HDLcarry

HDLcarry is a conceptual construct used in digital design to describe carry propagation within hardware description languages. It provides an abstraction for carry signals that can cross module boundaries, aiming to simplify the description of wide adders and arithmetic units in HDL code.

In practice, HDLcarry relies on a set of propagate and generate signals, mirroring carry-lookahead concepts. A width parameter defines the number of bits; the interface typically accepts two input vectors A and B and an input carry Cin, and yields a sum vector and a carry-out Cout. HDLcarry can be organized hierarchically to form fast carry chains across large widths, enabling synthesis tools to optimize the resulting hardware. The approach emphasizes readability and modularity, allowing designers to express arithmetic behavior independently of the exact implementation of the carry logic.

Implementation and portability: HDLcarry exists as community libraries or language-agnostic constructs and is not an official standard. It is designed to be portable across common HDLs such as Verilog and VHDL, though support varies by toolchain. The primary goal is to provide a clear description of arithmetic behavior while leaving optimization and hardware mapping to the synthesis process. HDLcarry components are typically parameterizable, facilitating reuse in different designs and projects.

Applications and limitations: HDLcarry is useful in CPUs, digital signal processors, and data-path accelerators where wide addition is frequent. Limitations include potential mismatches with vendor-specific optimizations or timing constraints, and the need for careful verification to ensure that abstract carry models translate correctly into efficient hardware. As a design abstraction, HDLcarry can improve readability but may require additional validation in performance-critical paths.

See also: carry-lookahead adder, ripple-carry adder, arithmetic logic unit, hardware description language.