added verilog support

default.nix

@@ -5,6 +5,8 @@
       llvmPackages.bintools
       rustup
       ghdl-llvm
+      verilator
+      python3
     ];
     RUSTC_VERSION = "nightly";
     # https://github.com/rust-lang/rust-bindgen#environment-variables

examples/example.v

@@ -0,0 +1,19 @@
+// Do not modify the following module interface.
+module circuit (
+    input  wire        clk,   // 500 Hz, period 2 ms
+    input  wire [31:0] btn,
+    input  wire [31:0] sw,
+    output reg  [31:0] led = 32'h00000000,
+    output wire [31:0] segv,
+    output wire [31:0] segs
+);
+    reg [31:0] counter = 32'h00000000;
+
+    assign segv = 32'h00000000;
+    assign segs = 32'h00000000;
+
+    always @(posedge clk) begin
+        counter <= counter + 32'd1;
+        led <= counter ^ sw ^ btn;
+    end
+endmodule

relay.nix

@@ -12,11 +12,11 @@ pkgs.rustPlatform.buildRustPackage {
   cargoBuildFlags = [ "-p" "relay" ];
 
   nativeBuildInputs = [ pkgs.makeWrapper ];
-  buildInputs = [ pkgs.ghdl-llvm pkgs.zlib ];
+  buildInputs = [ pkgs.ghdl-llvm pkgs.verilator pkgs.python3 pkgs.zlib ];
 
   postFixup = ''
     wrapProgram $out/bin/relay \
-      --prefix PATH : ${pkgs.lib.makeBinPath [ pkgs.ghdl-llvm pkgs.glib.dev ]} \
+      --prefix PATH : ${pkgs.lib.makeBinPath [ pkgs.ghdl-llvm pkgs.verilator pkgs.python3 pkgs.glib.dev ]} \
       --prefix LIBRARY_PATH : ${pkgs.lib.makeLibraryPath [ pkgs.zlib ]} \
       --prefix LD_LIBRARY_PATH : ${pkgs.lib.makeLibraryPath [ pkgs.zlib ]}
   '';

relay/shim/verilog.c

@@ -0,0 +1,35 @@
+#include <cstdint>
+#include "Vcircuit.h"
+#include "verilated.h"
+
+extern "C" void ffi_init();
+extern "C" std::uint32_t ffi_get_sw();
+extern "C" std::uint32_t ffi_get_btn();
+extern "C" void ffi_set_outputs(std::uint32_t led, std::uint32_t segv, std::uint32_t segs);
+
+int main(int argc, char** argv) {
+    Verilated::commandArgs(argc, argv);
+
+    Vcircuit top;
+    top.clk = 0;
+    top.btn = 0;
+    top.sw = 0;
+
+    ffi_init();
+
+    while (true) {
+        top.sw = ffi_get_sw();
+        top.btn = ffi_get_btn();
+
+        top.clk = 0;
+        top.eval();
+        ffi_set_outputs(top.led, top.segv, top.segs);
+
+        top.sw = ffi_get_sw();
+        top.btn = ffi_get_btn();
+
+        top.clk = 1;
+        top.eval();
+        ffi_set_outputs(top.led, top.segv, top.segs);
+    }
+}

relay/src/build.rs

@@ -9,7 +9,20 @@ use tokio::process::{Child, Command};
 use crate::HResult;
 
 const EMBEDDED_VHDL_UI_LIB: &[u8] = include_bytes!(env!("EMBEDDED_VHDL_CONN_LIB_PATH"));
-const EMBEDDED_TB_VHDL: &str = include_str!("../../rtl/tb.vhdl");
+const EMBEDDED_TB_VHDL: &str = include_str!("../shim/shim.vhdl");
+const EMBEDDED_TB_VERILATOR: &str = include_str!("../shim/verilog.c");
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum Simulator {
+    Ghdl,
+    Verilator,
+}
+
+#[derive(Debug, Clone)]
+pub struct BuildArtifact {
+    pub simulator: Simulator,
+    pub run_target: PathBuf,
+}
 
 async fn ensure_ok(child: Child) -> Result<(), Box<dyn std::error::Error + Send + Sync>> {
     let result = child.wait_with_output().await?;
@@ -23,6 +36,137 @@ async fn ensure_ok(child: Child) -> Result<(), Box<dyn std::error::Error + Send
     Ok(())
 }
 
+fn source_files(src: &Path) -> HResult<Vec<PathBuf>> {
+    let mut files = Vec::new();
+    for file in src.read_dir()?.flatten() {
+        let path = file.path();
+        if path.is_file() {
+            files.push(path.canonicalize()?);
+        }
+    }
+    files.sort();
+    Ok(files)
+}
+
+fn detect_simulator(files: &[PathBuf]) -> HResult<Simulator> {
+    let mut has_vhdl = false;
+    let mut has_verilog = false;
+
+    for file in files {
+        match file.extension().and_then(OsStr::to_str) {
+            Some("vhdl" | "vhd") => has_vhdl = true,
+            Some("v" | "sv") => has_verilog = true,
+            _ => {}
+        }
+    }
+
+    match (has_vhdl, has_verilog) {
+        (true, false) => Ok(Simulator::Ghdl),
+        (false, true) => Ok(Simulator::Verilator),
+        (true, true) => Err("mixed VHDL and Verilog sources are not supported yet".into()),
+        (false, false) => Err("no VHDL or Verilog source files found".into()),
+    }
+}
+
+async fn build_with_ghdl(
+    build: &Path,
+    files: &[PathBuf],
+    embedded_lib_path: &Path,
+) -> HResult<BuildArtifact> {
+    let embedded_tb_path = build.join("tb.vhdl");
+    std::fs::write(&embedded_tb_path, EMBEDDED_TB_VHDL)?;
+
+    let mut cmd = Command::new("ghdl");
+    cmd.kill_on_drop(true);
+    cmd.args(["-i", "-g", "--std=08"]);
+
+    for file in files {
+        if matches!(
+            file.extension().and_then(OsStr::to_str),
+            Some("vhdl" | "vhd")
+        ) {
+            cmd.arg(file);
+        }
+    }
+    cmd.arg(&embedded_tb_path.canonicalize()?);
+
+    cmd.stdin(std::process::Stdio::piped())
+        .stdout(std::process::Stdio::piped())
+        .stderr(std::process::Stdio::piped());
+
+    cmd.current_dir(build);
+    ensure_ok(cmd.spawn()?).await?;
+
+    let mut cmd = Command::new("ghdl");
+    cmd.kill_on_drop(true);
+    cmd.args(["-m", "--std=08"]);
+    cmd.arg(format!(
+        "-Wl,{}",
+        embedded_lib_path.canonicalize()?.display()
+    ));
+    cmd.arg("tb");
+    cmd.current_dir(build);
+    cmd.stdin(std::process::Stdio::piped())
+        .stdout(std::process::Stdio::piped())
+        .stderr(std::process::Stdio::piped());
+    ensure_ok(cmd.spawn()?).await?;
+
+    Ok(BuildArtifact {
+        simulator: Simulator::Ghdl,
+        run_target: build.join("tb"),
+    })
+}
+
+async fn build_with_verilator(
+    build: &Path,
+    files: &[PathBuf],
+    embedded_lib_path: &Path,
+) -> HResult<BuildArtifact> {
+    let embedded_tb_path = build.join("tb.cpp");
+    let obj_dir = build.join("obj_dir");
+    std::fs::write(&embedded_tb_path, EMBEDDED_TB_VERILATOR)?;
+    std::fs::create_dir_all(&obj_dir)?;
+
+    let mut cmd = Command::new("verilator");
+    cmd.kill_on_drop(true);
+    cmd.args(["--cc", "--exe", "--top-module", "circuit", "--Mdir"]);
+    cmd.arg(&obj_dir);
+    cmd.args(["-o", "tb"]);
+    cmd.args([
+        "-LDFLAGS",
+        &embedded_lib_path.canonicalize()?.display().to_string(),
+    ]);
+    cmd.arg(&embedded_tb_path);
+
+    for file in files {
+        if matches!(file.extension().and_then(OsStr::to_str), Some("v" | "sv")) {
+            cmd.arg(file);
+        }
+    }
+
+    cmd.current_dir(build);
+    cmd.stdin(std::process::Stdio::piped())
+        .stdout(std::process::Stdio::piped())
+        .stderr(std::process::Stdio::piped());
+    ensure_ok(cmd.spawn()?).await?;
+
+    let mut cmd = Command::new("make");
+    cmd.kill_on_drop(true);
+    cmd.args(["-C"]);
+    cmd.arg(&obj_dir);
+    cmd.args(["-f", "Vcircuit.mk", "-j", "1"]);
+    cmd.current_dir(build);
+    cmd.stdin(std::process::Stdio::piped())
+        .stdout(std::process::Stdio::piped())
+        .stderr(std::process::Stdio::piped());
+    ensure_ok(cmd.spawn()?).await?;
+
+    Ok(BuildArtifact {
+        simulator: Simulator::Verilator,
+        run_target: obj_dir.join("tb"),
+    })
+}
+
 pub struct TempDir(PathBuf);
 impl Drop for TempDir {
     fn drop(&mut self) {
@@ -49,7 +193,12 @@ impl AsRef<Path> for TempDir {
     }
 }
 
-pub async fn copy_and_build(files: HashMap<String, String>) -> HResult<TempDir> {
+pub struct TempBuild {
+    pub dir: TempDir,
+    pub artifact: BuildArtifact,
+}
+
+pub async fn copy_and_build(files: HashMap<String, String>) -> HResult<TempBuild> {
     use std::hash::*;
     let mut hasher = std::hash::DefaultHasher::default();
     for (key, value) in &files {
@@ -59,7 +208,7 @@ pub async fn copy_and_build(files: HashMap<String, String>) -> HResult<TempDir>
     let hash = hasher.finish();
 
     let mut work_dir = std::env::temp_dir();
-    work_dir.push(format!("ghdl-relay-{hash:x?}"));
+    work_dir.push(format!("hdl-relay-{hash:x?}"));
     std::fs::create_dir_all(&work_dir)?;
     let work_dir = TempDir(work_dir);
 
@@ -69,49 +218,23 @@ pub async fn copy_and_build(files: HashMap<String, String>) -> HResult<TempDir>
         std::fs::write(path, contents)?;
     }
 
-    build(&work_dir, &work_dir).await?;
+    let artifact = build(&work_dir, &work_dir).await?;
 
-    Ok(work_dir)
+    Ok(TempBuild {
+        dir: work_dir,
+        artifact,
+    })
 }
 
-pub async fn build(build: &Path, src: &Path) -> HResult<()> {
-    std::fs::create_dir_all(build)?;
+pub async fn build(build: &Path, src: &Path) -> HResult<BuildArtifact> {
+    let build = build.canonicalize()?;
+    let src = src.canonicalize()?;
+    std::fs::create_dir_all(&build)?;
     let embedded_lib_path = build.join("libvhdl_conn.a");
-    let embedded_tb_path = build.join("tb.vhdl");
     std::fs::write(&embedded_lib_path, EMBEDDED_VHDL_UI_LIB)?;
-    std::fs::write(&embedded_tb_path, EMBEDDED_TB_VHDL)?;
-
-    let mut cmd = Command::new("ghdl");
-    cmd.kill_on_drop(true);
-    cmd.args(["-i", "-g", "--std=08"]);
-
-    for file in src.read_dir().unwrap().flatten() {
-        if Path::new(&file.file_name()).extension() == Some(OsStr::new("vhdl")) {
-            cmd.arg(file.path().canonicalize()?);
-        }
+    let files = source_files(&src)?;
+    match detect_simulator(&files)? {
+        Simulator::Ghdl => build_with_ghdl(&build, &files, &embedded_lib_path).await,
+        Simulator::Verilator => build_with_verilator(&build, &files, &embedded_lib_path).await,
     }
-    cmd.arg(&embedded_tb_path.canonicalize()?);
-
-    cmd.stdin(std::process::Stdio::piped())
-        .stdout(std::process::Stdio::piped())
-        .stderr(std::process::Stdio::piped());
-
-    cmd.current_dir(build);
-    ensure_ok(cmd.spawn()?).await?;
-
-    let mut cmd = Command::new("ghdl");
-    cmd.kill_on_drop(true);
-    cmd.args(["-m", "--std=08"]);
-    cmd.arg(format!(
-        "-Wl,{}",
-        embedded_lib_path.canonicalize()?.display()
-    ));
-    cmd.arg("tb");
-    cmd.current_dir(build);
-    cmd.stdin(std::process::Stdio::piped())
-        .stdout(std::process::Stdio::piped())
-        .stderr(std::process::Stdio::piped());
-    ensure_ok(cmd.spawn()?).await?;
-
-    Ok(())
 }

relay/src/main.rs

@@ -7,8 +7,7 @@ use axum::{
 };
 use serde::{Deserialize, Serialize};
 use std::{
-    net::{IpAddr, SocketAddr},
-    time::Duration,
+    net::{IpAddr, SocketAddr}, path::PathBuf, time::Duration
 };
 
 pub mod build;
@@ -45,12 +44,13 @@ async fn not_found() -> impl IntoResponse {
     (StatusCode::NOT_FOUND, "not found")
 }
 
-#[derive(Clone, Copy, Debug)]
+#[derive(Clone, Debug)]
 struct Config {
     ip: IpAddr,
     port: u16,
     update_ms: u64,
     workspace_ws: bool,
+    workspace_src: PathBuf,
 }
 
 impl Default for Config {
@@ -59,7 +59,8 @@ impl Default for Config {
             ip: IpAddr::from([127, 0, 0, 1]),
             port: 8080,
             update_ms: 30,
-            workspace_ws: true,
+            workspace_ws: false,
+            workspace_src: "./src".into(),
         }
     }
 }
@@ -92,6 +93,10 @@ fn parse_config_from_args() -> Result<Config, String> {
             "--workspace" => {
                 cfg.workspace_ws = true;
             }
+            "--workspace-src" => {
+                cfg.workspace_ws = true;
+                cfg.workspace_src = args.next().ok_or("missing value for --workspace-src")?.into();
+            }
             "--help" | "-h" => {
                 return Err(
                     "usage: relay [--ip <ip>] [--port <port>] [--update-ms <ms>] [--workspace]"
@@ -144,8 +149,9 @@ async fn main() {
             "/ws/workspace",
             get(move |ws: WebSocketUpgrade| {
                 let update_interval = update_interval;
+                let workspace_src = cfg.workspace_src;
                 async move {
-                    ws.on_upgrade(move |socket| workspace::ws_handler(socket, update_interval))
+                    ws.on_upgrade(move |socket| workspace::ws_handler(socket, workspace_src.clone(), update_interval))
                 }
             }),
         );

relay/src/run.rs

@@ -2,6 +2,8 @@ use std::path::Path;
 
 use tokio::process::{Child, ChildStderr, ChildStdin, ChildStdout, Command};
 
+use crate::build::{BuildArtifact, Simulator};
+
 pub struct Process {
     pub child: Child,
     pub stdin: ChildStdin,
@@ -9,16 +11,26 @@ pub struct Process {
     pub stderr: ChildStderr,
 }
 
-pub async fn run(artifact_dir: &Path) -> Result<Process, Box<dyn std::error::Error + Send + Sync>> {
-    let mut cmd = Command::new("ghdl");
-    cmd.args([
-        "-r",
-        "--std=08",
-        "tb",
-        "--stop-delta=4294967296",
-        "--unbuffered",
-        "--",
-    ]);
+pub async fn run(
+    artifact_dir: &Path,
+    artifact: &BuildArtifact,
+) -> Result<Process, Box<dyn std::error::Error + Send + Sync>> {
+    let mut cmd = match artifact.simulator {
+        Simulator::Ghdl => {
+            let mut cmd = Command::new("ghdl");
+            cmd.args([
+                "-r",
+                "--std=08",
+                "tb",
+                "--stop-delta=4294967296",
+                "--unbuffered",
+                "--",
+            ]);
+            cmd
+        }
+        Simulator::Verilator => Command::new(&artifact.run_target),
+    };
+
     cmd.args(std::env::args_os());
     cmd.current_dir(artifact_dir);
     cmd.kill_on_drop(true);

relay/src/uploaded.rs

@@ -19,8 +19,8 @@ pub async fn ws_handler(socket: WebSocket, refresh_time: Duration) {
         return;
     };
 
-    let artifact_dir = match build::copy_and_build(files).await {
-        Ok(dir) => dir,
+    let temp_build = match build::copy_and_build(files).await {
+        Ok(build) => build,
         Err(err) => {
             _ = sender
                 .send(Message::Text(format!("Failed to build: {err}").into()))
@@ -29,7 +29,10 @@ pub async fn ws_handler(socket: WebSocket, refresh_time: Duration) {
         }
     };
 
-    let mut process = match run::run(&artifact_dir).await {
+    let artifact_dir = temp_build.dir;
+    let artifact = temp_build.artifact;
+
+    let mut process = match run::run(&artifact_dir, &artifact).await {
         Ok(process) => process,
         Err(err) => {
             _ = sender
@@ -71,7 +74,6 @@ pub async fn ws_handler(socket: WebSocket, refresh_time: Duration) {
                 out = sout.next_line() => {
                     match out{
                         Ok(Some(line)) => {
-                            
                             let msg = ServerMsg::Log {
                                 stream: "stdout",
                                 line: line.strip_prefix(artifact_prefix).unwrap_or(&line),
@@ -114,7 +116,7 @@ pub async fn ws_handler(socket: WebSocket, refresh_time: Duration) {
                     print_deadline += refresh_time;
                     process.stdin.write_all("\n".as_bytes()).await?;
                 }
-            }        
+            }
         }
         Ok(())
     }.await;

relay/src/workspace.rs

@@ -162,7 +162,7 @@ impl Handler {
                             Ok(Some(line)) => {
                                 let msg = if let Some(repr) = line.strip_prefix("led="){
                                     ServerMsg::Led(repr.parse().unwrap_or(0))
-                                }else if let Some(repr) = line.strip_prefix("seg=") 
+                                }else if let Some(repr) = line.strip_prefix("seg=")
                                     && let Some((val, idx)) = repr.split_once(";") {
                                     ServerMsg::Seg{
                                         value: val.parse().unwrap_or(0),
@@ -197,15 +197,15 @@ impl Handler {
     }
 
     async fn run_program(&mut self) {
-        match build::build(&self.build_dir, &self.src_dir).await {
-            Ok(_) => {}
+        let artifact = match build::build(&self.build_dir, &self.src_dir).await {
+            Ok(artifact) => artifact,
             Err(err) => {
                 _ = self.eprint(format!("Failed to build: {err}")).await;
                 return;
             }
         };
 
-        let process = match run::run(&self.build_dir).await {
+        let process = match run::run(&self.build_dir, &artifact).await {
             Ok(process) => process,
             Err(err) => {
                 self.eprint(format!("Failed to run: {err}")).await;
@@ -233,8 +233,8 @@ impl Handler {
     }
 }
 
-pub async fn ws_handler(socket: WebSocket, refresh_time: Duration) {
-    Handler::workspace(socket, "./target".into(), "./src".into(), refresh_time)
+pub async fn ws_handler(socket: WebSocket, workspace_src: PathBuf, refresh_time: Duration) {
+    Handler::workspace(socket, "./target".into(), workspace_src, refresh_time)
         .run()
         .await;
 }

relay/ui/app.js

@@ -1,4 +1,6 @@
 const LS_KEY_VHDL = "circuit_ui:circuit.vhdl";
+const LS_KEY_VERILOG = "circuit_ui:circuit.v";
+const LS_KEY_EDITOR_LANGUAGE = "circuit_ui:editor_language";
 const LS_KEY_MODE = "circuit_ui:mode";
 
 const EXAMPLE_VHDL_TEXT = `library ieee;
@@ -28,6 +30,27 @@ begin
   end process;
 end description;`;
 
+const EXAMPLE_VERILOG_TEXT = `// Do not modify the following module interface.
+module circuit (
+    input  wire        clk,   // 500 Hz, period 2 ms
+    input  wire [31:0] btn,
+    input  wire [31:0] sw,
+    output reg  [31:0] led = 32'h00000000,
+    output wire [31:0] segv,
+    output wire [31:0] segs
+);
+    reg [31:0] counter = 32'h00000000;
+
+    assign segv = 32'h00000000;
+    assign segs = 32'h00000000;
+
+    always @(posedge clk) begin
+        counter <= counter + 32'd1;
+        led <= counter ^ sw ^ btn;
+    end
+endmodule
+`;
+
 function getDomRefs() {
   return {
     statusPill: document.getElementById("statusPill"),
@@ -37,6 +60,7 @@ function getDomRefs() {
 
     editorSection: document.getElementById("editorSection"),
     vhdlEditor: document.getElementById("vhdlEditor"),
+    editorLanguage: document.getElementById("editorLanguage"),
     lineGutter: document.getElementById("lineGutter"),
     loadExampleBtn: document.getElementById("loadExampleBtn"),
 
@@ -147,9 +171,18 @@ class LogController {
 }
 
 class EditorController {
-  constructor({ editorSection, vhdlEditor, lineGutter, loadExampleBtn, enabled, externalFiles }) {
+  constructor({
+    editorSection,
+    vhdlEditor,
+    editorLanguage,
+    lineGutter,
+    loadExampleBtn,
+    enabled,
+    externalFiles,
+  }) {
     this.editorSection = editorSection;
     this.vhdlEditor = vhdlEditor;
+    this.editorLanguage = editorLanguage;
     this.lineGutter = lineGutter;
     this.loadExampleBtn = loadExampleBtn;
 
@@ -157,6 +190,7 @@ class EditorController {
     this.externalFiles = externalFiles && typeof externalFiles === "object" ? externalFiles : null;
     this.saveTimer = null;
     this.initialized = false;
+    this.language = "vhdl";
   }
 
   init() {
@@ -180,23 +214,32 @@ class EditorController {
     if (this.initialized) return;
     this.initialized = true;
 
-    const saved = this.loadFromLocalStorage();
-    this.vhdlEditor.value = saved !== null ? saved : EXAMPLE_VHDL_TEXT;
+    this.language = this.loadLanguageFromLocalStorage();
+    this.editorLanguage.value = this.language;
+    this.vhdlEditor.value = this.getCurrentBuffer();
 
     this.loadExampleBtn.addEventListener("click", () => {
-      this.vhdlEditor.value = EXAMPLE_VHDL_TEXT;
-      this.saveToLocalStorageDebounced();
+      this.vhdlEditor.value = this.getExampleText(this.language);
+      this.saveCurrentBufferDebounced();
       this.updateLineNumbers();
     });
 
     this.vhdlEditor.addEventListener("input", () => {
-      this.saveToLocalStorageDebounced();
+      this.saveCurrentBufferDebounced();
       this.updateLineNumbers();
     });
 
     this.vhdlEditor.addEventListener("scroll", () => {
       this.lineGutter.scrollTop = this.vhdlEditor.scrollTop;
     });
+
+    this.editorLanguage.addEventListener("change", () => {
+      this.saveCurrentBuffer();
+      this.language = this.editorLanguage.value === "verilog" ? "verilog" : "vhdl";
+      this.persistLanguage();
+      this.vhdlEditor.value = this.getCurrentBuffer();
+      this.updateLineNumbers();
+    });
   }
 
   getFilesPayload() {
@@ -204,9 +247,9 @@ class EditorController {
       return this.externalFiles ? { ...this.externalFiles } : {};
     }
 
-    return {
-      "circuit.vhdl": this.vhdlEditor.value ?? "",
-    };
+    return this.language === "verilog"
+      ? { "circuit.v": this.vhdlEditor.value ?? "" }
+      : { "circuit.vhdl": this.vhdlEditor.value ?? "" };
   }
 
   updateLineNumbers() {
@@ -221,27 +264,61 @@ class EditorController {
     this.lineGutter.textContent = gutterText;
   }
 
-  saveToLocalStorageDebounced() {
+  saveCurrentBufferDebounced() {
     if (this.saveTimer) clearTimeout(this.saveTimer);
 
     this.saveTimer = setTimeout(() => {
-      try {
-        localStorage.setItem(LS_KEY_VHDL, this.vhdlEditor.value ?? "");
-      } catch {
-        // Ignore localStorage failures.
-      }
+      this.saveCurrentBuffer();
     }, 250);
   }
 
-  loadFromLocalStorage() {
+  saveCurrentBuffer() {
+    const key = this.language === "verilog" ? LS_KEY_VERILOG : LS_KEY_VHDL;
     try {
-      const saved = localStorage.getItem(LS_KEY_VHDL);
+      localStorage.setItem(key, this.vhdlEditor.value ?? "");
+    } catch {
+      // Ignore localStorage failures.
+    }
+  }
+
+  loadLanguageFromLocalStorage() {
+    try {
+      const saved = localStorage.getItem(LS_KEY_EDITOR_LANGUAGE);
+      if (saved === "verilog" || saved === "vhdl") return saved;
+    } catch {
+      // Ignore localStorage failures.
+    }
+    return "vhdl";
+  }
+
+  persistLanguage() {
+    try {
+      localStorage.setItem(LS_KEY_EDITOR_LANGUAGE, this.language);
+    } catch {
+      // Ignore localStorage failures.
+    }
+  }
+
+  getCurrentBuffer() {
+    const saved = this.loadBufferFromLocalStorage(this.language);
+    if (saved !== null) return saved;
+    return this.getExampleText(this.language);
+  }
+
+  loadBufferFromLocalStorage(language) {
+    const key = language === "verilog" ? LS_KEY_VERILOG : LS_KEY_VHDL;
+    try {
+      const saved = localStorage.getItem(key);
       if (saved !== null) return saved;
     } catch {
       // Ignore localStorage failures.
     }
     return null;
   }
+
+  getExampleText(language) {
+    return language === "verilog" ? EXAMPLE_VERILOG_TEXT : EXAMPLE_VHDL_TEXT;
+  }
 }
 
 class OutputController {

relay/ui/index.html

@@ -10,8 +10,13 @@
   <main class="grid">
     <section id="editorSection" class="card editor">
       <div class="cardHeader">
-        <div class="cardTitle">VHDL</div>
+        <div class="cardTitle">HDL</div>
         <div class="cardActions">
+          <label class="editorLanguageLabel" for="editorLanguage">Language</label>
+          <select id="editorLanguage" class="editorLanguageSelect" aria-label="Select HDL language">
+            <option value="vhdl">VHDL</option>
+            <option value="verilog">Verilog</option>
+          </select>
           <button id="loadExampleBtn" class="secondary">Load example</button>
         </div>
       </div>

relay/ui/styles.css

@@ -300,6 +300,26 @@ textarea {
   background: var(--surface-soft);
 }
 
+.editorLanguageLabel {
+  font-size: 12px;
+  color: var(--text-muted);
+}
+
+.editorLanguageSelect {
+  min-width: 104px;
+  padding: 7px 10px;
+  border: 1px solid var(--border-soft);
+  border-radius: 10px;
+  color: var(--text);
+  background: var(--surface-soft);
+  font: inherit;
+}
+
+.editorLanguageSelect:focus {
+  outline: 2px solid var(--accent-soft);
+  outline-offset: 2px;
+}
+
 .lineGutter {
   width: 54px;
   margin: 0;

src_verilog/bcd.v

@@ -0,0 +1,101 @@
+module bcd (
+    input  wire              clk,
+    input  wire signed [22:0] num,  // sfixed(15 downto -7)
+    input  wire              en,
+    output reg  [63:0]       seg
+);
+
+    function [7:0] seg_encode;
+        input [3:0] d;
+        begin
+            case (d)
+                4'd0: seg_encode = 8'b00111111;
+                4'd1: seg_encode = 8'b00000110;
+                4'd2: seg_encode = 8'b01011011;
+                4'd3: seg_encode = 8'b01001111;
+                4'd4: seg_encode = 8'b01100110;
+                4'd5: seg_encode = 8'b01101101;
+                4'd6: seg_encode = 8'b01111101;
+                4'd7: seg_encode = 8'b00000111;
+                4'd8: seg_encode = 8'b01111111;
+                4'd9: seg_encode = 8'b01101111;
+                default: seg_encode = 8'b00000000;
+            endcase
+        end
+    endfunction
+
+    function [3:0] to_bcd_digit;
+        input integer value;
+        integer remainder;
+        begin
+            remainder = value % 10;
+            case (remainder)
+                0: to_bcd_digit = 4'd0;
+                1: to_bcd_digit = 4'd1;
+                2: to_bcd_digit = 4'd2;
+                3: to_bcd_digit = 4'd3;
+                4: to_bcd_digit = 4'd4;
+                5: to_bcd_digit = 4'd5;
+                6: to_bcd_digit = 4'd6;
+                7: to_bcd_digit = 4'd7;
+                8: to_bcd_digit = 4'd8;
+                9: to_bcd_digit = 4'd9;
+                default: to_bcd_digit = 4'd0;
+            endcase
+        end
+    endfunction
+
+    integer scaled_hundredths;
+    integer magnitude;
+    integer frac_hundredths;
+    integer tmp;
+    integer j;
+    reg negative;
+    reg [3:0] digits [0:7];
+    reg [63:0] out_seg;
+
+    always @* begin
+        if (!en) begin
+            seg = 64'b0;
+        end else begin
+            out_seg = 64'b0;
+
+            // num is Q16.7 fixed-point, so value = num / 128.
+            // Round to nearest hundredth.
+            if (num < 0) begin
+                scaled_hundredths = ((num * 100) - 64) / 128;
+            end else begin
+                scaled_hundredths = ((num * 100) + 64) / 128;
+            end
+
+            negative = (scaled_hundredths < 0);
+
+            if (negative) begin
+                magnitude = -scaled_hundredths;
+            end else begin
+                magnitude = scaled_hundredths;
+            end
+
+            frac_hundredths = magnitude % 100;
+            tmp = magnitude;
+
+            for (j = 0; j < 8; j = j + 1) begin
+                digits[j] = to_bcd_digit(tmp);
+                tmp = tmp / 10;
+            end
+
+            for (j = 0; j < 7; j = j + 1) begin
+                out_seg[(7 - j) * 8 +: 8] = seg_encode(digits[j]);
+            end
+
+            out_seg[5 * 8 + 7] = 1'b1;
+
+            if (negative) begin
+                out_seg[6] = 1'b1;
+            end
+
+            seg = out_seg;
+        end
+    end
+
+endmodule

src_verilog/example.v

@@ -0,0 +1,56 @@
+// Do not modify the following module interface.
+module circuit (
+    input  wire        clk,   // 500 Hz, period 2 ms
+    input  wire [31:0] btn,
+    input  wire [31:0] sw,
+    output wire [31:0] led,
+    output wire [31:0] segv,
+    output wire [31:0] segs
+);
+
+    wire [3:0] dig;
+    wire       dig_e;
+    wire       dot;
+    wire       eq;
+    wire [3:0] op;
+    wire       op_e;
+
+    wire [63:0] seg_a;
+    wire [2:0]  segs_mux;
+    wire signed [22:0] sw_fixed;
+
+    assign led = 32'b0;
+    assign segs = {29'b0, segs_mux};
+
+    // Convert signed 16-bit integer to signed Q16.7 fixed-point.
+    assign sw_fixed = {sw[15:0], 7'b0};
+
+    keypad_input keypad_input_inst (
+        .clk(clk),
+        .keypad(btn[15:8]),
+        .dig(dig),
+        .dig_e(dig_e),
+        .dot(dot),
+        .eq(eq),
+        .op(op),
+        .op_e(op_e)
+    );
+
+    bcd bcd_inst (
+        .clk(clk),
+        .num(sw_fixed),
+        .en(1'b1),
+        .seg(seg_a)
+    );
+
+    seg_plex seg_plex_inst (
+        .clk(clk),
+        .seg0(seg_a),
+        .seg1(64'h0000000000000000),
+        .seg2(64'h0000000000000000),
+        .seg3(64'h0000000000000000),
+        .segv(segv),
+        .segs(segs_mux)
+    );
+
+endmodule

src_verilog/keypad_input.v

@@ -0,0 +1,55 @@
+module keypad_input (
+    input  wire       clk,
+    input  wire [7:0] keypad,
+
+    output reg  [3:0] dig,
+    output reg        dig_e,
+
+    output reg        dot,
+    output reg        eq,
+
+    output reg  [3:0] op,
+    output reg        op_e
+);
+
+    reg [7:0] keypad_curr = 8'b00000000;
+
+    always @(posedge clk or negedge clk) begin
+        if (clk && (keypad != keypad_curr)) begin
+            case (keypad)
+                8'b10001000: begin op <= 4'b0001; op_e <= 1'b1; end
+                8'b10000100: begin eq <= 1'b1; end
+                8'b10000010: begin dot <= 1'b1; end
+                8'b10000001: begin dig <= 4'b0000; dig_e <= 1'b1; end
+
+                8'b01001000: begin op <= 4'b0010; op_e <= 1'b1; end
+                8'b01000100: begin dig <= 4'b0011; dig_e <= 1'b1; end
+                8'b01000010: begin dig <= 4'b0010; dig_e <= 1'b1; end
+                8'b01000001: begin dig <= 4'b0001; dig_e <= 1'b1; end
+
+                8'b00101000: begin op <= 4'b0011; op_e <= 1'b1; end
+                8'b00100100: begin dig <= 4'b0110; dig_e <= 1'b1; end
+                8'b00100010: begin dig <= 4'b0101; dig_e <= 1'b1; end
+                8'b00100001: begin dig <= 4'b0100; dig_e <= 1'b1; end
+
+                8'b00011000: begin op <= 4'b0100; op_e <= 1'b1; end
+                8'b00010100: begin dig <= 4'b1000; dig_e <= 1'b1; end
+                8'b00010010: begin dig <= 4'b1000; dig_e <= 1'b1; end
+                8'b00010001: begin dig <= 4'b0111; dig_e <= 1'b1; end
+
+                default: begin end
+            endcase
+            keypad_curr <= keypad;
+        end
+
+        if (!clk && (keypad != keypad_curr)) begin
+            dig   <= 4'b0000;
+            dig_e <= 1'b0;
+            eq    <= 1'b0;
+            dot   <= 1'b0;
+            op    <= 4'b0000;
+            op_e  <= 1'b0;
+        end
+    end
+
+endmodule

src_verilog/seg_plex.v

@@ -0,0 +1,32 @@
+module seg_plex (
+    input  wire        clk,
+
+    input  wire [63:0] seg0,
+    input  wire [63:0] seg1,
+    input  wire [63:0] seg2,
+    input  wire [63:0] seg3,
+
+    output reg  [31:0] segv,
+    output reg  [2:0]  segs
+);
+
+    reg [2:0] counter = 3'b000;
+
+    always @(posedge clk) begin
+        case (counter)
+            3'd0: segv <= seg0[31:0];
+            3'd1: segv <= seg0[63:32];
+            3'd2: segv <= seg1[31:0];
+            3'd3: segv <= seg1[63:32];
+            3'd4: segv <= seg2[31:0];
+            3'd5: segv <= seg2[63:32];
+            3'd6: segv <= seg3[31:0];
+            3'd7: segv <= seg3[63:32];
+            default: segv <= 32'b0;
+        endcase
+
+        counter <= counter + 3'd1;
+        segs <= counter;
+    end
+
+endmodule