What is hardware kernel in FPGA?

In FPGA land, a hardware kernel is a block of logic that implements a “kernel function” as dedicated hardware—instead of as instructions running on a CPU.

You’ll see this term a lot in HLS/OpenCL/SYCL/Vitis flows:

• Software kernel (CPU/GPU): a function is executed by many threads/cores over time.
• Hardware kernel (FPGA): the function is synthesized into circuits (pipelines, FSMs, DSP blocks, BRAM/URAM, etc.) that run in parallel and process streaming data at hardware speed.

What a hardware kernel usually looks like

A typical FPGA hardware kernel has:

• Control logic (finite state machine / handshaking)
• Compute datapath (adders, multipliers, DSP slices, LUT logic)
• Local memory (BRAM/URAM buffers, FIFOs)
• I/O interfaces (commonly AXI4 / AXI-Stream, or vendor-specific streaming links)
• Optional DMA/data movers to pull/push data from DDR/HBM

How it’s used (host + FPGA accelerator model)

Most frameworks treat the FPGA as an accelerator:

1. Host code (x86/ARM) sets arguments / buffer addresses.
2. Host launches the kernel.
3. The hardware kernel reads input data, computes, writes outputs.
4. Host reads results.

Why it’s powerful

Because it’s hardware, you can exploit:

• Pipelining: new input every clock once the pipeline fills.
• Parallelism: multiple compute units (CUs) or unrolled loops.
• Deterministic latency: very repeatable timing.
• Streaming: process data “in flight” without storing huge arrays.

Concrete example

If your kernel is “FIR filter” or “FFT stage”:

• CPU version: loops over samples.
• Hardware kernel: a pipeline of multiply-accumulate stages that can accept 1 sample per clock (depending on design), using DSP blocks and FIFOs.

Common places you’ll hear “hardware kernel”

• Xilinx/AMD Vitis: kernel is the HLS/OpenCL compute block compiled to an FPGA bitstream.
• Intel FPGA OpenCL/SYCL: kernels compile into hardware pipelines and memory interfaces.