Handel-C

Key takeaways

Handel-C belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Handel-C to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Handel-C from memory before moving on to harder problems.

Reference excerpt

Handel-C is a high-level hardware description language aimed at low-level hardware and is most commonly used in programming FPGAs. Handel-C is to hardware design what the first high-level programming languages were to programming CPUs. It is a turing-complete rich subset of the C programming language, with an emphasis on parallel computing. Unlike many other hardware design languages (HDL) that target a specific computer architecture Handel-C can be compiled to a number of HDLs and then synthesised to the corresponding hardware. This frees developers to concentrate on the programming task at hand rather than the idiosyncrasies of a specific design language and architecture.

Additional features Handel-C's subset of C includes all common C language features necessary to describe complex algorithms. Like many embedded C compilers, floating point data types were omitted. Floating point arithmetic is supported through external libraries that are very efficient.

Parallel programs In order to facilitate a way to describe parallel behavior some of the communicating sequential processes (CSP) keywords are used, along with the general file structure of the Occam programming language. For example:

Channels Channels provide a mechanism for message passing between parallel threads. Channels can be defined as asynchronous or synchronous (with or without an inferred storage element respectively). A thread writing to a synchronous channel will be immediately blocked until the corresponding listening thread is ready to receive the message. Likewise the receiving thread will block on a read statement until the sending thread executes the next send. Thus they may be used as a means of synchronizing threads.

Asynchronous channels provide a specified amount of storage for data passing through them in the form of a FIFO. Whilst this FIFO neither full nor empty, both sending and receiving threads may proceed without being blocked. However, when the FIFO is empty, the receiving thread will block at the next read. When it is full, the sending thread will block at the next send. A channel with actors in differing clock domains is automatically asynchronous due to the need for at least one element of storage to mitigate metastability. A thread may simultaneously wait on multiple channels, synchronous or asynchronous, acting upon the first one available given a specified order of priority or optionally executing an alternate path if none is ready.

Scope and variable sharing The scope of declarations are limited to the code blocks ({ ... }) in which they were declared, the scope is hierarchical in nature as declarations are in scope within sub blocks. For example:

Extensions to the C language In addition to the effects the standard semantics of C have on the timing of the program, the following keywords are reserved for describing the practicalities of the FPGA environment or for the language elements sourced from Occam:

Scheduling In Handel-C, assignment and the delay command take one cycle. All other operations are "free". This allows programmers to manually schedule tasks and create effective pipelines. By arranging loops in parallel with the correct delays, pipelines can massively increase data throughput, at the expense of increased hardware resource use.

History The historical roots of Handel-C are in a series of Oxford University Computing Laboratory hardware description languages developed by the hardware compilation group. Handel HDL evolved into Handel-C around early 1996. The technology developed at Oxford was spun off to mature as a cornerstone product for Embedded Solutions Limited (ESL) in 1996. ESL was renamed Celoxica in September 2000. Handel-C was adopted by many University Hardware Research groups after its release by ESL, as a result was able to establish itself as a hardware design tool of choice within the academic community, especially in the United Kingdom. In early 2008, Celoxica's ESL business was acquired by Agility, which developed and sold, among other products, ESL tools supporting Handel-C. In early 2009, Agility ceased operations after failing to obtain further capital investments or credit In January 2009, Mentor Graphics acquired Agility's C synthesis assets. Other subset C HDL's that developed around the same time are Transmogrifier C in 1994 at University of Toronto (now the FpgaC open source project) and Streams-C at Los Alamos National Laboratory (now licensed to Impulse Accelerated Technologies under the name Impulse C)

Worked examples

Example 1 — a first encounter with Handel-C

Start with the simplest possible case. Write down what Handel-C claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Handel-C before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Handel-C ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Handel-C

In research: Handel-C appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Handel-C in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Handel-C is common in secondary-school and first-year university syllabi. It links to neighbouring topics C programming language family, Electronic design automation, Hardware description languages, so understanding it makes those chapters shorter.
In everyday life: Look for Handel-C outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study Handel-C in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Handel-C means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Handel-C out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Handel-C in simple terms?

Handel-C is a high-level hardware description language aimed at low-level hardware and is most commonly used in programming FPGAs. Handel-C is to hardware design what the first high-level programming languages were to programming CPUs.

Why does Handel-C matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Handel-C?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Handel-C.