) begin
                found = 1'b1;
                win   = idx;
            end
        end

/app/dut.sv

− removed

            i2  = i[1:0];                // truncate loop var to 2 bits
            idx = ptr + i2;             // 2-bit add wraps mod 4

+ added

            i2  = i;                     // truncate loop var to 2 bits
            idx = ptr + i2;              // 2-bit add wraps mod 4

Now add the `i2` declaration and re-test.

/app/dut.sv

− removed

    logic [1:0] win;
    logic [1:0] idx;

+ added

    logic [1:0] win;
    logic [1:0] idx;
    logic [1:0] i2;

shell

iverilog -g2012 -o sim dut.sv tb_smoke.sv && vvp sim

output

grant=0000 gv=0
grant=0010 gv=1
grant=0100 gv=1
grant=1000 gv=1
grant=0001 gv=1
grant=0010 gv=1
lone grant=0100 gv=1
lone grant=0100 gv=1
lone grant=0100 gv=1
idle gv=0
resume grant=0000
hold t0 grant=0000
hold t1 grant=0010 (others high)
hold t2 grant=0010
drop grant=0010
after drop grant=0100
dead grant=0001 gv=1
resume grant=0100
tb_smoke.sv:32: $finish called at 216 (1s)

[stdout]
grant=0000 gv=0
grant=0010 gv=1
grant=0100 gv=1
grant=1000 gv=1
grant=0001 gv=1
grant=0010 gv=1
lone grant=0100 gv=1
lone grant=0100 gv=1
lone grant=0100 gv=1
idle gv=0
resume grant=0000
hold t0 grant=0000
hold t1 grant=0010 (others high)
hold t2 grant=0010
drop grant=0010
after drop grant=0100
dead grant=0001 gv=1
resume grant=0100
tb_smoke.sv:32: $finish called at 216 (1s)

Compiles clean now. The `0000` first-lines are a testbench read race (sampling combinational output in the same delta as driving inputs), not a DUT bug , the round-robin `1,2,3,0,1`, lone-winner, hold, drop, and dead-air sequences are all correct. Let me confirm with a race-free bench that samples inputs/outputs at consistent points.

shell

cat > tb2.sv <<'EOF'
module tb;
  logic clk=0, rst, lock; logic [3:0] req, grant; logic grant_valid;
  TopModule dut(.clk,.rst,.req,.lock,.grant,.grant_valid);
  always #5 clk=~clk;
  int errors=0;
  // drive inputs just after negedge, sample just before posedge
  task drive(input [3:0] r, input l, input rs); @(negedge clk); req=r; lock=l; rst=rs; endtask
  task chk(input [3:0] eg, input egv, input [127:0] tag);
    @(posedge clk); #0;
    if (grant!==eg || grant_valid!==egv) begin
      $display("FAIL %0s: grant=%b gv=%b exp %b %b",tag,grant,grant_valid,eg,egv); errors++;
    end else $display("ok   %0s: grant=%b gv=%b",tag,grant,grant_valid);
  endtask
  initial begin
    // reset
    drive(4'b0000,0,1); chk(4'b0000,0,"rst");
    // round robin 0,1,2,3,0
    drive(4'b1111,0,0); chk(4'b0001,1,"rr0");
    drive(4'b1111,0,0); chk(4'b0010,1,"rr1");
    drive(4'b1111,0,0); chk(4'b0100,1,"rr2");
    drive(4'b1111,0,0); chk(4'b1000,1,"rr3");
    drive(4'b1111,0,0); chk(4'b0001,1,"rr0b");
    // lone winner 2
    drive(4'b0100,0,0); chk(4'b0100,1,"lone");
    drive(4'b0100,0,0); chk(4'b0100,1,"lone2");
    // idle keeps place (last winner was 2 -> ptr=3)
    drive(4'b0000,0,0); chk(4'b0000,0,"idle");
    // resume: scan from 3: only req0 -> after 3 comes 0
    drive(4'b0001,0,0); chk(4'b0001,1,"resume0");
    // lock hold on 1
    drive(4'b1111,0,1); chk(4'b0010,1,"lk_start1"); // ptr was 1 (past 0)
    drive(4'b1111,0,1); chk(4'b0010,1,"lk_hold");   // others high, locked to 1
    drive(4'b1101,0,1); chk(4'b0010,1,"lk_req1_low_others"); // req1 still high
    drive(4'b1111,0,0); chk(4'b0010,1,"lk_drop_owns");        // lock drops, still owns
    drive(4'b1111,0,0); chk(4'b0100,1,"resume_past1");        // resume just past 1 -> 2
    // dead air: latch 0 with lock, then req0 low while lock high
    drive(4'b0001,0,0); // clear to known: winner0, ptr->1
    @(posedge clk);
    drive(4'b0010,0,1); // scan from 2? ptr after 0 =1 -> winner1 latched
    @(posedge clk);
    drive(4'b0000,0,1); chk(4'b0000,0,"deadair"); // held(1) req low -> dead
    drive(4'b0100,0,0); chk(4'b0100,1,"resume_past_dead"); // past dead 1 ->2
    $display("errors=%0d",errors); $finish;
  end
endmodule
EOF
iverilog -g2012 -o sim2 dut.sv tb2.sv && vvp sim2

output

ok   rst: grant=0000 gv=0
ok   rr0: grant=0001 gv=1
ok   rr1: grant=0010 gv=1
ok   rr2: grant=0100 gv=1
ok   rr3: grant=1000 gv=1
ok   rr0b: grant=0001 gv=1
ok   lone: grant=0100 gv=1
ok   lone2: grant=0100 gv=1
ok   idle: grant=0000 gv=0
ok   resume0: grant=0001 gv=1
FAIL lk_start1: grant=0000 gv=0 exp 0010 1
FAIL lk_hold: grant=0000 gv=0 exp 0010 1
FAIL _req1_low_others: grant=0000 gv=0 exp 0010 1
FAIL lk_drop_owns: grant=0001 gv=1 exp 0010 1
FAIL resume_past1: grant=0010 gv=1 exp 0100 1
ok   deadair: grant=0000 gv=0
ok   resume_past_dead: grant=0100 gv=1
errors=5
tb2.sv:43: $finish called at 195 (1s)

[stdout]
ok   rst: grant=0000 gv=0
ok   rr0: grant=0001 gv=1
ok   rr1: grant=0010 gv=1
ok   rr2: grant=0100 gv=1
ok   rr3: grant=1000 gv=1
ok   rr0b: grant=0001 gv=1
ok   lone: grant=0100 gv=1
ok   lone2: grant=0100 gv=1
ok   idle: grant=0000 gv=0
ok   resume0: grant=0001 gv=1
FAIL lk_start1: grant=0000 gv=0 exp 0010 1
FAIL lk_hold: grant=0000 gv=0 exp 0010 1
FAIL _req1_low_others: grant=0000 gv=0 exp 0010 1
FAIL lk_drop_owns: grant=0001 gv=1 exp 0010 1
FAIL resume_past1: grant=0010 gv=1 exp 0100 1
ok   deadair: grant=0000 gv=0
ok   resume_past_dead: grant=0100 gv=1
errors=5
tb2.sv:43: $finish called at 195 (1s)

The 5 failures are a bug in my **test bench**, not the DUT: my `drive(req, lock, rst)` calls for the lock section passed `1` as the third arg (`rst`) instead of setting `lock`. So `rst=1` gated grant to 0. The first tb (with explicit `lock=1`) already showed correct hold behavior. Let me fix the arg order and re-run.

shell

cat > tb3.sv <<'EOF'
module tb;
  logic clk=0, rst, lock; logic [3:0] req, grant; logic grant_valid;
  TopModule dut(.clk,.rst,.req,.lock,.grant,.grant_valid);
  always #5 clk=~clk;
  int errors=0;
  task drive(input [3:0] r, input l, input rs); @(negedge clk); req=r; lock=l; rst=rs; endtask
  task chk(input [3:0] eg, input egv, input [199:0] tag);
    @(posedge clk); #0;
    if (grant!==eg || grant_valid!==egv) begin
      $display("FAIL %0s: grant=%b gv=%b exp %b %b",tag,grant,grant_valid,eg,egv); errors++;
    end else $display("ok   %0s: grant=%b gv=%b",tag,grant,grant_valid);
  endtask
  initial begin
    drive(4'b0000,0,1); chk(4'b0000,0,"rst");
    drive(4'b1111,0,0); chk(4'b0001,1,"rr0"); // ptr->1
    // lock hold on 1
    drive(4'b1111,1,0); chk(4'b0010,1,"lk_start1");         // win1, latch (lock)
    drive(4'b1111,1,0); chk(4'b0010,1,"lk_hold");           // locked to 1
    drive(4'b1101,1,0); chk(4'b0010,1,"lk_others_low");     // req1 high, held
    drive(4'b1111,0,0); chk(4'b0010,1,"lk_drop_owns");      // lock drops, still owns; ptr->2
    drive(4'b1111,0,0); chk(4'b0100,1,"resume_past1");      // resume just past 1 -> 2; ptr->3
    // dead air with lock still high
    drive(4'b1000,1,0); chk(4'b1000,1,"latch3");            // scan from3: win3, latch (lock)
    drive(4'b0100,1,0); chk(4'b0000,0,"deadair");           // held3 req low -> dead air; ptr->0
    drive(4'b0100,1,0); chk(4'b0100,1,"reLatch2");          // lock still high, latch next winner 2 (from ptr0: req2)
    drive(4'b0000,1,0); chk(4'b0000,0,"dead_again");        // held2 req low -> dead; ptr->3
    drive(4'b1111,0,0); chk(4'b1000,1,"resume_past2");      // resume from3 -> win3
    // reset clears hold mid-lock
    drive(4'b0001,1,0); chk(4'b0001,1,"latch0b");           // ptr was 0 (past3): win0 latch
    drive(4'b0001,1,1); chk(4'b0000,0,"rst_clears");        // rst gates + clears hold, ptr->0
    drive(4'b1111,0,0); chk(4'b0001,1,"after_rst_0wins");   // order restarted: 0 wins
    // lock with no grant does nothing
    drive(4'b0000,1,0); chk(4'b0000,0,"lock_no_grant");
    drive(4'b1111,0,0); chk(4'b0010,1,"noHoldFromEmpty");   // ptr->1 (past0 from after_rst); no latch happened
    $display("errors=%0d",errors); $finish;
  end
endmodule
EOF
iverilog -g2012 -o sim3 dut.sv tb3.sv && vvp sim3

output

ok   rst: grant=0000 gv=0
ok   rr0: grant=0001 gv=1
ok   lk_start1: grant=0010 gv=1
ok   lk_hold: grant=0010 gv=1
FAIL lk_others_low: grant=0000 gv=0 exp 0010 1
FAIL lk_drop_owns: grant=0100 gv=1 exp 0010 1
FAIL resume_past1: grant=1000 gv=1 exp 0100 1
ok   latch3: grant=1000 gv=1
ok   deadair: grant=0000 gv=0
ok   reLatch2: grant=0100 gv=1
ok   dead_again: grant=0000 gv=0
ok   resume_past2: grant=1000 gv=1
ok   latch0b: grant=0001 gv=1
ok   rst_clears: grant=0000 gv=0
ok   after_rst_0wins: grant=0001 gv=1
ok   lock_no_grant: grant=0000 gv=0
ok   noHoldFromEmpty: grant=0010 gv=1
errors=3
tb3.sv:35: $finish called at 175 (1s)

[stdout]
ok   rst: grant=0000 gv=0
ok   rr0: grant=0001 gv=1
ok   lk_start1: grant=0010 gv=1
ok   lk_hold: grant=0010 gv=1
FAIL lk_others_low: grant=0000 gv=0 exp 0010 1
FAIL lk_drop_owns: grant=0100 gv=1 exp 0010 1
FAIL resume_past1: grant=1000 gv=1 exp 0100 1
ok   latch3: grant=1000 gv=1
ok   deadair: grant=0000 gv=0
ok   reLatch2: grant=0100 gv=1
ok   dead_again: grant=0000 gv=0
ok   resume_past2: grant=1000 gv=1
ok   latch0b: grant=0001 gv=1
ok   rst_clears: grant=0000 gv=0
ok   after_rst_0wins: grant=0001 gv=1
ok   lock_no_grant: grant=0000 gv=0
ok   noHoldFromEmpty: grant=0010 gv=1
errors=3
tb3.sv:35: $finish called at 175 (1s)

Two are my tb typo again: I wrote `4'b1101` (which has `req[1]=0`) when I meant "req[1] still high, others low" , so the DUT correctly produced dead air, which cascaded my hand-computed expectations off by one. That's correct DUT behavior. Let me fix that one stimulus line (`req[1]` high) so expectations realign.

shell

sed -i 's|drive(4.b1101,1,0); chk(4.b0010,1,"lk_others_low");.*|drive(4'"'"'b0010,1,0); chk(4'"'"'b0010,1,"lk_others_low");|' tb3.sv
iverilog -g2012 -o sim3 dut.sv tb3.sv && vvp sim3 | grep -E "FAIL|errors"

output

errors=0

[stdout]
errors=0

All pass. Final clean compile check with the grader's flags:

shell

iverilog -g2012 -o /dev/null dut.sv && echo "iverilog -g2012: clean compile"
grep -nE "\$|force|release|\`" dut.sv | grep -v "grant\$\|valid\$" || echo "no system tasks / preprocessor / force-release"
LC_ALL=C grep -nP '[^\x00-\x7F]' dut.sv && echo "NON-ASCII FOUND" || echo "ascii-only OK"

output

iverilog -g2012: clean compile
no system tasks / preprocessor / force-release
ascii-only OK

[stdout]
iverilog -g2012: clean compile
no system tasks / preprocessor / force-release
ascii-only OK

Done. `/app/dut.sv` compiles cleanly under `iverilog -g2012`, is ASCII-only, and uses no system tasks, preprocessor directives, or force/release. ## Design The arbiter keeps three registers of state: - `ptr` , where the circular scan starts (just past the last winner) - `hold_active` / `hold_who` , a latched, locked winner **Combinational output** = function of registered state + current `req` (deliberately *not* of current `lock`, so a held winner "still owns the cycle" when lock drops). `lock` only steers the *next-state transition*. Behavior covered and tested (all cases pass in `tb3.sv`, `errors=0`): - **Start rule**: reset makes `ptr=0`, so the first all-ask gives requester 0. - **Circular fairness**: `1111` forever → `0,1,2,3,0,1,…`; lone requester keeps winning; idle leaves `ptr` untouched. - **Lock hold**: a granted cycle with `lock` high latches that winner; grant stays nailed to it, `grant_valid` high, others wait, while lock stays high. Lock with no grant latches nothing. - **Lock drop**: held winner owns the drop cycle; sharing resumes next cycle from just past it. - **Dead air**: a held cycle with its `req` low (even the drop cycle) → `grant=0`, `grant_valid=0`, hold over, counts as served (`ptr` advances past it). A still-high lock needs no re-arm , it re-latches the next real winner by the start rule. - **Reset**: synchronous, active-high , gates outputs to 0, clears any hold, restarts the order. Note: the stray `0000` first-lines you'd see in a naive bench are a read race (sampling the combinational output in the same delta as the input change), not DUT behavior , the race-free bench confirms correctness.

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.HjNBHH/runtime/cb_verifier_349-2969-5541-19315_349/results.jsonl"}

Reproduce this trial: git checkout 2f94510 && PYTHONPATH=src python3 scripts/build_site.py , then open trial/trial_2b93dc7346a24a91. Re-running the agent live requires EVAL_PLATFORM_ENABLE_OAUTH_SMOKE=1 and is non-deterministic.

Trial trial_2b93dc7346a24a91 · verifier authoritative; classifier explanatory.