A mailbox provides a synchronized communication channel between two concurrent processes — typically a Generator and a Driver in a SystemVerilog testbench.

🧩 Concept Overview

• The Generator creates and sends transactions.
• The Driver receives and processes those transactions.
• A mailbox connects them, allowing asynchronous and decoupled communication.

This pattern models a realistic testbench environment where different verification components operate at their own pace.

Exchanging Objects Using a Mailbox

program mailbox_example(bus_if.TB bus, ...);

    // -----------------------
    // Generator Class
    // -----------------------
    class Generator;
        Transaction tr;
        mailbox mbx;

        function new(mailbox mbx);
            this.mbx = mbx;
        endfunction

        task run;
            repeat (10) begin
                tr = new;
                assert(tr.randomize);
                mbx.put(tr); // Send transaction to mailbox
            end
        endtask
    endclass


    // -----------------------
    // Driver Class
    // -----------------------
    class Driver;
        Transaction tr;
        mailbox mbx;

        function new(mailbox mbx);
            this.mbx = mbx;
        endfunction

        task run;
            repeat (10) begin
                mbx.get(tr); // Receive transaction from mailbox
                @(posedge busif.cb.ack);
                bus.cb.kind <= tr.kind;
                ...
            end
        endtask
    endclass


    // -----------------------
    // Testbench Setup
    // -----------------------
    mailbox mbx;   // Shared mailbox between generator & driver
    Generator gen;
    Driver drv;

    initial begin
        mbx = new;           // Create the mailbox
        gen = new(mbx);      // Pass mailbox handle to generator
        drv = new(mbx);      // Pass mailbox handle to driver

        fork
            gen.run();       // Spawn generator thread
            drv.run();       // Spawn driver thread
        join
    end

endprogram

🧠 Explanation

• The mailbox synchronizes the flow of data:

  • The generator may be faster — it blocks if the mailbox is full.
  • The driver may be slower — it blocks if the mailbox is empty.

• This ensures that both components run asynchronously but safely without data loss.

🔄 Data Flow Summary

Generator ── put(tr) ─▶ [ Mailbox FIFO ] ── get(tr) ─▶ Driver

• put() inserts a transaction (blocks if full)
• get() retrieves a transaction (blocks if empty)

Both operations automatically handle synchronization between threads.

🧱 Key Takeaways

• Mailboxes provide a simple and safe way to exchange data between concurrent processes.
• They decouple components — neither side needs to know the other’s hierarchy or timing.
• The mailbox acts as a FIFO buffer between producer (Generator) and consumer (Driver).
• This pattern forms the foundation of transaction-level communication in testbench design.