Difference bw UVM and OVM methodologies

1. 1 Introduction
The Universal Verification Methodology (UVM) is a standardized hybrid methodology for verifying complex design in the semiconductor
industry. It has superseded the Open Verification Methodology which was an Open Source verification methodology was supported by both
Cadence and Mentor. UVM has full industry wide support and standardised under the Accellera Systems Initiative.
In this paper TVS describes in detail the differences between the Open Verification Methodology (version2.1.2) and the Universal Verification
Methodology (version 1.b). It is intended to help engineers to understand the implications of moving from OVM to UVM.
The paper starts with a short history of OVM and UVM to set the context. A detailed comparison then follows looking at phases, managing the
end of test, component configuration and finally register modeling.
About the Authors:
Suresh Babu has been involved in hardware verification for 10 years. Currently he is a project lead @ TVS with responsibility for OVM, UVM
and eRM Based test bench and VIP development, customer support and metric driven based verification signoff using asureSign™.
Dr. Mike Bartley founded TVS in 2008 after spending over 20 years working in both hardware verification and software testing.
About Test and Verification Solutions
TVS delivers tailored solutions for hardware verification and software testing. TVS is an independent company providing both services and
products (VIP & asureSign™) from offices around the world. TVS prides itself on having the flexibility to meet diverse client requirements.
To learn more about our offerings, visit www.testandverification.com or write to us at info@testandverification.com

2. 2 Overview of UVM History
UVM is built on System Verilog and the history of that language is shown in Figure 1: History of System Verilog below.
Figure 1: History of System Verilog
Verification methodologies came into existence soon after the first dedicated HVLs (Hardware Verification Languages) appeared (see . The main
advantages of adopting a methodology (such as UVM) are
• Reusability through test bench re‐use and verification IP allowing plug and play
• A proven methodology with industry wide support and availability of engineers with existing knowledge/experience
• Simulator and vendor independence
HiLo Verilog
1980 1990 2000 2005
e
C
Vera
System
Verilog
VHDL
SuperLog

3.
Figure 2: History of Verification Methodologies
2.1 OVM and UVM Availability
The following releases are available in http://verificationacademy.com/verification‐methodology
UVM 1.1b (tar.gz) ‐ Accellera
UVM 1.1b User Guide‐ Accellera
UVM 1.1a (tar.gz) ‐ Accellera
UVM 1.0 (tar.gz) ‐ Accellera
OVM 2.1.2 (.zip)
OVM 2.1.2 (tar.gz)
UVM Register Kit for OVM 2.1.2 (tar.gz)
OVM<‐>VMM reference library, examples and documentation
Vera RVM
VMM
AVM
URM
System
Verilog
OVM
VMM 1.2
UVM
e eRM
Open Source

4. 3 OVM Phases vs. UVM Phases
In this section we look at the main changes in the way phases are handled in UVM. There are 2 changes in the methods (see section 3.1) and
changes in the actual numbers of phases (see section 3.2). Note that as these changes are significant and not backwards compatible then there is
a way to invoke OVM style semantics.
3.1 Changes in phase methods
There are 2 changes in the interface to the phase methods. These are summarised below with the detail in “Table 1: Summary of the changes in
phase methods”.
1. Method name changed into “<phase_name>_phase.
2. Argument added in all the phase methods.
OVM UVM
class xbus_env extends ovm_env;
// Virtual Interface variable
protected virtual interface xbus_if xi0;
‐‐‐‐
‐‐‐‐
function void build();
string inst_name;
super.build();
if(has_bus_monitor == 1) begin
bus_monitor = xbus_bus_monitor::type_id::create("bus_monitor",
this);
end
‐‐‐‐
‐‐‐‐
Endfunction : build
class ubus_env extends uvm_env;
// Virtual Interface variable
protected virtual interface ubus_if vif;
‐‐‐‐
‐‐‐‐
function void build_phase(uvm_phase phase);
string inst_name;
super.build_phase(phase);
if(!uvm_config_db#(virtual ubus_if)::get(this, "", "vif", vif))
`uvm_fatal("NOVIF",{"virtual interface must be set for:
",get_full_name(),".vif"});
‐‐‐
‐‐‐
Endfunction : build_phase
If you add +UVM_USE_OVM_RUN_SEMANTIC in the command line it will cause the run phase to use old OVM style run semantics.

5.
// implement run task
task run;
fork
update_vif_enables();
join
endtask : run
function void end_of_elaboration();
$display("%0t: %0s: end_of_elaboration", $time, get_full_name());
endfunction
function void start_of_simulation();
$display("%0t: %0s: start_of_simulation", $time, get_full_name());
endfunction
function void extract();
$display("%0t: %0s: extract", $time, get_full_name());
endfunction
function void check();
$display("%0t: %0s: check", $time, get_full_name());
endfunction
function void report();
$display("%0t: %0s: report", $time, get_full_name());
Endfunction
// implement run task
task run_phase(uvm_phase phase);
fork
update_vif_enables();
join
endtask : run_phase
function void end_of_elaboration_phase(uvm_phase phase);
$display("%0t: %0s: end_of_elaboration", $time, get_full_name());
endfunction
function void start_of_simulation_phase(uvm_phase phase);
$display("%0t: %0s: start_of_simulation", $time, get_full_name());
endfunction
function void extract_phase(uvm_phase phase);
$display("%0t: %0s: extract", $time, get_full_name());
endfunction
function void check_phase(uvm_phase phase);
$display("%0t: %0s: check", $time, get_full_name());
endfunction
Table 1: Summary of the changes in phase methods
3.2 Additional phases in UVM
UVM saw the introduction of a large number of new phases to give finer control over the simulation. These are summarised in “Error! Reference
source not found.” below.

6.
OVM UVM
Table 2: Additional phases in UVM

7. 4 Managing the End of Test
Modern test benches provide a way for components and objects to synchronize their testing activity and indicate it is safe to end the phase and
the simulation. UVM (and OVM) provides a built‐in objection for each phase which allows a component to “object” to the phase ending. This
objection mechanism gives a structured way for hierarchical test bench components status to communicate their status. For example, a
component may raise an objection when it starts a transaction with the DUT (“Design Under Test”) and not drop that objection under the
transaction is complete. Or a component expecting a response from the DUT will keep an objection raised until the response is received.
Note that the “uvm_test_done” objection also works in UVM, but it is not the recommended way of managing the end of test. In UVM it is
recommended to use the available time consuming phases, so using a global variable is no longer a robust mechanism.
In OVM, calling “global_stop_request” was not recommended but it was not deprecated.
OVM UVM
task run();
seq.start( m_virtual_sequencer );
global_stop_request();
endtask
task run_phase( uvm_phase phase );
phase.raise_objection( this );
seq.start( m_virtual_sequencer );
phase.lower_objection( this );
endtask
task run();
ovm_test_done.raise_objection();
seq.start( m_virtual_sequencer );
ovm_test_done.drop_objection();
endtask
task run_phase( uvm_phase phase );
phase.raise_objection( this , "started sequence"
);
seq.start( m_virtual_sequencer );
phase.drop_objection( this , "finished
sequence");
endtask
Table 3: Comparing end of test in OVM and UVM

8. 5 Configuring Component
In UVM it is recommended to use “uvm_config_db” method for configuring components. OVM used the “[set,get]_config_[int,string,object]“
methods for configuring components. The UVM equivalents of these methods are available, but not recommended.
The “uvm_config_db” is parameterized by the type of object that is being configured.
OVM UVM
class my_env extends ovm_env;
...
function void build();
ahb_cfg = ahb_config::type_id::create("ahb_cfg");
ahb_cfg.width = 16;
// set additional fields
set_config_object("*","ahb_cfg",ahb_cfg);
endfunction
...
endclass
class my_ahb_agent extends ovm_component;
...
function void build();
ovm_object cfg;
ahb_config my_cfg;
assert(get_config_object("ahb_cfg",cfg,0);
if (!$cast(my_cfg, cfg))
ovm_report_error(...);
...
endfunction
...
endclass
class my_env extends uvm_env;
...
function void build();
ahb_cfg = ahb_config::type_id::create("ahb_cfg");
ahb_cfg.width = 16;
// set additional fields
uvm_config_db#(ahb_config)::set(
this,"ahb_agent","ahb_cfg",ahb_cfg);
endfunction
...
endclass
class my_ahb_agent extends uvm_component;
...
function void build();
ahb_config my_cfg;
if (!uvm_config_db::ahb_config::get(
this,"","ahb_cfg",my_cfg);
`uvm_error(...)
...
endfunction
...
endclass
Table 4: Configuring components in OVM and UVM

9. It is recommended to avoid using “assign_vi” function that takes a virtual interface handle as an argument and calls an equivalent function on
one or more child component. This is repeated down until the last component reached.
Following approach is not recommended
XBUS ENV
function void assign_vi(virtual interface xbus_if xi);
xi0 = xi;
if( bus_monitor != null) begin
bus_monitor.assign_vi(xi);
end
for(int i = 0; i < num_masters; i++) begin
masters[i].assign_vi(xi);
end
for(int i = 0; i < num_slaves; i++) begin
slaves[i].assign_vi(xi);
end
endfunction : assign_vi
AGENT
function void assign_vi(virtual interface xbus_if xmi);
monitor.assign_vi(xmi);
if (is_active == UVM_ACTIVE) begin
sequencer.assign_vi(xmi);
driver.assign_vi(xmi);
end
endfunction : assign_vi
Table 5: Avoid using “assign_vi”

10. Figure 3: The steps involved in using the register model
6 UVM Register layer
Constrained random test benches are required to model the DUT behaviour to predict expected behaviours. This includes models of the
registers and/or memories within the DUT. The UVM provides register layer classes to create a high‐level, object‐oriented model for memory‐
mapped registers and memories in a design under verification. The following methodology features are key to building and using such a model.
• Create an abstract model of the registers and memories in DUT
o To maintain a “mirror” of the DUT registers.
• Create a hierarchy that is analogous to the DUT hierarchy
o Register Block
o Register File
o Memory
o Register
o Field
• Provide access to the register through a defined API
o Address‐independent instance/string names
• Model the address map
o Model access via specific interface
Figure 3 opposite shows the steps involved in using the register
model.

11. 6.1 Access API
“Table 6: Register access API” below gives an overview of the register access API and how it should be used.
Command Description
Read()/Write()
Generate Physical Read from the DUT
Generate physical Write to the DUT
Peek()/poke()
Peek() or poke() methods Read/write directly to the register
Get()/set()
Get() or set() methods read/write directly to the desired value

12. Update()
Update() the DUT with desired value in the model
Mirror()
Read the DUT register and check/update the model value
Table 6: Register access API
7 Summary
Over the years various verification methodologies have been introduced in order to optimise use of scarce verification resources. The
methodologies have followed an evolution that has brought us naturally to UVM – an industry wide methodology built on an open source
language and library.
In this paper we have shown what is required in order to transition from OVM to UVM.