I was going through basics of UVM tutorials. Everywhere I read the transaction objects are always extended from uvm_sequence_item and not uvm_transaction since uvm_sequence_item has additional features like transaction id, etc. If that is the case, why is the uvm_transaction class even there in the UVM class hierarchy?
Who is using uvm_transaction other than uvm_sequence_item extending from it?
Is it because of legacy?
This is what the UVM Class Reference says about this:
"The uvm_transaction class is the root base class for UVM transactions. Inheriting all the methods of uvm_object, uvm_transaction adds a timing and recording interface.
This class provides timestamp properties, notification events, and transaction recording support.
Use of this class as a base for user-defined transactions is deprecated. Its subtype, uvm_sequence_item, shall be used as the base class for all user-defined transaction types."
If you refer uvm class hierarchy (Link:[https://www.google.co.in/search?biw=1366&bih=620&tbm=isch&sa=1&btnG=Search&q=uvm+class+hierarchy#imgrc=Laxc9UWNpnGTpM%3A][1] ) then you find out that uvm_transaction parent class while uvm_sequence is child class.
So, child class can access all the property of parent class. But parent class can not access child class property.
uvm_sequence_item has its own functionality like get_sequencer_id,set_sequencer_id, get_root_sequence.
These methods used by sequencer internally in case of layer sequences.
You call sequence by start method, uvm_do family or config_db.
Each of these method call start_item(req) and finish_item(req).
task start_item(uvm_sequence_item item)
task finish_item(uvm_sequence_item item)
If you observes data type in first argument in both the function is uvm_sequence_item.
There are total eight uvm classes.
(1) uvm_driver.svh
(2) uvm_push_driver.svh
(3) uvm_sequencer.svh
(4) uvm_push_sequencer.svh
(5) uvm_sequencer_library.svh
(6) uvm_sequencer_analysis_fifo.svh
(7) uvm_sequence.svh
(8) uvm_sequencer_param_base.svh
These classes are parameterized with uvm_sequence_item not with uvm_transaction.
If you use uvm_transaction instead of uvm_sequence_item then ti will shout an error(set_sequence_id not found which is property of uvm_sequence_item not uvm_transaction ) from uvm_sequencer_param_base.svh.
So, communication of sequence,sequencer and driver is not completed.
In most of the cases which I observe if you have a code in which you are not going to use uvm_sequencer then you can use uvm_transaction it will not shout an error.
But if your code contains uvm_sequener and you use uvm_transaction then it will shout an error (Could not find member 'set_sequence_id').
Note that from uvm_transaction class, uvm_sequence_item as well as uvm_objects are inherited. So, the static behaviour and non-static behaviour (so to say) class hierarchy all starts from uvm_transaction.
Now, I can add whatever functionality I want to, to uvm_sequence_item so that every inheritor of uvm_sequence_item can get this. I can do this without modifying uvm_transction_item. Otherwise, any change in uvm_transaction would lead to unwanted functionality in the component class hierarchy (static behaviour) and can even lead to unintended side effects.
PS: one of the difference between other OOPs languages and SV is that multiple inheritance is not allowed in SV. For example, in case of C++, I can inherit from 2 classes in a new class. This is not allowed in SV. So, the only way one can get the properties of uvm_transaction as well as from uvm_sequence_item is by inheriting from uvm_sequence_item. This maybe the source of your confusion.
