# Introduction

The Universal Verification Methodology (UVM) is the dominant RTL verification network across the IC industry. The UVM is popular because it allows engineers to reuse testbench components across testbenches. It also provides a standardized testbench structure that allows engineers to understand existing testbenches as these engineers move across projects, companies, and industries. If you apply for a job as a verification engineer, it is likely that the hiring team uses the UVM and will test your UVM knowledge.

IEEE defined the UVM in the [*IEEE Standard for Universal Verification Methodology Language Reference Manual*](https://ieeexplore.ieee.org/document/9195920), also know as the [IEEE 1800.2 standard](https://ieeexplore.ieee.org/document/9195920).

While the industry defined the 1800.2 standard in terms of SystemVerilog, there is little in the standard that requires us to implement the UVM library in SystemVerilog.  It could be implemented in any language with sufficient object oriented support---for example, Python.


## API Documentation

You can read the API documentation for **pyuvm** on [GitHub Pages](https://pyuvm.github.io/pyuvm/).

## Python and IEEE 1800.2

**pyuvm** is the Universal Verification Methodology implemented in Python instead of SystemVerilog. **pyuvm** uses [**cocotb**][cocotbLink] to interact with simulators and schedule simulation events.

**pyuvm** takes advantage of Python's ease of use and object-oriented power to implement the most-often used parts of the IEEE 1800.2 standard.  It is easier to write UVM code in Python because Python does not have strict typing and does not require parameterized classes.  Python also supports important object-oriented programming (OOP) concepts such as [multiple-inheritance][multipleInh] that are missing in SystemVerilog.

## IEEE 1800.2 and pyuvm

**pyuvm** is a clean implementation of IEEE 1800.2 in Python.  It implements the following sections from 1800.2:

|Section|Name|Description|
|-------|----|-----------|
|5|Base Classes|Basic classes such as `uvm_void` and `uvm_object`|
|6|Reporting Classes|**pyuvm** uses the **logging** package to implement reporting, but integrates it within some of the UVM reporting functionality.|
|8|Factory Classes|**pyuvm** implements all the UVM factory functionality without using the macros needed in SystemVerilog.  The factory supports any class extended from `uvm_void`.|
|9|Phasing|IEEE 1800.2 describes basic phasing that everyone uses and a complicated custom phasing system that almost nobody uses.  **pyuvm** only implmenents the phasing that everyone uses, but you can extend phasing using Python OOP techniques.|
|12|UVM TLM Interfaces|**pyuvm** fully implements the UVM *Transaction Level Modeling* (TLM) system. |
|13|Predefined Component Classes|**pyuvm** implements uvm_component with hierarchy, the uvm_root singleton, and the run_test() task. It simplifies the `uvm_config_db` to the Python-friendly `Config` class.
|14 & 15|Sequences, sequencer, sequence_item|**pyuvm** refactored the sequencer functionality to create a simpler implementation of the UVM Sequence functionality.|
|17|UVM Register Enum|**pyuvm** implements all the basic Enum types in the **pyuvm** *Register Access Layer* (RAL)|
|18|UVM Register Block|**pyuvm** implements the RAL register block classes|
|19|UVM Register Field|**pyuvm** implements register fields as defined in IEEE 18002.  There are still few functionalities missing like atomic Backdoor access, Field byte access, and single Field access during read or write operation|
|20|UVM Register|Main register class is implemented but is still missing Backdoor and used Backdoor to be leveraged from cocotb force. Byte access and single field access yet to be implemented|
|21|UVM Register Map|Main register map class, should be refactored to guarantee simplicity and backdoor access, extension class in Read and Write to be implemented|
|22|UVM Memory|Not Implemented|
|23|Register Item|Register Item used across multiple classes|
|24|Register include file|Includes other Enum and types to be merged with s17|
|25|UVM register adapter|Main register adapter|
|26|UVM register predictor|Main register predictor, should be disabled if auto_prediction is not set|
|27|Register Package|Main PKG if included should behave similarly to uvm_reg_pkg|

# Installation

You can install **pyuvm** using `pip`. This will also install **cocotb** as a requirement for **pyuvm**.

```bash
% pip install pyuvm
```

Then you can run a simple test:

```bash
% python
>>> from pyuvm import *
>>> my_object = uvm_object("my_object")
>>> type(my_object)
<class 's05_base_classes.uvm_object'>
>>> print("object name:", my_object.get_name())
object name: my_object
```

## Running from a cloned repository

You can run pyuvm from a cloned repository by installing the cloned repository using pip.

```bash
% cd <pyuvm repo directory>
% pip install -e .
```

# Usage

This section demonstrates running an example simulation and then shows how the example has been put together demonstrating what the UVM looks like in Python.

## Running the simulation

The TinyALU is, as its name implies, a tiny ALU. It has four operations: ADD, AND, NOT, and MUL. This example shows us running the Verilog version of the design, but there is also a VHDL version.

**cocotb** uses a Makefile to run its simulation. We see it in `examples/TinyALU`:

```makefile
CWD=$(shell pwd)
COCOTB_REDUCED_LOG_FMT = True
SIM ?= icarus
VERILOG_SOURCES =$(CWD)/hdl/verilog/tinyalu.sv
MODULE := testbench
TOPLEVEL=tinyalu
TOPLEVEL_LANG=verilog
COCOTB_HDL_TIMEUNIT=1us
COCOTB_HDL_TIMEPRECISION=1us
include $(shell cocotb-config --makefiles)/Makefile.sim
```

You can learn more about the Makefile targets at [cocotb.org](https://docs.cocotb.org/en/stable/building.html).  The `cocotb-config` command on the last line points to the **cocotb** Makefile locations and launches the `sim` target.

Modify the `SIM` variable to match your simulator. All the simulator types are in `cocotb/share/makefiles/simulators/makefile.$(SIM)`.

You should be able to run the simulation like this:

```bash
% cd <path>/pyuvm/examples/TinyALU
% make sim
```

**cocotb** will present a lot of messages, but in the middle of them you will see these UVM messages. It runs four examples with one for each command with randomized operands.

```text
250000.00ns INFO     testbench.py(209)[uvm_test_top.env.scoreboard]: PASSED: 0x34 ADD 0x23 = 0x0057
250000.00ns INFO     testbench.py(209)[uvm_test_top.env.scoreboard]: PASSED: 0xf9 AND 0x29 = 0x0029
250000.00ns INFO     testbench.py(209)[uvm_test_top.env.scoreboard]: PASSED: 0x71 XOR 0x01 = 0x0070
250000.00ns INFO     testbench.py(209)[uvm_test_top.env.scoreboard]: PASSED: 0xb8 MUL 0x47 = 0x3308
250000.00ns INFO     testbench.py(209)[uvm_test_top.env.scoreboard]: PASSED: 0xff ADD 0xff = 0x01fe
250000.00ns INFO     testbench.py(209)[uvm_test_top.env.scoreboard]: PASSED: 0xff AND 0xff = 0x00ff
250000.00ns INFO     testbench.py(209)[uvm_test_top.env.scoreboard]: PASSED: 0xff XOR 0xff = 0x0000
250000.00ns INFO     testbench.py(209)[uvm_test_top.env.scoreboard]: PASSED: 0xff MUL 0xff = 0xfe01
```

## The `TinyAluBfm` in `tinyalu_utils.py`

The `TinyAluBfm` is a singleton that uses **cocotb** to communicate with the TinyALU.  The BFM exposes three coroutines to the user: `send_op()`, `get_cmd()`, and `get_result()`.

The singleton uses the `cocotb.top` variable to get the handle to the DUT.  This is the handle that we normally pass to a `cocotb.test()` coroutine.

The `TinyAluBfm` is defined in `tinyalu_utils.py` and imported into our testbench.

# The **pyuvm** testbench

The `testbench.py` contains the entire UVM testbench and connects to the TinyALU through a `TinyAluBfm` object defined in `tinyalu_utils.py`.  We'll examine the `testbench.py` file and enough of the **cocotb** test to run the simulation.

## Importing **pyuvm**

Testbenches written in the SystemVerilog UVM usually import the package like this:

```systemverilog
import uvm_pkg::*;
```

This gives you access to the class names without needing a package path.  To get
similar behavior with **pyuvm** us the `from` import syntax. We import **pyuvm** to distinguish the `@pyuvm.test()` decorator from the `@cocotb.test()` decorator:

```python
import pyuvm
from pyuvm import *
```

## The AluTest classes

We're going to examine the UVM classes from the top, the test, to the bottom, the sequences.

**pyuvm** names the UVM classes as they are named in the specification. Therefore **pyuvm** uses underscore naming as is done in SystemVerilog and not camel-casing.

We extend `uvm_test` to create the `AluTest`, using camel-casing in our code even if **pyuvm** does not use it:

You'll see the following in the test:

* We define a class that extends `uvm_test`.

* We use the `@pyuvm.test()` decorator to notify **cocotb** that this is a test.

* There is no `uvm_component_utils()` macro. **pyuvm** automatically registers classes that extend `uvm_void` with the factory.

* The phases do not have a `phase` variable. Phasing has been refactored to support only the *common phases* as described in the specification.

* We create the environment using the `create()` method and the factory. Notice that `create()` is now a simple class method. There is no typing-driven incantation.

* `raise_objection()` is now a `uvm_component` method. There is no longer a `phase` variable.

* The `ConfigDB()` singleton acts the same way as the `uvm_config_db` interface in the SystemVerilog UVM. **pyuvm** refactored away the `uvm_resource_db` as there are no issues with classes to manage.

* **pyuvm** leverages the Python logging system and does not implement the UVM reporting system. Every descendent of `uvm_report_object` has a `logger` data member.

* Sequences work as they do in the SystemVerilog UVM.

```python
@pyuvm.test()
class AluTest(uvm_test):
    def build_phase(self):
        self.env = AluEnv("env", self)

    def end_of_elaboration_phase(self):
        self.test_all = TestAllSeq.create("test_all")

    async def run_phase(self):
        self.raise_objection()
        await self.test_all.start()
        self.drop_objection()
```

We extend the `AluTest` class to create a parallel version of the test and a Fibonacci program. You can find these sequences in `testbench.py`

```python
@pyuvm.test()
class ParallelTest(AluTest):
    def build_phase(self):
        uvm_factory().set_type_override_by_type(TestAllSeq, TestAllForkSeq)
        super().build_phase()

@pyuvm.test()
class FibonacciTest(AluTest):
    def build_phase(self):
        ConfigDB().set(None, "*", "DISABLE_COVERAGE_ERRORS", True)
        uvm_factory().set_type_override_by_type(TestAllSeq, FibonacciSeq)
        return super().build_phase()


```

All the familiar pieces of a UVM testbench are in **pyuvm**.

## The ALUEnv Class

The `uvm_env` class is a container for the components that make up the testbench.  There are four component classes instantiated:

* `Monitor`—There are actually two monitors instantiated, one to monitor commands (`self.cmd_mod`) and the other to monitor results (`self.result_mon`).  The `Monitor` code is the same for both. We pass them the name of the proxy function that gets the data they monitor.
* `Scoreboard`—The scoreboard gathers all the commands and results and compares predicted results to actual results.
* `Coverage`—The coverage class checks that we've covered all the kinds of operations and issues an error if we did not.
* `Driver`—This `uvm_driver` processes sequences items.
* `uvm_sequencer`—The `uvm_sequencer` queues sequence items and passes them to the `Driver`
* We store `self.seqr` in the `ConfigDB()` so test can get it as we see above.

The `AluEnv` creates all these components in `build_phase()` and connects the exports to the ports in `connect_phase()`. The `build_phase()` is a top-down phase and the `connect_phase()` is a bottom up phase.

```python
class AluEnv(uvm_env):

    def build_phase(self):
        self.seqr = uvm_sequencer("seqr", self)
        ConfigDB().set(None, "*", "SEQR", self.seqr)
        self.driver = Driver.create("driver", self)
        self.cmd_mon = Monitor("cmd_mon", self, "get_cmd")
        self.coverage = Coverage("coverage", self)
        self.scoreboard = Scoreboard("scoreboard", self)

    def connect_phase(self):
        self.driver.seq_item_port.connect(self.seqr.seq_item_export)
        self.cmd_mon.ap.connect(self.scoreboard.cmd_export)
        self.cmd_mon.ap.connect(self.coverage.analysis_export)
        self.driver.ap.connect(self.scoreboard.result_export)
```

## The Monitor

The `Monitor` extends `uvm_component`. Takes the name of a `CocotProxy` method name as an argument.  It uses the name to find the method in the proxy and then calls the method. You cannot do this in SystemVerilog as there is no introspection.

The `Monitor` creates an analysis port and writes the data it gets into the analysis port.

Notice in the `run_phase()` we use the `await` keyword to wait for the `get_cmd` coroutine.  Unlike SystemVerilog, Python makes it clear when you are calling a time-consuming task vs a function. Also notice that the `run_phase()` has the `async` keyword to designate that it is a coroutine. (A task in SystemVerilog.)

```python
class Monitor(uvm_component):
    def __init__(self, name, parent, method_name):
        super().__init__(name, parent)
        self.method_name = method_name

    def build_phase(self):
        self.ap = uvm_analysis_port("ap", self)
        self.bfm = TinyAluBfm()
        self.get_method = getattr(self.bfm, self.method_name)

    async def run_phase(self):
        while True:
            datum = await self.get_method()
            self.logger.debug(f"MONITORED {datum}")
            self.ap.write(datum)
```

## The Scoreboard

The scoreboard receives commands from the command monitor and results from the results monitor in the same order.  It uses the commands to predict the results and compares them.

* The `build_phase` uses `uvm_tlm_analysis_fifos` to receive data from the monitors and store it.
* The scoreboard exposes the FIFO exports by copying them into class data members.  As we see in the environment above, this allows us to connect the exports without reaching into the `Scoreboard's` inner workings.
* We connect the exports in the `connect_phase()`
* The `check_phase()` runs after the `run_phase()`.  At this point the scoreboard has all operations and results. It loops through the operations and predicts the result, then it compares the predicted and actual result.
* Notice that we do not use UVM reporting. Instead we us the Python `logging` module. Every `uvm_report_object` and its children has its own logger stored in `self.logger.`

```python
class Scoreboard(uvm_component):

    def build_phase(self):
        self.cmd_fifo = uvm_tlm_analysis_fifo("cmd_fifo", self)
        self.result_fifo = uvm_tlm_analysis_fifo("result_fifo", self)
        self.cmd_get_port = uvm_get_port("cmd_get_port", self)
        self.result_get_port = uvm_get_port("result_get_port", self)
        self.cmd_export = self.cmd_fifo.analysis_export
        self.result_export = self.result_fifo.analysis_export

    def connect_phase(self):
        self.cmd_get_port.connect(self.cmd_fifo.get_export)
        self.result_get_port.connect(self.result_fifo.get_export)

    def check_phase(self):
        while self.result_get_port.can_get():
            _, actual_result = self.result_get_port.try_get()
            cmd_success, cmd = self.cmd_get_port.try_get()
            if not cmd_success:
                self.logger.critical(f"result {actual_result} had no command")
            else:
                (A, B, op_numb) = cmd
                op = Ops(op_numb)
                predicted_result = alu_prediction(A, B, op)
                if predicted_result == actual_result:
                    self.logger.info(f"PASSED: 0x{A:02x} {op.name} 0x{B:02x} ="
                                     f" 0x{actual_result:04x}")
                else:
                    self.logger.error(f"FAILED: 0x{A:02x} {op.name} 0x{B:02x} "
                                      f"= 0x{actual_result:04x} "
                                      f"expected 0x{predicted_result:04x}")

```

## Coverage

The `Coverage` Class extends `uvm_subscriber` which extends `uvm_analysis_export`.  As we see in the `AluEnv` above, this allows us to pass the object directly to the `connect()` method to connect it to an analysis port.

The `Coverage` Class overrides the `write()` method expected of a `uvm_subscriber`. If it didn't you'd get a runtime error.  The `Coverage` class uses a set to store all the operations seen. Then it subtracts that from the set of all operations. If the result has a length longer than `0` it issues an error.

Since this tesbench loops through all the operations you will not see this error.

```python
class Coverage(uvm_subscriber):

    def end_of_elaboration_phase(self):
        self.cvg = set()

    def write(self, cmd):
        (_, _, op) = cmd
        self.cvg.add(op)

    def report_phase(self):
        try:
            disable_errors = ConfigDB().get(
                self, "", "DISABLE_COVERAGE_ERRORS")
        except UVMConfigItemNotFound:
            disable_errors = False
        if not disable_errors:
            if len(set(Ops) - self.cvg) > 0:
                self.logger.error(
                    f"Functional coverage error. Missed: {set(Ops)-self.cvg}")
                assert False
            else:
                self.logger.info("Covered all operations")
                assert True
```

## Driver

The `Driver` extends `uvm_driver` and so it works with sequences and sequence items.

The `connect_phase()` gets the proxy from the `ConfigDB()` and the `run_phase()` uses it to get items and process them by calling `send_op`.  We use `while True` because we do this forever. **cocotb** will shut down the `run_phase` coroutine at the end of simulation.

```python
class Driver(uvm_driver):
    def build_phase(self):
        self.ap = uvm_analysis_port("ap", self)

    def start_of_simulation_phase(self):
        self.bfm = TinyAluBfm()

    async def launch_tb(self):
        await self.bfm.reset()
        self.bfm.start_tasks()

    async def run_phase(self):
        await self.launch_tb()
        while True:
            cmd = await self.seq_item_port.get_next_item()
            await self.bfm.send_op(cmd.A, cmd.B, cmd.op)
            result = await self.bfm.get_result()
            self.ap.write(result)
            cmd.result = result
            self.seq_item_port.item_done()

```

### The ALU Sequence

The ALU Sequence creates sequence items, randomizes them and sends them to the `Driver`. It inherits `start_item` and `finish_item` from `uvm_sequence`.

It is clear that `start_item` and `finish_item` block because we call them using the `await` keyword.  `start_item` waits for it's turn to use the sequencer, and `finish_item` sends the sequence_item to the driver and returns when the driver calls `item_done()`

```python
class TestAllSeq(uvm_sequence):

    async def body(self):
        seqr = ConfigDB().get(None, "", "SEQR")
        random = RandomSeq("random")
        max = MaxSeq("max")
        await random.start(seqr)
        await max.start(seqr)

```

This virtual sequence launches two other sequences: `RandomSeq` and `MaxSeq`. `RandomSeq` randomizes the operands.

```python
class RandomSeq(uvm_sequence):
    async def body(self):
        for op in list(Ops):
            cmd_tr = AluSeqItem("cmd_tr", None, None, op)
            await self.start_item(cmd_tr)
            cmd_tr.randomize_operands()
            await self.finish_item(cmd_tr)
```

`MaxSeq` sets the operands to `0xff`:

```python
class MaxSeq(uvm_sequence):
    async def body(self):
        for op in list(Ops):
            cmd_tr = AluSeqItem("cmd_tr", 0xff, 0xff, op)
            await self.start_item(cmd_tr)
            await self.finish_item(cmd_tr)
```

## ALU Sequence Item

The `AluSeqItem` contains the TinyALU commands.  It has two operands and an operation.

The SystemVerilog `uvm_sequence_item` class uses `convert2string()` to convert the item to a string and `do_compare()` to compare the item to another item.  We do not use these in **pyuvm** because Python has magic methods that do these functions.

`__eq__()`—This does the same thing as `do_compare()` It returns `True` if the items are equal.  This method works with the `==` operator.

`__str__()`—This does the same thing as `convert2string()`.  It returns a string version of the item. The `print` function calls this method automatically.

```python
class AluSeqItem(uvm_sequence_item):

    def __init__(self, name, aa, bb, op):
        super().__init__(name)
        self.A = aa
        self.B = bb
        self.op = Ops(op)

    def randomize_operands(self):
        self.A = random.randint(0, 255)
        self.B = random.randint(0, 255)

    def randomize(self):
        self.randomize_operands()
        self.op = random.choice(list(Ops))

    def __eq__(self, other):
        same = self.A == other.A and self.B == other.B and self.op == other.op
        return same

    def __str__(self):
        return f"{self.get_name()} : A: 0x{self.A:02x} \
        OP: {self.op.name} ({self.op.value}) B: 0x{self.B:02x}"

```


# Contributing

You can contribute to **pyuvm** by forking this repository and submitting pull requests.

### Development Environment Setup

The pyuvm project uses forked Github rebase flow.
The first step is to [fork the project](https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/fork-a-repo).
Then clone the repo to your machine.

```sh
git clone git@github.com/{your_username}/pyuvm.git
cd pyuvm/
```

It is recommended to make a virtual environment and work with that environment activated.
This can be done with the built-in [`venv`](https://docs.python.org/3/library/venv.html) module in Python,
or with an alternative like [uv](https://docs.astral.sh/uv/).

```sh
python -m venv .venv
source .venv/bin/activate
```

Next install the main development tools using `pip`.

```sh
pip install .[dev]
```

### Linting

Linting is done with various tools: ruff, pre-commit, codespell, and others.
These tools are all run using [pre-commit](https://pre-commit.com/).
To run pre-commit lints on all sources run the following in the repo root.

```sh
pre-commit run -a
```

You can also set pre-commit to run as a git commit hook, so lints are done before every `git commit`, by running the following in the repo root.

```sh
pre-commit install
```


### Testing

The repository runs all needed tests using [tox](https://tox.wiki/en/stable/).
This will run the full set of tests for all supported operating systems, Python versions, and cocotb versions.
But it will automatically skip any tests that don't apply to your system (OS and Python version).

```sh
tox
```

You can also run tests manually without the use of tox.

There are [pytest](https://docs.pytest.org/en/stable/) tests that test features that don't use coroutines.
You can run these by using calling `pytest` directly at the repo root.

```sh
pytest
```

The rest of the tests are in `tests/cocotb_tests` and use cocotb, thus requiring a simulator.
You can run these by calling `make` in the repo root.
You can change which simulator to use by setting `VERILOG_SIM` and `VHDL_SIM` to set the Verilog and VHDL simulator of choice, respectively.
These `*_SIM` variables take the values the cocotb `SIM` make variable would take.

```sh
make VERILOG_SIM=icarus VHDL_SIM=nvc
```

# Credits

* Ray Salemi—Original author, created as an employee of Siemens.
* IEEE 1800.2 Specification
* Siemens for supporting me in this effort.

# License

Copyright 2020 Siemens EDA and Ray Salemi

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

[http://www.apache.org/licenses/LICENSE-2.0](http://www.apache.org/licenses/LICENSE-2.0)

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.


[cocotbLink]: https://cocotb.org
[multipleInh]: https://www.geeksforgeeks.org/multiple-inheritance-in-python/