UVM Out-of-Order Scoreboard | Tagging & queues

The Pipelined Matching Challenge

In simple protocols like APB, transactions appear on the bus in the exact order they were requested. Verifying this is easy: you use a uvm_tlm_analysis_fifo and compare items as they arrive.

However, advanced SoC protocols like AXI or PCIe support Out-of-Order (OoO) completions. A read request with ID=5 might finish after a read request with ID=10 because of interconnect latency or memory controller arbitration.

The FIFO Trap:

If you use a standard FIFO queue for an OoO protocol, the first transaction that returns out of sequence will trigger a mismatch error, even though the data might be perfectly correct. You need a Searchable Buffer instead.

Strategy: Associative Array Matching

The most robust way to build an OoO scoreboard is to use a SystemVerilog Associative Array (Hash Map) as your internal "expectation memory." You index this memory using a unique Transaction ID or Tag.

The Algorithm:

Predict: When a request is observed at the Master side, clone it and store it in the array: expected_mem[tr.id] = tr;
Observe: When a response arrives at the Slave side, retrieve the original request: tr_exp = expected_mem[tr_actual.id];
Compare: Perform the data check and then delete the entry from the array to free memory.

Implementation Flow

OoO Scoreboard Logic


class axi_scoreboard extends uvm_scoreboard;
    // Expectation Buffer: Key = Transaction ID
    axi_item exp_buffer[int];
    // Input from Master Monitor (Request)
    virtual function void write_request(axi_item tr);
        axi_item tr_clone;
        $cast(tr_clone, tr.clone());
        exp_buffer[tr_clone.id] = tr_clone;
    endfunction
    // Input from Slave Monitor (Response)
    virtual function void write_response(axi_item tr_actual);
        if (exp_buffer.exists(tr_actual.id)) begin
            axi_item tr_exp = exp_buffer[tr_actual.id];
            if (!tr_actual.compare(tr_exp))
                `uvm_error("SCB", "Data Mismatch!")
            // Clean up to prevent memory leak
            exp_buffer.delete(tr_actual.id);
        end else begin
            `uvm_error("SCB", "Unexpected Response ID!")
        end
    endfunction
endclass

Best Practices for OoO Checkers

ID Reuse: Ensure your scoreboarding logic handles ID reuse. If the DUT reuses an ID before the previous transaction finished, your code should flag an error or use a more complex key (e.g., ID + Address).
Memory Leak Check: Always use the check_phase to verify the associative array is empty. Any remaining items represent "dropped" transactions that the DUT never completed.
Clone Everything: Transaction objects are passed by reference. If you don't clone() the request before storing it, future updates to the object in the monitor will corrupt your scoreboard memory.

Out-of-Order Scoreboarding