Register Design Languages – DSL or not?

Recently, Gary Stringham has been running a series of interviews with providers of register design tools on his website. Register design tools seems to be an active area with several small companies (and some open-source tools) fighting for the market. I have written about Gary Stringham and register designs before, and it is an area that keeps fascinating me. There is something about the task of register design that keeps it separate from the main hardware design languages, tools, and flows.The different approaches taken by the tools supporting the register design task also illustrates some points about programming language standards, domain-specific languages, and exchange formats that I want to address.

First of all, what makes register design such a special task? My guess is because it is a higher-level aspect of the design: it describes an interface, which can be implemented in many different ways in hardware. Regular HDLs do not help you reason about register layouts in a good way. The register specification should also be used in other places than the hardware: in address definitions for device drivers, in manuals, and other documentation. All of this indicates that the information needs to be explicit, high-level, and in a format that facilitates automatic processing by tools. It basically screams for a domain-specific language (DSL) at some level of ambition (read my old post about how languages are grown from problems for more background on just how simple a DSL can be).

If you look at the register design tool vendors that Gary interviewed, you can essentially see two different approaches to how to support the task of register design. In particular, the input language differs quite radically between the tools.

There is domain-specific programming language approach. A register-design DSL makes it easy to work with register designs, and pretty hard to do anything else (imagine writing a sorting algorithm in a register-design language – I don’t think it can be done, and if it can be done, the result has to be absolutely contorted). The DSL approach assumes a willingness  on the part of a user to learn a new language, but it seems that for this domain, the languages are simple enough that learning them saves you time in the end. Especially if you maintain large register maps that keep changing or is updated and extended across generations of chips (indeed, ease of maintenance is an undersold aspect of most DSLs in my experience).

The underlying assumption of the DSL approach is that the entry language is not too important, as long as you can export data into other formats. To me, this makes sense. From a single source with sufficient descriptive power, you can ideally generate both HDL code to implement the decoder in hardware, as well as documentation, header files, and maybe even skeletons for virtual platform models. And standard formats like IP-XACT to feed into any other tool.

It does not matter if there is a single tool using each input language, since the outputs are what matters. This leaves the vendors free to invent on the input side, and provide a really powerful tool.

The second approach is transformation-based. Its idea is to not use a dedicated input language, but rather powerful import functions to read whatever specifications already exist in text files, Microsoft Word documents, Microsoft Excel spreadsheets, Framemaker documentation files, IP-XACT, or whatever you can come up with.  The assumption is that the tool is really about transforming data and not about compiling a language. Register designs kind of already exist, and since there is no standard language to compile, you just import whatever there is. It makes the assumption that the user base is not willing to learn a new language to handle register design. A funky side-effect is that we might end up actually doing programming work in Word docs.

Still, the transformation approach ends up generating the same outputs as the DSL approach. In  both cases, some of the outputs can be considered “industry standards”, like IP-XACT, while others are decidedly ad-hoc, like Excel sheets. To me, this demonstrates that the true value in standards is to enable tool interoperability, and not so much as a way to provide inputs.

Indeed, there is a clear difference between good input languages and good exchange formats. An input language tends to drive for expressive power and human readability, and it is OK to have a heavy compilation process associated with it. An exchange format like IP-XACT does not need to be human-readable, nor efficient to code in. It needs to be easy to parse and compile, so that as many tools as possible can work on it.

A typical example from the field of register design is dealing with repeated groups of registers (such as banks of registers for a DMA channel) and parametrized register designs. The input languages used by the tools that Gary Stringham covers all allow this – while the standard for register design exchange, IP-XACT, only uses a flat compiled map. It just lists all registers with sizes and offsets, not how these offsets were arrived at or if there is some logical grouping or iterated structure. Quite OK for a tool to work on, but not very useful as a repository for the actual information.

This distinction is worth to keep in mind.

I definitely side with the DSL idea, as that fits my idea of a good tool, but I can see why many people find the transformation from other formats attractive. In all cases, the goal is to have an original design specification that is as clear, succinct, and flexible as possible.



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