SHDL

SHDL (Simple Hardware Description Language) is an educational, gate-level hardware description language and simulation toolchain. Its founding principle is fidelity: circuits simulate gate by gate, one propagation level per cycle (the unit-delay model), and no tool in the chain is allowed to collapse or shortcut that structure. You describe hardware out of AND/OR/NOT/XOR; you watch signals ripple through it.

The two languages

SHDL is the authoring language: reusable hierarchical components, multi-bit ports, compile-time parameters, generators and conditionals for repetitive structure, bit slices and concatenation, named constants, optional initial state, and imports. Specified in docs/shdl.md.

Base SHDL is the intermediate representation every tool consumes: a flat netlist of single-bit wires over exactly six primitives — AND, OR, NOT, XOR, __VCC__, __GND__ — plus a JSON metadata section (multi-bit port groups, hierarchy, source maps, timing, constants, init seeds). Specified in docs/base_shdl.md.

Pipeline

 .shdl source
      │
      ▼
  Flattener        six phases: strip, monomorphize, expand generators,
      │            expand slices, materialize constants, flatten hierarchy
      ▼
  Base SHDL        single-bit primitive netlist + JSON metadata
      │
      ▼
  SHDLC            generates C (two-buffer compute/commit cycle),
      │            builds a shared library with a stable ABI
      ▼
  libcircuit       reset() / poke() / peek() / step()
                   (+ step_settle() / run_batch() throughput paths)

Quickstart

Requires Python ≥ 3.14 and a C compiler (clang or gcc).

Install the released package from PyPI to get the SHDL Python package and the shdl / shdlc / shdl-flatten / shdl-conformance CLIs:

pip install PySHDL          # or: uv add PySHDL

Or work from a clone with uv:

uv sync

# Flatten SHDL to Base SHDL (inspect the IR)
uv run shdl-flatten examples/fullAdder.shdl

# Compile straight from SHDL source to a shared library
uv run shdlc examples/fullAdder.shdl -o fullAdder.dylib

Drive a circuit from Python with PySHDL — one class, Circuit, runs the whole pipeline (flatten → compile → build → load) in-process and exposes poke/peek/step plus dict access and a context manager:

from SHDL import Circuit

with Circuit("examples/adder8.shdl") as c:
    c["A"] = 100            # dict-style poke; bit 0 is the LSB
    c["B"] = 55
    c.settle()             # advance every gate level (combinational depth)
    print(c["Sum"])         # 155

See docs/pyshdl.md for the full guide, and examples/interacting.py for a runnable walkthrough (uv run python examples/interacting.py). The underlying ABI is callable from any language; see docs/shdlc_goals.md §3 for the full contract (masking, lazy evaluation, init-seeded power-on state).

Projects & packages: the shdl CLI

The shdl command (installed with the package) manages whole projects and installs circuit libraries from Circuit Circus, the SHDL package index:

shdl new counter && cd counter   # scaffold shdl.toml, src/, tests/ — it compiles
shdl add arith                   # vendor a package (+ its deps) into shdl_modules/
shdl build && shdl test          # flatten + compile, then run the test vectors
shdl run                         # poke/peek/step REPL over the live circuit

use arith::{RippleAdder};        # vendored packages are directly importable

Dependencies are pinned in a committed shdl.lock (builds never touch the network), and shdl publish walks you through contributing a package back. See docs/shdl_cli.md.

Repository layout

The flattener and shdlc are deliberately decoupled: Base SHDL text is the only contract between them, and each side carries its own independent reference interpreter so the two can cross-check each other.

Testing

uv run pytest                    # full suite
uv run shdl-conformance run      # golden corpus: flatten + compile + trace lockstep

The suite covers per-phase units, per-raise-site diagnostics (every error's exact position is pinned), differential fuzzing against two independent oracles, byte-determinism, ABI contracts under ASAN/UBSAN, and instruction- level lockstep of the example CPU against a golden model.

Documentation

• docs/SHDL_Project.md — project charter: architecture, ecosystem, build sequence.
• docs/shdl.md — the SHDL language specification.
• docs/base_shdl.md — the Base SHDL IR specification.
• docs/shdlc_goals.md — the compiler's obligations and ABI contract.
• docs/golden_tests.md — the verification map tying every spec obligation to tests.

Status

The flattener, the SHDLC compiler (release ABI), the conformance suite, PySHDL — the user-facing Python driver (from SHDL import Circuit) — and the shdl CLI (projects + the Circuit Circus package registry) are complete and green. Next up are the debug build and the SHDB debugger — see the build sequence in the charter.