SyncValsverifier → artifact → classifier → verdict
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 output reports `TEST reference_equivalence: PASS (all 4583 samples matched reference)` and `reward=1.0`. The agent trajectory shows it independently wrote `/app/dut.sv`, debugged local timing/iverilog issues, and verified round-robin, lock hold, dead-air, reset, and idle behavior without reading hidden `tests/` or `solution/` artifacts.
Root causeThe task was well-specified: the instruction described the required round-robin and lock semantics, and the hidden tests checked behavioral equivalence to a reference over deterministic and randomized scenarios. The agent implemented a legitimate arbiter matching those requirements.
RecommendationN/A - task is fine
Trajectory
Tool-by-tool agent trajectory
14 tool calls · 2 tool types · 28 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 design this arbiter carefully, then write it. Let me reason through the state machine before writing. **State:** `ptr` (scan start = just past last winner, init 0 so first all-ask → 0 wins), `held`/`held_id` (active lock hold). Outputs are combinational (same-cycle response to req/lock → Mealy), reset only affects state.
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
);

    // Circular-queue arbiter state.
    // ptr     : position to begin scanning (just past the last winner)
    // held    : a locked hold is active from a previous granted cycle
    // held_id : the requester the hold is nailed to
    logic [1:0] ptr;
    logic       held;
    logic [1:0] held_id;

    // next-state
    logic [1:0] ptr_n;
    logic       held_n;
    logic [1:0] held_id_n;

    // Scan up from ptr, wrapping 3->0, granting the first asker.
    logic       found;
    logic [1:0] winner;
    always_comb begin
        found  = 1'b0;
        winner = 2'b00;
        for (int i = 0; i < 4; i = i + 1) begin
            if (!found && req[ptr + i[1:0]]) begin
                found  = 1'b1;
                winner = ptr + i[1:0];
            end
        end
    end

    // Outputs (Mealy) and next state.
    always_comb begin
        grant       = 4'b0000;
        grant_valid = 1'b0;
        ptr_n       = ptr;
        held_n      = 1'b0;
        held_id_n   = held_id;

        if (held) begin
            if (req[held_id]) begin
                // Held requester keeps asking: grant stays nailed to it.
                grant       = 4'b0001 << held_id;
                grant_valid = 1'b1;
                ptr_n       = held_id + 2'd1;
                if (lock) begin
                    // Lock still high: hold continues.
                    held_n    = 1'b1;
                    held_id_n = held_id;
                end else begin
                    // Lock dropped: this cycle still owned, sharing resumes next.
                    held_n = 1'b0;
                end
            end else begin
                // Held requester dropped its req: dead air, hold is over.
                // Counts as served; sharing resumes just past it.
                grant       = 4'b0000;
                grant_valid = 1'b0;
                held_n      = 1'b0;
                ptr_n       = held_id + 2'd1;
            end
        end else begin
            // Normal arbitration by the start rule.
            if (found) begin
                grant       = 4'b0001 << winner;
                grant_valid = 1'b1;
                ptr_n       = winner + 2'd1;
                if (lock) begin
                    // Lock high on a granted cycle latches this winner.
                    held_n    = 1'b1;
                    held_id_n = winner;
                end
            end else begin
                // No asker: idle keeps your place (lock with no grant does nothing).
                grant_valid = 1'b0;
                ptr_n       = ptr;
            end
        end
    end

    always_ff @(posedge clk) begin
        if (rst) begin
            // Synchronous: clear any hold, restart the order.
            ptr     <= 2'd0;
            held    <= 1'b0;
            held_id <= 2'd0;
        end else begin
            ptr     <= ptr_n;
            held    <= held_n;
            held_id <= held_id_n;
        end
    end

endmodule
Let me verify it compiles with iverilog and passes a quick sanity testbench.
shell
cat > tb_sanity.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; $display("req=%b lock=%b -> grant=%b gv=%b", req,lock,grant,grant_valid); endtask
  initial begin
    rst=1; req=0; lock=0; @(posedge clk); #1;
    rst=0;
    $display("-- all four forever (expect 0,1,2,3,0):");
    req=4'b1111; lock=0;
    step; step; step; step; step;
    $display("-- lone requester 2 keeps winning:");
    req=4'b0100; step; step;
    $display("-- lock hold on req1: latch then nail while lock high:");
    req=4'b1111; lock=1; step;   // grants someone, latches
    step; step;                   // nailed to that winner
    $display("-- drop lock, req still high (owns cycle then resume):");
    lock=0; step; step;
    $display("-- dead air: hold then held req goes low:");
    req=4'b1111; lock=1; step;    // latch a winner w
    // now drop that winner's req: need to know w; just drop all but keep others
    lock=1; req=4'b1111; step;    // nailed
    $finish;
  end
endmodule
EOF
iverilog -g2012 -o sim dut.sv tb_sanity.sv && vvp sim
output
dut.sv:26: sorry: constant selects in always_* processes are not currently supported (all bits will be included).
-- all four forever (expect 0,1,2,3,0):
req=1111 lock=0 -> grant=0010 gv=1
req=1111 lock=0 -> grant=0100 gv=1
req=1111 lock=0 -> grant=1000 gv=1
req=1111 lock=0 -> grant=0001 gv=1
req=1111 lock=0 -> grant=0010 gv=1
-- lone requester 2 keeps winning:
req=0100 lock=0 -> grant=0100 gv=1
req=0100 lock=0 -> grant=0100 gv=1
-- lock hold on req1: latch then nail while lock high:
req=1111 lock=1 -> grant=1000 gv=1
req=1111 lock=1 -> grant=1000 gv=1
req=1111 lock=1 -> grant=1000 gv=1
-- drop lock, req still high (owns cycle then resume):
req=1111 lock=0 -> grant=0001 gv=1
req=1111 lock=0 -> grant=0010 gv=1
-- dead air: hold then held req goes low:
req=1111 lock=1 -> grant=0010 gv=1
req=1111 lock=1 -> grant=0010 gv=1
tb_sanity.sv:26: $finish called at 146 (1s)

[stdout]
dut.sv:26: sorry: constant selects in always_* processes are not currently supported (all bits will be included).
-- all four forever (expect 0,1,2,3,0):
req=1111 lock=0 -> grant=0010 gv=1
req=1111 lock=0 -> grant=0100 gv=1
req=1111 lock=0 -> grant=1000 gv=1
req=1111 lock=0 -> grant=0001 gv=1
req=1111 lock=0 -> grant=0010 gv=1
-- lone requester 2 keeps winning:
req=0100 lock=0 -> grant=0100 gv=1
req=0100 lock=0 -> grant=0100 gv=1
-- lock hold on req1: latch then nail while lock high:
req=1111 lock=1 -> grant=1000 gv=1
req=1111 lock=1 -> grant=1000 gv=1
req=1111 lock=1 -> grant=1000 gv=1
-- drop lock, req still high (owns cycle then resume):
req=1111 lock=0 -> grant=0001 gv=1
req=1111 lock=0 -> grant=0010 gv=1
-- dead air: hold then held req goes low:
req=1111 lock=1 -> grant=0010 gv=1
req=1111 lock=1 -> grant=0010 gv=1
tb_sanity.sv:26: $finish called at 146 (1s)
The logic is correct (the TB samples one cycle late, but the rotation `0,1,2,3` is right and hold behavior works). However, iverilog printed a `sorry: constant selects in always_* processes are not currently supported` warning for the dynamic bit-select `req[ptr + i]` inside the process , that's an unsupported construct and risky for grading. Let me rewrite the indexing to avoid dynamic bit-selects inside `always` blocks.
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
);

    // Circular-queue arbiter state.
    // ptr     : position to begin scanning (just past the last winner)
    // held    : a locked hold is active from a previous granted cycle
    // held_id : the requester the hold is nailed to
    logic [1:0] ptr;
    logic       held;
    logic [1:0] held_id;

    // next-state
    logic [1:0] ptr_n;
    logic       held_n;
    logic [1:0] held_id_n;

    // Rotate req so that rr[i] corresponds to actual requester (ptr+i) mod 4.
    logic [3:0] rr;
    always_comb begin
        case (ptr)
            2'd0:    rr = req;
            2'd1:    rr = {req[0],  req[3:1]};
            2'd2:    rr = {req[1:0], req[3:2]};
            default: rr = {req[2:0], req[3]};
        endcase
    end

    // Priority scan on the rotated vector: first asker starting just past
    // the last winner, wrapping 3 -> 0.
    logic       found;
    logic [1:0] rel;
    logic [1:0] winner;
    always_comb begin
        found = |rr;
        if      (rr[0]) rel = 2'd0;
        else if (rr[1]) rel = 2'd1;
        else if (rr[2]) rel = 2'd2;
        else            rel = 2'd3;
        winner = ptr + rel;
    end

    // Whether the currently held requester is still asking (index-safe).
    logic req_held;
    assign req_held = req[held_id];

    // Outputs (Mealy) and next state.
    always_comb begin
        grant       = 4'b0000;
        grant_valid = 1'b0;
        ptr_n       = ptr;
        held_n      = 1'b0;
        held_id_n   = held_id;

        if (held) begin
            if (req_held) begin
                // Held requester keeps asking: grant stays nailed to it.
                grant       = 4'b0001 << held_id;
                grant_valid = 1'b1;
                ptr_n       = held_id + 2'd1;
                if (lock) begin
                    // Lock still high: hold continues.
                    held_n    = 1'b1;
                    held_id_n = held_id;
                end else begin
                    // Lock dropped: this cycle still owned, sharing resumes next.
                    held_n = 1'b0;
                end
            end else begin
                // Held requester dropped its req: dead air, hold is over.
                // Counts as served; sharing resumes just past it.
                grant       = 4'b0000;
                grant_valid = 1'b0;
                held_n      = 1'b0;
                ptr_n       = held_id + 2'd1;
            end
        end else begin
            // Normal arbitration by the start rule.
            if (found) begin
                grant       = 4'b0001 << winner;
                grant_valid = 1'b1;
                ptr_n       = winner + 2'd1;
                if (lock) begin
                    // Lock high on a granted cycle latches this winner.
                    held_n    = 1'b1;
                    held_id_n = winner;
                end
            end else begin
                // No asker: idle keeps your place (lock with no grant does nothing).
                grant_valid = 1'b0;
                ptr_n       = ptr;
            end
        end
    end

    always_ff @(posedge clk) begin
        if (rst) begin
            // Synchronous: clear any hold, restart the order.
            ptr     <= 2'd0;
            held    <= 1'b0;
            held_id <= 2'd0;
        end else begin
            ptr     <= ptr_n;
            held    <= held_n;
            held_id <= held_id_n;
        end
    end

endmodule
Now let me verify it compiles cleanly (no warnings) and behaves correctly with a properly-timed testbench that samples outputs *before* the clock edge.
shell
cat > tb2.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;
  // drive inputs just after edge; sample outputs just before next edge
  task automatic cyc(input [3:0] r, input l, input string note);
    req=r; lock=l; #8;  // settle within the cycle, before posedge at +? 
    $display("%-40s req=%b lock=%b | grant=%b gv=%b", note, req, lock, grant, grant_valid);
    @(posedge clk);
  endtask
  initial begin
    rst=1; req=0; lock=0; @(posedge clk);
    rst=0;
    cyc(4'b1111,0,"all4 #1 (exp grant0)");
    cyc(4'b1111,0,"all4 #2 (exp grant1)");
    cyc(4'b1111,0,"all4 #3 (exp grant2)");
    cyc(4'b1111,0,"all4 #4 (exp grant3)");
    cyc(4'b1111,0,"all4 #5 (exp grant0)");
    cyc(4'b0100,0,"lone2 (exp grant2)");
    cyc(4'b0100,0,"lone2 (exp grant2)");
    cyc(4'b0000,0,"idle (exp gv=0, keep place ptr=3)");
    cyc(4'b1000,0,"only3 (exp grant3, ptr was 3)");
    // lock/hold: next winner after ptr=0 among 1111 is 0; latch it
    cyc(4'b1111,1,"lock latch (exp grant0, held)");
    cyc(4'b1111,1,"held nailed (exp grant0)");
    cyc(4'b1111,1,"held nailed (exp grant0)");
    cyc(4'b1111,0,"lock drop, req0 hi (exp grant0, owns)");
    cyc(4'b1111,0,"resume just past (exp grant1)");
    // dead air: latch a winner then drop its req
    cyc(4'b1111,1,"lock latch (exp grant2, held)");
    cyc(4'b1011,1,"held req2 low -> dead air (exp gv=0)");
    cyc(4'b1011,1,"resume, lock still hi, re-latch (exp grant3)");
    $finish;
  end
endmodule
EOF
iverilog -g2012 -Wall -o sim2 dut.sv tb2.sv && echo "=== compiled clean ===" && vvp sim2
output
dut.sv:25: sorry: constant selects in always_* processes are not currently supported (all bits will be included).
dut.sv:25: sorry: constant selects in always_* processes are not currently supported (all bits will be included).
dut.sv:25: sorry: constant selects in always_* processes are not currently supported (all bits will be included).
dut.sv:25: sorry: constant selects in always_* processes are not currently supported (all bits will be included).
dut.sv:25: sorry: constant selects in always_* processes are not currently supported (all bits will be included).
dut.sv:25: sorry: constant selects in always_* processes are not currently supported (all bits will be included).
dut.sv:39: sorry: constant selects in always_* processes are not currently supported (all bits will be included).
dut.sv:39: sorry: constant selects in always_* processes are not currently supported (all bits will be included).
dut.sv:39: sorry: constant selects in always_* processes are not currently supported (all bits will be included).
=== compiled clean ===
all4 #1 (exp grant0)                     req=1111 lock=0 | grant=xxxx gv=1
all4 #2 (exp grant1)                     req=1111 lock=0 | grant=xxxx gv=1
all4 #3 (exp grant2)                     req=1111 lock=0 | grant=xxxx gv=1
all4 #4 (exp grant3)                     req=1111 lock=0 | grant=xxxx gv=1
all4 #5 (exp grant0)                     req=1111 lock=0 | grant=xxxx gv=1
lone2 (exp grant2)                       req=0100 lock=0 | grant=xxxx gv=1
lone2 (exp grant2)                       req=0100 lock=0 | grant=xxxx gv=1
idle (exp gv=0, keep place ptr=3)        req=0000 lock=0 | grant=0000 gv=0
only3 (exp grant3, ptr was 3)            req=1000 lock=0 | grant=xxxx gv=1
lock latch (exp grant0, held)            req=1111 lock=1 | grant=xxxx gv=1
held nailed (exp grant0)                 req=1111 lock=1 | grant=0000 gv=0
held nailed (exp grant0)                 req=1111 lock=1 | grant=xxxx gv=1
lock drop, req0 hi (exp grant0, owns)    req=1111 lock=0 | grant=0000 gv=0
resume just past (exp grant1)            req=1111 lock=0 | grant=xxxx gv=1
lock latch (exp grant2, held)            req=1111 lock=1 | grant=xxxx gv=1
held req2 low -> dead air (exp gv=0)     req=1011 lock=1 | grant=0000 gv=0
resume, lock still hi, re-latch (exp grant3) req=1011 lock=1 | grant=xxxx gv=1
tb2.sv:36: $finish called at 175 (1s)

[stdout]
dut.sv:25: sorry: constant selects in always_* processes are not currently supported (all bits will be included).
dut.sv:25: sorry: constant selects in always_* processes are not currently supported (all bits will be included).
dut.sv:25: sorry: constant selects in always_* processes are not currently supported (all bits will be included).
dut.sv:25: sorry: constant selects in always_* processes are not currently supported (all bits will be included).
dut.sv:25: sorry: constant selects in always_* processes are not currently supported (all bits will be included).
dut.sv:25: sorry: constant selects in always_* processes are not currently supported (all bits will be included).
dut.sv:39: sorry: constant selects in always_* processes are not currently supported (all bits will be included).
dut.sv:39: sorry: constant selects in always_* processes are not currently supported (all bits will be included).
dut.sv:39: sorry: constant selects in always_* processes are not currently supported (all bits will be included).
=== compiled clean ===
all4 #1 (exp grant0)                     req=1111 lock=0 | grant=xxxx gv=1
all4 #2 (exp grant1)                     req=1111 lock=0 | grant=xxxx gv=1
all4 #3 (exp grant2)                     req=1111 lock=0 | grant=xxxx gv=1
all4 #4 (exp grant3)                     req=1111 lock=0 | grant=xxxx gv=1
all4 #5 (exp grant0)                     req=1111 lock=0 | grant=xxxx gv=1
lone2 (exp grant2)                       req=0100 lock=0 | grant=xxxx gv=1
lone2 (exp grant2)                       req=0100 lock=0 | grant=xxxx gv=1
idle (exp gv=0, keep place ptr=3)        req=0000 lock=0 | grant=0000 gv=0
only3 (exp grant3, ptr was 3)            req=1000 lock=0 | grant=xxxx gv=1
lock latch (exp grant0, held)            req=1111 lock=1 | grant=xxxx gv=1
held nailed (exp grant0)                 req=1111 lock=1 | grant=0000 gv=0
held nailed (exp grant0)                 req=1111 lock=1 | grant=xxxx gv=1
lock drop, req0 hi (exp grant0, owns)    req=1111 lock=0 | grant=0000 gv=0
resume just past (exp grant1)            req=1111 lock=0 | grant=xxxx gv=1
lock latch (exp grant2, held)            req=1111 lock=1 | grant=xxxx gv=1
held req2 low -> dead air (exp gv=0)     req=1011 lock=1 | grant=0000 gv=0
resume, lock still hi, re-latch (exp grant3) req=1011 lock=1 | grant=xxxx gv=1
tb2.sv:36: $finish called at 175 (1s)
Interesting , now `grant=xxxx`. The part-selects `req[3:1]` inside `always_comb` are triggering the same iverilog limitation and producing X. Let me check the iverilog version and probe exactly which constructs it accepts.
shell
iverilog -V | head -1
cat > probe.sv <<'EOF'
module probe(input logic [3:0] req, input logic [1:0] p, output logic [3:0] o1,o2,o3,o4, output logic b);
  // part select in always_comb
  always_comb begin
    case(p)
      2'd0: o1 = req;
      2'd1: o1 = {req[0], req[3:1]};
      default: o1 = 4'b0;
    endcase
  end
  // continuous assign part select
  assign o2 = {req[2:0], req[3]};
  // dynamic bit index in assign
  assign b = req[p];
  // shift in always
  always_comb o3 = 4'b0001 << p;
  // part select feeding priority in always
  always_comb begin
    if (req[0]) o4 = 4'd1; else o4 = 4'd2;
  end
endmodule
EOF
iverilog -g2012 -Wall -o probe probe.sv 2>&1 | head; echo "rc=$?"
output
Icarus Verilog version 12.0 (stable) ()
Unable to get version from "/usr/lib/aarch64-linux-gnu/ivl/ivlpp -V"
Unable to get version from "/usr/lib/aarch64-linux-gnu/ivl/ivl -V -C"/tmp/ivrlh35bc73e7" -C"/usr/lib/aarch64-linux-gnu/ivl/vvp.conf""
probe.sv:3: sorry: constant selects in always_* processes are not currently supported (all bits will be included).
probe.sv:3: sorry: constant selects in always_* processes are not currently supported (all bits will be included).
probe.sv:17: sorry: constant selects in always_* processes are not currently supported (all bits will be included).
rc=0

[stdout]
Icarus Verilog version 12.0 (stable) ()
Unable to get version from "/usr/lib/aarch64-linux-gnu/ivl/ivlpp -V"
Unable to get version from "/usr/lib/aarch64-linux-gnu/ivl/ivl -V -C"/tmp/ivrlh35bc73e7" -C"/usr/lib/aarch64-linux-gnu/ivl/vvp.conf""
probe.sv:3: sorry: constant selects in always_* processes are not currently supported (all bits will be included).
probe.sv:3: sorry: constant selects in always_* processes are not currently supported (all bits will be included).
probe.sv:17: sorry: constant selects in always_* processes are not currently supported (all bits will be included).
rc=0
iverilog 12.0 flags bit/part-selects of a signal inside `always_*` processes (lines 3 and 17) but is fine with them in continuous `assign`. Let me confirm whether it produces *wrong* results, and find the clean pattern , the safest fix is to move all bit-selection into continuous `assign`s and keep the process purely on whole-vector/scalar signals.
shell
cat > probe2.sv <<'EOF'
module probe2(input logic [3:0] req, output logic hit0, hit2);
  // does select-in-always give WRONG results?
  always_comb hit0 = req[0];
  // clean: select in assign
  assign hit2 = req[2];
endmodule
module tb;
  logic [3:0] req; logic hit0,hit2;
  probe2 d(.*);
  initial begin
    for (int k=0;k<16;k++) begin req=k; #1;
      $display("req=%b req[0]=%b(always) req[2]=%b(assign)", req, hit0, hit2);
    end
  end
endmodule
EOF
iverilog -g2012 -o p2 probe2.sv 2>/dev/null && vvp p2
output
req=0000 req[0]=0(always) req[2]=0(assign)
req=0001 req[0]=1(always) req[2]=0(assign)
req=0010 req[0]=0(always) req[2]=0(assign)
req=0011 req[0]=1(always) req[2]=0(assign)
req=0100 req[0]=0(always) req[2]=1(assign)
req=0101 req[0]=1(always) req[2]=1(assign)
req=0110 req[0]=0(always) req[2]=1(assign)
req=0111 req[0]=1(always) req[2]=1(assign)
req=1000 req[0]=0(always) req[2]=0(assign)
req=1001 req[0]=1(always) req[2]=0(assign)
req=1010 req[0]=0(always) req[2]=0(assign)
req=1011 req[0]=1(always) req[2]=0(assign)
req=1100 req[0]=0(always) req[2]=1(assign)
req=1101 req[0]=1(always) req[2]=1(assign)
req=1110 req[0]=0(always) req[2]=1(assign)
req=1111 req[0]=1(always) req[2]=1(assign)

[stdout]
req=0000 req[0]=0(always) req[2]=0(assign)
req=0001 req[0]=1(always) req[2]=0(assign)
req=0010 req[0]=0(always) req[2]=0(assign)
req=0011 req[0]=1(always) req[2]=0(assign)
req=0100 req[0]=0(always) req[2]=1(assign)
req=0101 req[0]=1(always) req[2]=1(assign)
req=0110 req[0]=0(always) req[2]=1(assign)
req=0111 req[0]=1(always) req[2]=1(assign)
req=1000 req[0]=0(always) req[2]=0(assign)
req=1001 req[0]=1(always) req[2]=0(assign)
req=1010 req[0]=0(always) req[2]=0(assign)
req=1011 req[0]=1(always) req[2]=0(assign)
req=1100 req[0]=0(always) req[2]=1(assign)
req=1101 req[0]=1(always) req[2]=1(assign)
req=1110 req[0]=0(always) req[2]=1(assign)
req=1111 req[0]=1(always) req[2]=1(assign)
`req[0]` in an `always` actually simulates correctly , the warning is benign for simple selects, but the part-selects/concats in my `rr` case gave X. The robust fix: do **all** bit-selection in continuous `assign`s (dynamic index in `assign` compiled cleanly), and keep the `always` blocks on whole scalars/vectors + shifts only.
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
);

    // Circular-queue arbiter state.
    // ptr     : position to begin scanning (just past the last winner)
    // held    : a locked hold is active from a previous granted cycle
    // held_id : the requester the hold is nailed to
    logic [1:0] ptr;
    logic       held;
    logic [1:0] held_id;

    // next-state
    logic [1:0] ptr_n;
    logic       held_n;
    logic [1:0] held_id_n;

    // Rotated request: rr[i] is requester (ptr + i) mod 4.  All bit selection
    // is done in continuous assigns (safe indexing).
    logic [3:0] rr;
    assign rr[0] = req[ptr];
    assign rr[1] = req[ptr + 2'd1];
    assign rr[2] = req[ptr + 2'd2];
    assign rr[3] = req[ptr + 2'd3];

    // Priority scan on the rotated vector: first asker starting just past the
    // last winner, wrapping 3 -> 0.
    logic       found;
    logic [1:0] rel;
    logic [1:0] winner;
    assign found  = |rr;
    assign rel    = rr[0] ? 2'd0 : rr[1] ? 2'd1 : rr[2] ? 2'd2 : 2'd3;
    assign winner = ptr + rel;

    // Whether the currently held requester is still asking.
    logic req_held;
    assign req_held = req[held_id];

    // Outputs (Mealy) and next state.  Uses only scalars / whole vectors and
    // shifts -- no bit-select of req inside the process.
    always_comb begin
        grant       = 4'b0000;
        grant_valid = 1'b0;
        ptr_n       = ptr;
        held_n      = 1'b0;
        held_id_n   = held_id;

        if (held) begin
            if (req_held) begin
                // Held requester keeps asking: grant stays nailed to it.
                grant       = 4'b0001 << held_id;
                grant_valid = 1'b1;
                ptr_n       = held_id + 2'd1;
                if (lock) begin
                    // Lock still high: hold continues.
                    held_n    = 1'b1;
                    held_id_n = held_id;
                end else begin
                    // Lock dropped: this cycle still owned, sharing resumes next.
                    held_n = 1'b0;
                end
            end else begin
                // Held requester dropped its req: dead air, hold is over.
                // Counts as served; sharing resumes just past it.
                grant       = 4'b0000;
                grant_valid = 1'b0;
                held_n      = 1'b0;
                ptr_n       = held_id + 2'd1;
            end
        end else begin
            // Normal arbitration by the start rule.
            if (found) begin
                grant       = 4'b0001 << winner;
                grant_valid = 1'b1;
                ptr_n       = winner + 2'd1;
                if (lock) begin
                    // Lock high on a granted cycle latches this winner.
                    held_n    = 1'b1;
                    held_id_n = winner;
                end
            end else begin
                // No asker: idle keeps your place (lock with no grant does nothing).
                grant_valid = 1'b0;
                ptr_n       = ptr;
            end
        end
    end

    always_ff @(posedge clk) begin
        if (rst) begin
            // Synchronous: clear any hold, restart the order.
            ptr     <= 2'd0;
            held    <= 1'b0;
            held_id <= 2'd0;
        end else begin
            ptr     <= ptr_n;
            held    <= held_n;
            held_id <= held_id_n;
        end
    end

endmodule
shell
iverilog -g2012 -Wall -o sim2 dut.sv tb2.sv 2>&1 | grep -v "Unable to get version"; echo "=== end compile output ==="; vvp sim2
output
=== end compile output ===
all4 #1 (exp grant0)                     req=1111 lock=0 | grant=0000 gv=0
all4 #2 (exp grant1)                     req=1111 lock=0 | grant=0000 gv=0
all4 #3 (exp grant2)                     req=1111 lock=0 | grant=0000 gv=0
all4 #4 (exp grant3)                     req=1111 lock=0 | grant=0000 gv=0
all4 #5 (exp grant0)                     req=1111 lock=0 | grant=0000 gv=0
lone2 (exp grant2)                       req=0100 lock=0 | grant=0000 gv=0
lone2 (exp grant2)                       req=0100 lock=0 | grant=0000 gv=0
idle (exp gv=0, keep place ptr=3)        req=0000 lock=0 | grant=0000 gv=0
only3 (exp grant3, ptr was 3)            req=1000 lock=0 | grant=0000 gv=0
lock latch (exp grant0, held)            req=1111 lock=1 | grant=0000 gv=0
held nailed (exp grant0)                 req=1111 lock=1 | grant=0000 gv=0
held nailed (exp grant0)                 req=1111 lock=1 | grant=0000 gv=0
lock drop, req0 hi (exp grant0, owns)    req=1111 lock=0 | grant=0000 gv=0
resume just past (exp grant1)            req=1111 lock=0 | grant=0000 gv=0
lock latch (exp grant2, held)            req=1111 lock=1 | grant=0000 gv=0
held req2 low -> dead air (exp gv=0)     req=1011 lock=1 | grant=0000 gv=0
resume, lock still hi, re-latch (exp grant3) req=1011 lock=1 | grant=0000 gv=0
tb2.sv:36: $finish called at 175 (1s)

[stdout]
=== end compile output ===
all4 #1 (exp grant0)                     req=1111 lock=0 | grant=0000 gv=0
all4 #2 (exp grant1)                     req=1111 lock=0 | grant=0000 gv=0
all4 #3 (exp grant2)                     req=1111 lock=0 | grant=0000 gv=0
all4 #4 (exp grant3)                     req=1111 lock=0 | grant=0000 gv=0
all4 #5 (exp grant0)                     req=1111 lock=0 | grant=0000 gv=0
lone2 (exp grant2)                       req=0100 lock=0 | grant=0000 gv=0
lone2 (exp grant2)                       req=0100 lock=0 | grant=0000 gv=0
idle (exp gv=0, keep place ptr=3)        req=0000 lock=0 | grant=0000 gv=0
only3 (exp grant3, ptr was 3)            req=1000 lock=0 | grant=0000 gv=0
lock latch (exp grant0, held)            req=1111 lock=1 | grant=0000 gv=0
held nailed (exp grant0)                 req=1111 lock=1 | grant=0000 gv=0
held nailed (exp grant0)                 req=1111 lock=1 | grant=0000 gv=0
lock drop, req0 hi (exp grant0, owns)    req=1111 lock=0 | grant=0000 gv=0
resume just past (exp grant1)            req=1111 lock=0 | grant=0000 gv=0
lock latch (exp grant2, held)            req=1111 lock=1 | grant=0000 gv=0
held req2 low -> dead air (exp gv=0)     req=1011 lock=1 | grant=0000 gv=0
resume, lock still hi, re-latch (exp grant3) req=1011 lock=1 | grant=0000 gv=0
tb2.sv:36: $finish called at 175 (1s)
Clean compile now (no warnings). The all-zeros output is a flawed TB sampling window (`#8` straddles the edge). Let me use a correct TB that applies inputs right after the reset edge and samples the combinational grant during each cycle.
shell
cat > tb3.sv <<'EOF'
module tb;
  logic clk=0, rst, lock;
  logic [3:0] req; logic [3:0] grant; logic grant_valid;
  TopModule dut(.*);
  always #5 clk=~clk;
  task automatic cyc(input [3:0] r, input l, input string note);
    req=r; lock=l; #1;               // apply inputs for this cycle, settle
    $display("%-42s req=%b lock=%b | grant=%b gv=%b", note, req, lock, grant, grant_valid);
    @(posedge clk);                  // consume the cycle
  endtask
  initial begin
    rst=1; req=0; lock=0; #1; @(posedge clk);   // synchronous reset edge
    rst=0;
    cyc(4'b1111,0,"all4 #1 (exp 0001)");
    cyc(4'b1111,0,"all4 #2 (exp 0010)");
    cyc(4'b1111,0,"all4 #3 (exp 0100)");
    cyc(4'b1111,0,"all4 #4 (exp 1000)");
    cyc(4'b1111,0,"all4 #5 (exp 0001)");
    cyc(4'b0100,0,"lone2 (exp 0100)");
    cyc(4'b0100,0,"lone2 (exp 0100)");
    cyc(4'b0000,0,"idle (exp gv0, keep ptr=3)");
    cyc(4'b1000,0,"only3 (exp 1000)");
    cyc(4'b1111,1,"lock latch @ptr0 (exp 0001 held)");
    cyc(4'b1111,1,"held nailed (exp 0001)");
    cyc(4'b1111,1,"held nailed (exp 0001)");
    cyc(4'b1111,0,"lock drop req0 hi (exp 0001 owns)");
    cyc(4'b1111,0,"resume just past (exp 0010)");
    cyc(4'b1111,1,"lock latch @ptr2 (exp 0100 held)");
    cyc(4'b1011,1,"held req2 low -> dead air (exp gv0)");
    cyc(4'b1011,1,"resume lock hi re-latch (exp 1000)");
    cyc(4'b1011,1,"held nailed on 3 (exp 1000)");
    $finish;
  end
endmodule
EOF
iverilog -g2012 -Wall -o sim3 dut.sv tb3.sv 2>&1 | grep -v "Unable to get version"; vvp sim3
output
all4 #1 (exp 0001)                         req=1111 lock=0 | grant=0000 gv=0
all4 #2 (exp 0010)                         req=1111 lock=0 | grant=0000 gv=0
all4 #3 (exp 0100)                         req=1111 lock=0 | grant=0000 gv=0
all4 #4 (exp 1000)                         req=1111 lock=0 | grant=0000 gv=0
all4 #5 (exp 0001)                         req=1111 lock=0 | grant=0000 gv=0
lone2 (exp 0100)                           req=0100 lock=0 | grant=0000 gv=0
lone2 (exp 0100)                           req=0100 lock=0 | grant=0000 gv=0
idle (exp gv0, keep ptr=3)                 req=0000 lock=0 | grant=0000 gv=0
only3 (exp 1000)                           req=1000 lock=0 | grant=0000 gv=0
lock latch @ptr0 (exp 0001 held)           req=1111 lock=1 | grant=0000 gv=0
held nailed (exp 0001)                     req=1111 lock=1 | grant=0000 gv=0
held nailed (exp 0001)                     req=1111 lock=1 | grant=0000 gv=0
lock drop req0 hi (exp 0001 owns)          req=1111 lock=0 | grant=0000 gv=0
resume just past (exp 0010)                req=1111 lock=0 | grant=0000 gv=0
lock latch @ptr2 (exp 0100 held)           req=1111 lock=1 | grant=0000 gv=0
held req2 low -> dead air (exp gv0)        req=1011 lock=1 | grant=0000 gv=0
resume lock hi re-latch (exp 1000)         req=1011 lock=1 | grant=0000 gv=0
held nailed on 3 (exp 1000)                req=1011 lock=1 | grant=0000 gv=0
tb3.sv:32: $finish called at 185 (1s)

[stdout]
all4 #1 (exp 0001)                         req=1111 lock=0 | grant=0000 gv=0
all4 #2 (exp 0010)                         req=1111 lock=0 | grant=0000 gv=0
all4 #3 (exp 0100)                         req=1111 lock=0 | grant=0000 gv=0
all4 #4 (exp 1000)                         req=1111 lock=0 | grant=0000 gv=0
all4 #5 (exp 0001)                         req=1111 lock=0 | grant=0000 gv=0
lone2 (exp 0100)                           req=0100 lock=0 | grant=0000 gv=0
lone2 (exp 0100)                           req=0100 lock=0 | grant=0000 gv=0
idle (exp gv0, keep ptr=3)                 req=0000 lock=0 | grant=0000 gv=0
only3 (exp 1000)                           req=1000 lock=0 | grant=0000 gv=0
lock latch @ptr0 (exp 0001 held)           req=1111 lock=1 | grant=0000 gv=0
held nailed (exp 0001)                     req=1111 lock=1 | grant=0000 gv=0
held nailed (exp 0001)                     req=1111 lock=1 | grant=0000 gv=0
lock drop req0 hi (exp 0001 owns)          req=1111 lock=0 | grant=0000 gv=0
resume just past (exp 0010)                req=1111 lock=0 | grant=0000 gv=0
lock latch @ptr2 (exp 0100 held)           req=1111 lock=1 | grant=0000 gv=0
held req2 low -> dead air (exp gv0)        req=1011 lock=1 | grant=0000 gv=0
resume lock hi re-latch (exp 1000)         req=1011 lock=1 | grant=0000 gv=0
held nailed on 3 (exp 1000)                req=1011 lock=1 | grant=0000 gv=0
tb3.sv:32: $finish called at 185 (1s)
Still zeros , the DUT itself is broken now. Likely the four separate continuous assigns to individual bits of a `logic [3:0] rr` don't resolve in iverilog. Let me probe internals.
shell
cat > tb4.sv <<'EOF'
module tb;
  logic clk=0, rst, lock; logic [3:0] req; logic [3:0] grant; logic grant_valid;
  TopModule dut(.*);
  always #5 clk=~clk;
  initial begin
    rst=1; req=0; lock=0; #1; @(posedge clk); rst=0;
    req=4'b1111; lock=0; #1;
    $display("ptr=%0d rr=%b found=%b rel=%0d winner=%0d grant=%b gv=%b",
      dut.ptr, dut.rr, dut.found, dut.rel, dut.winner, grant, grant_valid);
    $finish;
  end
endmodule
EOF
iverilog -g2012 -o sim4 dut.sv tb4.sv 2>/dev/null && vvp sim4
output
ptr=x rr=xxxx found=x rel=x winner=x grant=0000 gv=0
tb4.sv:10: $finish called at 6 (1s)

[stdout]
ptr=x rr=xxxx found=x rel=x winner=x grant=0000 gv=0
tb4.sv:10: $finish called at 6 (1s)
shell
iverilog -g2012 -Wall -o sim4 dut.sv tb4.sv 2>&1 | grep -v "Unable to get version"
echo "--- run with more cycles ---"
cat > tb5.sv <<'EOF'
module tb;
  logic clk=0, rst, lock; logic [3:0] req; logic [3:0] grant; logic grant_valid;
  TopModule dut(.*);
  always #5 clk=~clk;
  initial begin
    rst=1; req=4'b1111; lock=0;
    repeat(3) @(posedge clk);
    $display("after3edges rst=1: ptr=%0d held=%b rr=%b", dut.ptr, dut.held, dut.rr);
    rst=0; #1;
    $display("rst=0 t: ptr=%0d rr=%b found=%b winner=%0d grant=%b gv=%b",
      dut.ptr, dut.rr, dut.found, dut.winner, grant, grant_valid);
    @(posedge clk); #1;
    $display("next: ptr=%0d grant=%b gv=%b", dut.ptr, grant, grant_valid);
    $finish;
  end
endmodule
EOF
iverilog -g2012 -o sim5 dut.sv tb5.sv 2>/dev/null && vvp sim5
output
--- run with more cycles ---
after3edges rst=1: ptr=0 held=0 rr=1111
rst=0 t: ptr=1 rr=1111 found=1 winner=1 grant=0010 gv=1
next: ptr=2 grant=0100 gv=1
tb5.sv:14: $finish called at 36 (1s)

[stdout]
--- run with more cycles ---
after3edges rst=1: ptr=0 held=0 rr=1111
rst=0 t: ptr=1 rr=1111 found=1 winner=1 grant=0010 gv=1
next: ptr=2 grant=0100 gv=1
tb5.sv:14: $finish called at 36 (1s)
The DUT works , reset gives `ptr=0`, `rr=1111`, and arbitration produces correct one-hots. The earlier all-zeros were a testbench race (deasserting `rst` exactly on the clock edge). Let me write a proper TB that drives inputs on the negedge (away from the sampling edge) and run the full functional check.
shell
cat > tb.sv <<'EOF'
module tb;
  logic clk=0, rst, lock; logic [3:0] req; logic [3:0] grant; logic grant_valid;
  TopModule dut(.*);
  always #5 clk=~clk;
  int errors=0;
  // apply inputs on negedge; outputs valid through the next posedge (which consumes them)
  task automatic cyc(input [3:0] r, input l, input [3:0] eg, input egv, input string note);
    @(negedge clk); req=r; lock=l; #1;
    if (grant!==eg || grant_valid!==egv) begin
      errors++;
      $display("MISMATCH %-38s req=%b lock=%b | grant=%b(exp %b) gv=%b(exp %b)",
        note, req, lock, grant, eg, grant_valid, egv);
    end else
      $display("ok       %-38s req=%b lock=%b | grant=%b gv=%b", note, req, lock, grant, grant_valid);
  endtask
  initial begin
    rst=1; req=0; lock=0;
    @(negedge clk); @(posedge clk);   // reset applied on this edge, rst deasserted at negedge next
    // round-robin
    cyc(4'b1111,0,4'b0001,1,"all4 #1");
    cyc(4'b1111,0,4'b0010,1,"all4 #2");
    cyc(4'b1111,0,4'b0100,1,"all4 #3");
    cyc(4'b1111,0,4'b1000,1,"all4 #4");
    cyc(4'b1111,0,4'b0001,1,"all4 #5 wrap");
    // lone requester keeps winning
    cyc(4'b0100,0,4'b0100,1,"lone2 a");
    cyc(4'b0100,0,4'b0100,1,"lone2 b");
    // idle keeps place (ptr stays at 3, since last winner was 2)
    cyc(4'b0000,0,4'b0000,0,"idle");
    cyc(4'b1000,0,4'b1000,1,"only3 -> grant3");
    // ptr now 0. lock latch on winner 0
    cyc(4'b1111,1,4'b0001,1,"lock latch @0");
    cyc(4'b1111,1,4'b0001,1,"held nailed 0");
    cyc(4'b0001,1,4'b0001,1,"held, only 0 asks");
    cyc(4'b1111,1,4'b0001,1,"held nailed 0 again");
    // drop lock while req0 still high: owns this cycle, then resume
    cyc(4'b1111,0,4'b0001,1,"lock drop, 0 owns");
    cyc(4'b1111,0,4'b0010,1,"resume -> grant1");
    // lock latch on winner 2 (ptr now 2)
    cyc(4'b1111,1,4'b0100,1,"lock latch @2");
    cyc(4'b1111,1,4'b0100,1,"held nailed 2");
    // held req2 goes low while lock high -> dead air
    cyc(4'b1011,1,4'b0000,0,"dead air (req2 low)");
    // resume just past dead winner (ptr=3), lock still high re-latches 3
    cyc(4'b1011,1,4'b1000,1,"re-latch @3");
    cyc(4'b1011,1,4'b1000,1,"held nailed 3");
    // held 3 drops on the cycle lock also drops -> still dead air
    cyc(4'b0011,0,4'b0000,0,"dead air on lock-drop cycle");
    // resume just past 3 -> ptr0; grant0
    cyc(4'b0011,0,4'b0001,1,"resume -> grant0");
    // lock with no grant does nothing (no req)
    cyc(4'b0000,1,4'b0000,0,"lock no grant");
    if (errors==0) $display("\nALL PASS"); else $display("\n%0d ERRORS", errors);
    $finish;
  end
endmodule
EOF
iverilog -g2012 -Wall -o sim dut.sv tb.sv 2>&1 | grep -v "Unable to get version"; vvp sim
output
ok       all4 #1                                req=1111 lock=0 | grant=0001 gv=1
MISMATCH all4 #2                                req=1111 lock=0 | grant=0001(exp 0010) gv=1(exp 1)
MISMATCH all4 #3                                req=1111 lock=0 | grant=0001(exp 0100) gv=1(exp 1)
MISMATCH all4 #4                                req=1111 lock=0 | grant=0001(exp 1000) gv=1(exp 1)
ok       all4 #5 wrap                           req=1111 lock=0 | grant=0001 gv=1
ok       lone2 a                                req=0100 lock=0 | grant=0100 gv=1
ok       lone2 b                                req=0100 lock=0 | grant=0100 gv=1
ok       idle                                   req=0000 lock=0 | grant=0000 gv=0
ok       only3 -> grant3                        req=1000 lock=0 | grant=1000 gv=1
ok       lock latch @0                          req=1111 lock=1 | grant=0001 gv=1
ok       held nailed 0                          req=1111 lock=1 | grant=0001 gv=1
ok       held, only 0 asks                      req=0001 lock=1 | grant=0001 gv=1
ok       held nailed 0 again                    req=1111 lock=1 | grant=0001 gv=1
ok       lock drop, 0 owns                      req=1111 lock=0 | grant=0001 gv=1
MISMATCH resume -> grant1                       req=1111 lock=0 | grant=0001(exp 0010) gv=1(exp 1)
MISMATCH lock latch @2                          req=1111 lock=1 | grant=0001(exp 0100) gv=1(exp 1)
MISMATCH held nailed 2                          req=1111 lock=1 | grant=0001(exp 0100) gv=1(exp 1)
MISMATCH dead air (req2 low)                    req=1011 lock=1 | grant=0001(exp 0000) gv=1(exp 0)
MISMATCH re-latch @3                            req=1011 lock=1 | grant=0001(exp 1000) gv=1(exp 1)
MISMATCH held nailed 3                          req=1011 lock=1 | grant=0001(exp 1000) gv=1(exp 1)
MISMATCH dead air on lock-drop cycle            req=0011 lock=0 | grant=0001(exp 0000) gv=1(exp 0)
ok       resume -> grant0                       req=0011 lock=0 | grant=0001 gv=1
ok       lock no grant                          req=0000 lock=1 | grant=0000 gv=0

10 ERRORS
tb.sv:54: $finish called at 241 (1s)

[stdout]
ok       all4 #1                                req=1111 lock=0 | grant=0001 gv=1
MISMATCH all4 #2                                req=1111 lock=0 | grant=0001(exp 0010) gv=1(exp 1)
MISMATCH all4 #3                                req=1111 lock=0 | grant=0001(exp 0100) gv=1(exp 1)
MISMATCH all4 #4                                req=1111 lock=0 | grant=0001(exp 1000) gv=1(exp 1)
ok       all4 #5 wrap                           req=1111 lock=0 | grant=0001 gv=1
ok       lone2 a                                req=0100 lock=0 | grant=0100 gv=1
ok       lone2 b                                req=0100 lock=0 | grant=0100 gv=1
ok       idle                                   req=0000 lock=0 | grant=0000 gv=0
ok       only3 -> grant3                        req=1000 lock=0 | grant=1000 gv=1
ok       lock latch @0                          req=1111 lock=1 | grant=0001 gv=1
ok       held nailed 0                          req=1111 lock=1 | grant=0001 gv=1
ok       held, only 0 asks                      req=0001 lock=1 | grant=0001 gv=1
ok       held nailed 0 again                    req=1111 lock=1 | grant=0001 gv=1
ok       lock drop, 0 owns                      req=1111 lock=0 | grant=0001 gv=1
MISMATCH resume -> grant1                       req=1111 lock=0 | grant=0001(exp 0010) gv=1(exp 1)
MISMATCH lock latch @2                          req=1111 lock=1 | grant=0001(exp 0100) gv=1(exp 1)
MISMATCH held nailed 2                          req=1111 lock=1 | grant=0001(exp 0100) gv=1(exp 1)
MISMATCH dead air (req2 low)                    req=1011 lock=1 | grant=0001(exp 0000) gv=1(exp 0)
MISMATCH re-latch @3                            req=1011 lock=1 | grant=0001(exp 1000) gv=1(exp 1)
MISMATCH held nailed 3                          req=1011 lock=1 | grant=0001(exp 1000) gv=1(exp 1)
MISMATCH dead air on lock-drop cycle            req=0011 lock=0 | grant=0001(exp 0000) gv=1(exp 0)
ok       resume -> grant0                       req=0011 lock=0 | grant=0001 gv=1
ok       lock no grant                          req=0000 lock=1 | grant=0000 gv=0

10 ERRORS
tb.sv:54: $finish called at 241 (1s)
I forgot to deassert `rst` in the TB , it stays high, resetting `ptr` to 0 every cycle. Let me fix that one line.
shell
sed -i 's|    @(negedge clk); @(posedge clk);   // reset applied on this edge, rst deasserted at negedge next|    @(negedge clk); @(posedge clk); @(negedge clk); rst=0;   // reset applied, then deassert away from edge|' tb.sv
iverilog -g2012 -Wall -o sim dut.sv tb.sv 2>&1 | grep -v "Unable to get version"; vvp sim
output
ok       all4 #1                                req=1111 lock=0 | grant=0001 gv=1
ok       all4 #2                                req=1111 lock=0 | grant=0010 gv=1
ok       all4 #3                                req=1111 lock=0 | grant=0100 gv=1
ok       all4 #4                                req=1111 lock=0 | grant=1000 gv=1
ok       all4 #5 wrap                           req=1111 lock=0 | grant=0001 gv=1
ok       lone2 a                                req=0100 lock=0 | grant=0100 gv=1
ok       lone2 b                                req=0100 lock=0 | grant=0100 gv=1
ok       idle                                   req=0000 lock=0 | grant=0000 gv=0
ok       only3 -> grant3                        req=1000 lock=0 | grant=1000 gv=1
ok       lock latch @0                          req=1111 lock=1 | grant=0001 gv=1
ok       held nailed 0                          req=1111 lock=1 | grant=0001 gv=1
ok       held, only 0 asks                      req=0001 lock=1 | grant=0001 gv=1
ok       held nailed 0 again                    req=1111 lock=1 | grant=0001 gv=1
ok       lock drop, 0 owns                      req=1111 lock=0 | grant=0001 gv=1
ok       resume -> grant1                       req=1111 lock=0 | grant=0010 gv=1
ok       lock latch @2                          req=1111 lock=1 | grant=0100 gv=1
ok       held nailed 2                          req=1111 lock=1 | grant=0100 gv=1
ok       dead air (req2 low)                    req=1011 lock=1 | grant=0000 gv=0
ok       re-latch @3                            req=1011 lock=1 | grant=1000 gv=1
ok       held nailed 3                          req=1011 lock=1 | grant=1000 gv=1
ok       dead air on lock-drop cycle            req=0011 lock=0 | grant=0000 gv=0
ok       resume -> grant0                       req=0011 lock=0 | grant=0001 gv=1
ok       lock no grant                          req=0000 lock=1 | grant=0000 gv=0

ALL PASS
tb.sv:54: $finish called at 251 (1s)

[stdout]
ok       all4 #1                                req=1111 lock=0 | grant=0001 gv=1
ok       all4 #2                                req=1111 lock=0 | grant=0010 gv=1
ok       all4 #3                                req=1111 lock=0 | grant=0100 gv=1
ok       all4 #4                                req=1111 lock=0 | grant=1000 gv=1
ok       all4 #5 wrap                           req=1111 lock=0 | grant=0001 gv=1
ok       lone2 a                                req=0100 lock=0 | grant=0100 gv=1
ok       lone2 b                                req=0100 lock=0 | grant=0100 gv=1
ok       idle                                   req=0000 lock=0 | grant=0000 gv=0
ok       only3 -> grant3                        req=1000 lock=0 | grant=1000 gv=1
ok       lock latch @0                          req=1111 lock=1 | grant=0001 gv=1
ok       held nailed 0                          req=1111 lock=1 | grant=0001 gv=1
ok       held, only 0 asks                      req=0001 lock=1 | grant=0001 gv=1
ok       held nailed 0 again                    req=1111 lock=1 | grant=0001 gv=1
ok       lock drop, 0 owns                      req=1111 lock=0 | grant=0001 gv=1
ok       resume -> grant1                       req=1111 lock=0 | grant=0010 gv=1
ok       lock latch @2                          req=1111 lock=1 | grant=0100 gv=1
ok       held nailed 2                          req=1111 lock=1 | grant=0100 gv=1
ok       dead air (req2 low)                    req=1011 lock=1 | grant=0000 gv=0
ok       re-latch @3                            req=1011 lock=1 | grant=1000 gv=1
ok       held nailed 3                          req=1011 lock=1 | grant=1000 gv=1
ok       dead air on lock-drop cycle            req=0011 lock=0 | grant=0000 gv=0
ok       resume -> grant0                       req=0011 lock=0 | grant=0001 gv=1
ok       lock no grant                          req=0000 lock=1 | grant=0000 gv=0

ALL PASS
tb.sv:54: $finish called at 251 (1s)
All 23 scenarios pass. Let me clean up the scratch test files, leaving only `dut.sv`.
shell
rm -f tb.sv tb2.sv tb3.sv tb4.sv tb5.sv tb_sanity.sv probe.sv probe2.sv sim sim2 sim3 sim4 sim5 p2 probe && ls -la /app
output
total 12
drwxrwxrwx 1 root root 4096 Jul  5 01:17 .
drwxr-xr-x 1 root root 4096 Jul  5 01:08 ..
-rw-r--r-- 1 root root 3577 Jul  5 01:15 dut.sv

[stdout]
total 12
drwxrwxrwx 1 root root 4096 Jul  5 01:17 .
drwxr-xr-x 1 root root 4096 Jul  5 01:08 ..
-rw-r--r-- 1 root root 3577 Jul  5 01:15 dut.sv
Done. `TopModule` is written to `/app/dut.sv` , compiles cleanly under `iverilog -g2012 -Wall` (no warnings, no preprocessor directives, system tasks, or force/release; ASCII only) and passes a testbench covering every rule in the spec. ## Design A Mealy round-robin arbiter. State is three synchronously-reset registers: - `ptr` , where to start scanning (just past the last winner); reset to 0, so the first all-four-ask cycle grants requester 0. - `held` / `held_id` , an active lock hold and who it's nailed to. **Key detail I had to work around:** iverilog 12.0 emits a `sorry: constant selects in always_* processes` warning (and can produce wrong/`x` results) for bit- and part-selects of a signal *inside* `always_*` blocks. So all `req` indexing is done in continuous `assign`s (which handle dynamic indices correctly), and the `always` blocks operate only on whole vectors/scalars plus shifts: - `rr[i] = req[(ptr+i) mod 4]` , request rotated to scan order. - `winner = ptr + (priority-encode rr)` , first asker walking up from `ptr`. - `req_held = req[held_id]` , whether the held requester still asks. **Behavior verified against the spec:** - `0,1,2,3,0…` under constant full demand; lone requester keeps winning; idle keeps `ptr` in place. - Lock on a granted cycle latches that winner; grant stays nailed while lock stays high. - Lock drop with the winner still asking → it owns that cycle; sharing resumes next cycle just past it. - Held requester dropping its `req` (including on the lock-drop cycle) → dead air (`grant=0`, `grant_valid=0`), counts as served, `ptr` advances past it; a still-high lock re-latches the next real winner with no re-arm. - Lock with no grant does nothing; synchronous `rst` clears the hold and restarts at requester 0.
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.on7gpY/runtime/cb_verifier_401-18346-3248-27931_401/results.jsonl"}

Reproduce this trial: git checkout 2f94510 && PYTHONPATH=src python3 scripts/build_site.py , then open trial/trial_110528fc8758473c. Re-running the agent live requires EVAL_PLATFORM_ENABLE_OAUTH_SMOKE=1 and is non-deterministic.

Trial trial_110528fc8758473c · verifier authoritative; classifier explanatory.