behavioral & RTL

Multi-cycle functionality:
It is a fundamental characteristic of synthesizable RTL code that the complete functionality of each clocked process must be performed within a single clock cycle. Behavioral synthesis lifts this restriction. Clocked processes in synthesizable behavioral code may contain functionality that takes more than one clock cycle to execute.

The behavioral synthesis algorithms will create a schedule that determines how many clock cycles will be used. The behavioral synthesis tool automatically creates the finite state machine (FSM) that is required to implement this multi-cycle behavior in the generated RTL code.

In a traditional RTL design process, the designer is responsible for manually decomposing multi-cycle functionality into a set of single-cycle processes. Typically this entails the creation of multiple processes to implement the finite state machine, and the creation of processes for each operation and each output.

A behavioral synthesis tool performs this decomposition for the designer. The multi-cycle behavior can be expressed in a natural way in a single process leading to more efficient design specification and debug.

Most algorithms include looping structures. Traditional RTL design imposes severe restrictions on the use of loops, or prohibits them outright. Some RTL logic synthesis tools permit for loops with fixed loop indices only. The loop body is restricted to being executed in a single cycle. Parallel hardware is inferred for each loop iteration.

These restrictions require the designer to transform the algorithm into a multi-cycle FSM adding substantial complexity to the designer's task. Behavioral design manages this complexity for the designer by permitting free use of loops. "While" loops and "for" loops with data-dependent loop indices are fully supported in a behavioral design flow. Loop termination constructs such as the C language "break" and "continue" keywords are permitted.

Memory access:
In general, reading and writing to memories requires complex multi-cycle protocols. In RTL design these are implemented as explicit FSMs. Worse, these accesses must usually be incorporated in an already complex FSM implementing an algorithm.

Behavioral synthesis permits them to be represented in an intuitive way as simple array accesses. An array is declared in the native syntax of the behavioral language in use, tool directives are provided to control the mapping of the array to a physical memory element, and the array elements are referenced using the array indexing syntax of the language. The behavioral synthesis tool instantiates the memory element and connects it to the rest of the circuit. It also develops of the FSM for the memory access protocol and integrates this FSM with the rest of the algorithm.

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