Are Program Blocks Necessary?
Program blocks came directly from donation of the Vera language to SystemVerilog by Synopsys , and try to mimic the scheduling semantics that a PLI application has interacting with a Verilog simulator. So coming from a Vera background, program blocks make perfect sense and do help people transitioning from Vera to SV. But looking at SV from scratch, they are just extra language baggage.
Who would have ever thought we’d be having language wars within the same language!
As far as I can tell, a program block by itself only addresses two race conditions between the testbench and DUT, both of which are covered by using a clocking block by itself.
- Erroneous use of blocking assignments for sequential logic. You have a race within your DUT regardless of the race between your testbench and DUT.
- Erroneous use of non-blocking assignments in combinational gated clock logic. You may have a race within your DUT regardless of the race between your testbench and DUT.
As a user, if you don’t understand why these create races within your DUT, you’re going to have the same races within your testbench, and there’s nothing a program block does that prevent races within your testbench. There lays the false sense of security of having a race-free testbench.
Using a clocking block by itself takes care of the same testbench to DUT races that a program block addresses, plus it takes care of the races caused by non-zero delay skews introduced by gate-level propagation. It does this by the use of the input skews for sampling and output skews for driving.
Now, in addition to the false sense of security, and the redundancy with clocking blocks, here are some additional reasons why I don’t recommend using program blocks
- If you have legacy Verilog testbench code, sometimes you want to share legacy BFM tasks by having your “class” based testbench call those BFM tasks. You’re going to run into nasty timing problems if that task was designed to be scheduled in the active region, and now is scheduled in the re-active. Sampling will be off by a clock cycle. You’ll have even nastier problems if some tasks are called from a program block, and other are still called from a module.
- One person’s Design IP is another person’s Verification IP. At the system level (ESL), there is less of a distinction between models written to represent higher level abstractions of the design, versus part of a testbench. You can’t have differences in scheduling just because one time it’s called from a program, and another time it’s called from a module. Same problem with C code called from a program block or module.
- Unless you’re an experienced Vera user, there is the unexpected surprise that your simulation exits immediately after the thread in your program block ends. Again this is an issue with mixing legacy testbenches, or mixed-language testbenches.
- Most advanced users can barely understand the scheduling semantics of SystemVerilog even without using program blocks. Why introduce unnecessary complexity. Many other enviroments, like SystemC and VHDL have been in production for years without needing the kind of scheduling semantics the program block introduces. Quick quiz: How can you get an assertion pass and fail in the same time slot?
The SystemVerilog language has had many hands involved with its development, including yours and mine. I’m not dismissing anyone efforts, but sometimes you have to take a step back and realize how bloated the language has become. Just because some feature exists in the LRM doesn’t justify that it needs to be used. Let me tell you about virtual interfaces…
Posted May 7th, 2009, by Dave Rich
- Loading tweets...
- Loading tweets...
- Portable and Productive Test Creation with Graph-Based Stimulus
- Supporting A Season of Learning
- DVCon Goes Global!
- Better Late Than Never: Magical Verification Horizons DAC Edition
- Accellera Approves UVM 1.2
- Getting More Value from your Stimulus Constraints
- The FPGA Verification Window Is Open
- UVM DVCon 2014 Tutorial Video Online
- Mentor Enterprise Verification Platform Debuts
- New Verification Academy ABV Course
- September 2014 (2)
- August 2014 (2)
- July 2014 (1)
- May 2014 (2)
- April 2014 (2)
- March 2014 (2)
- February 2014 (5)
- January 2014 (1)
- November 2013 (2)
- October 2013 (3)
- September 2013 (2)
- August 2013 (4)
- July 2013 (6)
- Part 7: The 2012 Wilson Research Group Functional Verification Study
- Walking in the Desert or Drinking from a Fire Hose?
- Part 6: The 2012 Wilson Research Group Functional Verification Study
- A Short Class on SystemVerilog Classes
- Part 5: The 2012 Wilson Research Group Functional Verification Study
- Part 4: The 2012 Wilson Research Group Functional Verification Study
- June 2013 (2)
- May 2013 (4)
- April 2013 (2)
- March 2013 (2)
- February 2013 (5)
- January 2013 (1)
- December 2012 (1)
- November 2012 (1)
- October 2012 (4)
- September 2012 (1)
- August 2012 (1)
- July 2012 (6)
- June 2012 (1)
- May 2012 (3)
- March 2012 (1)
- February 2012 (6)
- January 2012 (2)
- December 2011 (2)
- November 2011 (2)
- October 2011 (3)
- September 2011 (1)
- July 2011 (3)
- June 2011 (6)
- Intelligent Testbench Automation Delivers 10X to 100X Faster Functional Verification
- Part 9: The 2010 Wilson Research Group Functional Verification Study
- Verification Horizons DAC Issue Now Available Online
- Accellera & OSCI Unite
- The IEEE’s Most Popular EDA Standards
- UVM Register Kit Available for OVM 2.1.2
- May 2011 (2)
- April 2011 (7)
- User-2-User’s Functional Verification Track
- Part 7: The 2010 Wilson Research Group Functional Verification Study
- Part 6: The 2010 Wilson Research Group Functional Verification Study
- SystemC Day 2011 Videos Available Now
- Part 5: The 2010 Wilson Research Group Functional Verification Study
- Part 4: The 2010 Wilson Research Group Functional Verification Study
- Part 3: The 2010 Wilson Research Group Functional Verification Study
- March 2011 (5)
- February 2011 (4)
- January 2011 (1)
- December 2010 (2)
- October 2010 (3)
- September 2010 (4)
- August 2010 (1)
- July 2010 (3)
- June 2010 (9)
- The reports of OVM’s death are greatly exaggerated (with apologies to Mark Twain)
- New Verification Academy Advanced OVM (&UVM) Module
- OVM/UVM @DAC: The Dog That Didn’t Bark
- DAC: Day 1; An Ode to an Old Friend
- UVM: Joint Statement Issued by Mentor, Cadence & Synopsys
- Static Verification
- OVM/UVM at DAC 2010
- DAC Panel: Bridging Pre-Silicon Verification and Post-Silicon Validation
- Accellera’s DAC Breakfast & Panel Discussion
- May 2010 (9)
- Easier UVM Testbench Construction – UVM Sequence Layering
- North American SystemC User Group (NASCUG) Meeting at DAC
- An Extension to UVM: The UVM Container
- UVM Register Package 2.0 Available for Download
- Accellera’s OVM: Omnimodus Verification Methodology
- High-Level Design Validation and Test (HLDVT) 2010
- New OVM Sequence Layering Package – For Easier Tests
- OVM 2.0 Register Package Released
- OVM Extensions for Testbench Reuse
- April 2010 (6)
- SystemC Day Videos from DVCon Available Now
- On Committees and Motivations
- The Final Signatures (the meeting during the meeting)
- UVM Adoption: Go Native-UVM or use OVM Compatibility Kit?
- UVM-EA (Early Adopter) Starter Kit Available for Download
- Accellera Adopts OVM 2.1.1 for its Universal Verification Methodology (UVM)
- March 2010 (4)
- February 2010 (5)
- January 2010 (5)
- December 2009 (15)
- A Cliffhanger ABV Seminar, Jan 19, Santa Clara, CA
- Truth in Labeling: VMM2.0
- IEEE Std. 1800™-2009 (SystemVerilog) Ready for Purchase & Download
- December Verification Horizons Issue Out
- Evolution is a tinkerer
- It Is Better to Give than It Is to Receive
- Zombie Alert! (Can the CEDA DTC “User Voice” Be Heard When They Won’t Let You Listen)
- DVCon is Just Around the Corner
- The “Standards Corner” Becomes a Blog
- I Am Honored to Honor
- IEEE Standards Association Awards Ceremony
- ABV and being from Missouri…
- Time hogs, blogs, and evolving underdogs…
- Full House – and this is no gamble!
- Welcome to the Verification Horizons Blog!
- September 2009 (2)
- July 2009 (1)
- May 2009 (1)