Transaction Recording In Verilog Or System Verilog

As there is not yet a standard for transaction recording in Verilog or VHDL, ModelSim includes a set of system tasks to perform transaction recording into a WLF file. Transaction modeling allows users to raise the level of description, analysis and debugging of their designs to the transaction level. A transaction represents a transfer of high-level data or control information between the test bench and the design under test (DUT) over an interface or any sequence of signal transitions recorded in the simulation database as a transaction.


The API is the same for Verilog and SystemVerilog. As stated previously, the name "Verilog" refers both to Verilog and SystemVerilog unless otherwise noted.
The recording APIs for Verilog and VHDL are a bit simpler than the SCV API. Specifically, in Verilog and VHDL:
  • There is no database object as there is in SCV; the database is always WLF format (a .wlf file).
  • There is no concept of begin and end attributes All attributes are recorded with the system task $add_attribute() or add_attribute.

  • Your design code must free the transaction handle once the transaction is complete and all use of the handle for relations or attribute recording is complete. (In most cases, SystemC designs ignore this step since SCV frees the handle automatically.)
A transaction has a begin time, an end time, and attributes. Examples of transactions include read operations, write operations, and packet transmissions. The transaction level is the level at which many users think about the design, so it is the level at which you can verify the design most effectively.

Transactions are recorded on a stream. A stream is a collection of transactions, recorded over time. A stream has a name, and usually exists somewhere within the test bench hierarchy – for example a driver might have a stream which represents all the transactions that have occurred on that driver. Each driver defines a collection of attributes ( transaction items ) which are defined by users, and which are meaningful to the transaction. The values of attributes are set for each transaction. Finally, transactions can be linked to each other. A link has a direction and a user-defined name, and specifies a relation between the two transactions.


module top;
    integer stream, tr;
    initial begin
        stream = $create_transaction_stream("Stream");
        #10;
        tr = $begin_transaction(stream, "Tran1");
        $add_attribute(tr, 10, "beg");
        $add_attribute(tr, 12, "special");
        $add_attribute(tr, 14, "end");
        #4;
        $end_transaction(tr);
        $free_transaction(tr);
    end

endmodule

Here,

1. $create_transaction_stream() is used to define a transaction stream. You can use this system task to create one or more stream objects.

 module top;
        integer hStream

        initial begin
            hStream = $create_transaction_stream("stream", "transaction");
            .
            .
        end
        .
        .
    endmodule

2. $begin_transaction is used to start a transaction by providing a valid handle of the transaction as shown below.


integer hTrans;
.
.
 hTrans = $begin_transaction(hstream, "READ");


In this example, we begin a transaction named "READ" on the stream already created. The $begin_transaction system function accepts other parameters to specify: the start time for the transaction, and any relationship information, including its designation as a phase transaction.
The return value is the handle for the transaction. It is needed to end the transaction or record attribute.

3. $end_transaction has a single required argument – the handle of the transaction that is to be ended. It also has a single optional argument, the time in the past that this transaction ended. After a transaction has been ended, the transaction handle can still be used to add attributes and create relations.

$end_transaction( handle transaction [, time endTime])

4. $free_transaction has a single argument – the handle of the transaction to be deleted. Once a transaction is deleted the handle becomes invalid. It cannot be used in any other recording interfaces.

$free_transaction (handle transaction)

5. $add_attribute has two required arguments – a transaction handle on which the attribute is to be created and the attribute that is to be recorded. There is one optional argument of type string named attributeName. This attributeName specifies an alias name for the attribute. If not specified, the name used for the attribute is the actual name of the SystemVerilog object.

$add_attribute( handle transaction,  object attributeValue  [, string attributeName])

6. $add_relation has three arguments – the first two are the two transaction handles which are related. The third argument is the string name of the relation.

$add_relation( handle sourceTransaction,  handle targetTransaction,  string relationshipName)

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