Libre RISC-V M-Class

A 100% libre RISC-V + 3D GPU chip for mobile devices

Jul 29, 2019

Project update 7 of 10

Purism Donation

by Luke Kenneth Casson Leighton

We are delighted to be able to announce additional sponsorship by Purism, through NLNet.

Purism Sponsorship

As a social purpose corporation, Purism is empowered to balance ethics, social enterprise, and profitable business. I am delighted that they chose to fund the Libre RISC-V hybrid CPU/GPU through the NLNet Foundation. Their donation provides us some extra flexibility in how we reach the goal of bringing to market a hybrid CPU, VPU, and GPU that is libre to the bedrock.

Purism started with a Crowd Supply campaign to deliver a modern laptop with full software support and a coreboot BIOS. I know that, after this initial success, they worked hard to try to solve the "NSA backdoor co-processor" issue, known as the "Management Engine". Ironically, inspired by Purism, Intel’s internal efforts became moot, as a 3rd party reverse engineered an Intel BIOS and discovered the nsa_me_off_switch parameter, designed to be used by the NSA when Intel equipment is deployed within NSA premises.

Purism then moved quickly to provide a BIOS update to disable this "feature," eliminating the last and most important barrier to being able to declare a full privacy software stack.

Purism deserves our respect and gratitude for this type of brave and strategic decision-making to kick the trend towards privacy-invading hardware "by default."

However, just as NLNet recognises, Purism also appreciates that we cannot stop at just the software. Profit-maximising corporations just do not take the brave decisions that can compromise profits, particularly when faced with competition: it’s too much. This is why being a Social Purpose Corporation is so critically important. Socially-responsible decisions do not get undermined by profit-maximisation.

So, we are extremely grateful for their donation, managed through NLnet.


So much has happened already, since the last update, it is hard to know where to begin.


Multi-issue is absolutely critical for this CPU/VPU/GPU because the SimpleV engine critically relies on being able to turn one "vector" operation into multiple "scalar element" instructions, in every cycle. The simplest way to do this is to throw equivalent scalar opcodes into a multi-issue execution engine, and let the engine sort it out.

Regarding the dependency matrices: thanks to Mitch Alsup’s absolutely invaluable input, we now know how to do multi-issue. On top of a precise 6600 style dependency matrix it is almost comically trivial.

The key insight Mitch gave us was that instruction dependencies are transitive. In other words, if there are four instructions to be issued, the second instruction may have the dependencies of the first added to it, the 3rd may accumulate the dependencies of the first and second, and so on.

Where this trick does not work well (or takes significant hardware to implement) is when, for example with the Tomasulo Algorithm (or the original 6600 Q-Table), the register dependency hazards are expressed in binary (r5 = 0b00101, r3=0b00011). If instead the registers are expressed in unary (r5 = 0b00010000, r3= 0b00000100) then it should be pretty obvious that in a multi-issue design, all that is needed in each clock cycle is to OR the cumulative register dependencies in a cascading fashion. Aside from now also needing to increase the number of register ports and other resources to cope with the increased workload, amazingly that’s all it takes!

To achieve the same trick with a Tomasulo reorder buffer (ROB) requires the addition of an entire extra CAM per every extra issue to be added to the architecture: four-way multi-issue would require four ROB CAMs! The power consumption and gate count would be prohibitively expensive, and resolving the commits of multiple parallel operations is also fraught.


What began ironically as "simple" still bears some vestige of its original name, in that the ISA needs no new opcodes: any scalar RISC-V implementation may be turned parallel through the addition of SV at the instruction issue phase.

However, one of the major drawbacks of the initial draft spec was that the use of CSRs took a huge number of instructions just to set up and then tear down the vectorisation context. This had to be fixed.

The idea which came to mind was to embed RISC-V opcodes within a longer, variable-length encoding, which we’ve called the VBLOCK format. At the beginning of this new format, the vectorisation and predication context could be embedded, which "changes" the standard scalar opcodes to become "parallel" (multi-issue) operations.

The advantage of this approach is that, firstly, the context is much smaller: the actual CSR opcodes are gone, leaving only the "data," which is now batched together. Secondly, there is no need to "reset" (tear down) the vectorisation context, because that automatically goes when the long-format ends.

The other issue that needed to be fixed is that we really need a SETVL instruction. This is really unfortunate as it breaks the "no new opcodes" paradigm. However, what we are going to do is simply to reuse the RVV SETVL opcode, now that RVV has reached its last anticipated draft before ratification. Secondly: it’s not an actual instruction related to elements (it doesn’t perform a parallel add, for example). It’s more an "infrastructure support" instruction.

The reason for needing SETVL is complex. It is down to the fact that, unlike in RVV, the maximum vector length is not an architectural hard design parameter, it is a runtime dynamic one. Thus, it is absolutely crucial that not only VL is set on every loop (or SV prefix instruction), but that MVL is also set.

This means SV has two additional instructions for any algorithm, when compared to RVV, and this kind of penalty is just not acceptable. The solution therefore was to create a special SV.SETVL opcode that always takes the MVL as an additional extra parameter over and above those provided to the RV equivalent opcode. That basically puts SV on par with RV as far as instruction count is concerned.

Fail on First

The other really nice addition, which came with a small reorganisation of the vector and predicate contexts, is data dependent "fail on first."

ARM’s SVE, RVV, and the Mill architecture all have an incredibly neat feature where, if data is being loaded from memory in parallel, and the LD operations run off the end of a page boundary, this may be detected and the legal parallel operations may complete, all without needing to drop into "scalar" mode.

In the case of the Mill architecture, this is achieved through the extremely innovative feature of simply marking the result of the operation as "invalid," and that "tag" cascades through all subsequent operations. Thus, any attempts to ADD or STORE the data will result in the invalid data being simply ignored.

RV instead detects the point at which the LD became invalid, "fails" at the "first" such illegal memory access, and truncates all subsequent vector operations to within that limit, by changing VL. This is an extremely effective and very simple idea worth adding to SV.

However, when doing so, the idea sprang to mind: why not extend the "fail on first" concept to not just cover LD/ST operations, but to cover actual ALU operations as well? Why not, if any of the the results from a sequence of parallel operations is zero ("fail"), similarly truncate VL?

This idea was tested out on strncpy (the typical canonical function used to test out data-dependent ISA concepts), and it worked! So, that is going into SV as well. It does mean that after every ALU operation, a comparator against zero will be optionally activated: given that it is optional and under the control of the first bit, it is not a power penalty on every single instruction.


There is so much to do, and so much that has already been achieved, it is almost overwhelming. We still cannot lose sight of the fact there is an enormous amount that we do not yet know, yet at the same time, never let that stop us from moving forward. A journey starts with a first step, and continues with each step.

With help from NLNet and companies like Purism, we can look forward to actually paying people to contribute to solving what was formerly considered an impossible task.

It is worthwhile emphasising: any individual or corporation wishing to see this project succeed (so that you can use it as the basis for one of your products, for example), donations through NLNet, as a registered charitable foundation, are tax deductible.

Likewise, for anyone who would like to help with the project’s milestones, payments from NLnet are donations, and, depending on jurisdiction, may also be tax deductible (i.e., not classed as "earnings"). If you are interested to learn more, do get in touch.

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