diff --git a/flake.nix b/flake.nix
index 746bf7e0..59640821 100644
--- a/flake.nix
+++ b/flake.nix
@@ -45,6 +45,14 @@
           ];
         };
     in {
+      legacyPackages.sw = let
+        rules = import ./rules {inherit pkgs;};
+        common = pkgs.callPackage sw/c/common {inherit rules;};
+        demo = pkgs.callPackage sw/c/demo {inherit rules common;};
+      in {
+        inherit common demo;
+      };
+
       devShells.default = pkgs.mkShellNoCC {
         name = "labenv";
         buildInputs = with pkgs; [
diff --git a/rules/cc.nix b/rules/cc.nix
new file mode 100644
index 00000000..b5c55756
--- /dev/null
+++ b/rules/cc.nix
@@ -0,0 +1,175 @@
+{
+  pkgs,
+  lib,
+  stdenv,
+  toolchain ? {
+    cc = "/no-toolchain-defined";
+    cflags = [];
+    ld = "/no-toolchain-defined";
+    ldflags = [];
+    asm = "/no-toolchain-defined";
+    asmflags = [];
+  },
+}: let
+  include = {
+    src,
+    deps ? [],
+  }:
+    pkgs.runCommandLocal (builtins.baseNameOf src) {} ''
+      mkdir -p $out/include
+
+      ARGS=()
+      for dep in $deps; do
+        if [ -d $dep/include ]; then
+          ln -s $src/include/* $out/include/
+        fi
+      done
+
+      ln -s ${src} $out/include/${builtins.baseNameOf src}
+    '';
+
+  # If a raw file path is specified, turn that into a header derivation.
+  autowrap_header = f:
+    if !(builtins.isPath f)
+    then f
+    else if lib.hasSuffix ".h" f
+    then include {src = f;}
+    else throw "unknown file type: ${f}";
+
+  # Turn all raw file paths into derivations.
+  # Files will be grouped so that source files can depend on header files.
+  autowrap_deps = deps: let
+    src = builtins.filter (f: builtins.isPath f && !(lib.hasSuffix ".h" f)) deps;
+    other = builtins.filter (f: !(builtins.isPath f) || lib.hasSuffix ".h" f) deps;
+    wrapped = map autowrap_header other;
+    src_drv =
+      map (
+        f:
+          if lib.hasSuffix ".S" f
+          then
+            asm {
+              src = f;
+              deps = wrapped;
+            }
+          else if lib.hasSuffix ".c" f
+          then
+            object {
+              src = f;
+              deps = wrapped;
+            }
+          else throw "unknown file type: ${f}"
+      )
+      src;
+  in
+    src_drv ++ wrapped;
+
+  asm = {
+    src,
+    deps ? [],
+    extra-asmflags ? [],
+    asmflags ? toolchain.asmflags ++ extra-asmflags,
+  }:
+    stdenv.mkDerivation {
+      name = (lib.removeSuffix ".S" (builtins.baseNameOf src)) + ".o";
+      inherit src asmflags;
+      inherit (toolchain) asm;
+      deps = autowrap_deps deps;
+      dontUnpack = true;
+      buildPhase = ''
+        ARGS=()
+        for dep in $deps; do
+          if [ -d $dep/include ]; then
+            ARGS+=(-I$dep/include)
+          fi
+        done
+
+        mkdir -p $out/obj
+        $asm $asmflags -c -o $out/obj/$name $src ''${ARGS[@]}
+      '';
+    };
+
+  object = {
+    src,
+    deps ? [],
+    extra-cflags ? [],
+    cflags ? toolchain.cflags ++ extra-cflags,
+  }:
+    stdenv.mkDerivation {
+      name = (lib.removeSuffix ".c" (builtins.baseNameOf src)) + ".o";
+      inherit src cflags;
+      inherit (toolchain) cc;
+      deps = autowrap_deps deps;
+      dontUnpack = true;
+      buildPhase = ''
+        ARGS=()
+        for dep in $deps; do
+          if [ -d $dep/include ]; then
+            ARGS+=(-I$dep/include)
+          fi
+        done
+
+        mkdir -p $out/obj
+        $cc $cflags -c -o $out/obj/$name $src ''${ARGS[@]}
+      '';
+    };
+
+  static = {
+    name,
+    deps,
+  }:
+    stdenv.mkDerivation {
+      inherit name;
+      srcs = autowrap_deps deps;
+      dontUnpack = true;
+      buildPhase = ''
+        ARGS=()
+        for src in $srcs; do
+          if [ -d $src/obj ]; then
+            ARGS+=($src/obj/*)
+          fi
+          if [ -d $src/include ]; then
+            mkdir -p $out/include
+            ln -s $src/include/* $out/include/
+          fi
+        done
+
+        mkdir -p $out/lib
+        ar rcs $out/lib/$name ''${ARGS[@]}
+      '';
+    };
+
+  binary = {
+    name,
+    deps ? [],
+    extra-ldflags ? [],
+    ldflags ? toolchain.ldflags ++ extra-ldflags,
+  }:
+    stdenv.mkDerivation {
+      inherit name ldflags;
+      inherit (toolchain) ld;
+      srcs = autowrap_deps deps;
+      dontUnpack = true;
+      buildPhase = ''
+        ARGS=()
+        for src in $srcs; do
+          if [ -d $src/obj ]; then
+            ARGS+=($src/obj/*)
+          fi
+          if [ -d $src/lib ]; then
+            ARGS+=($src/lib/*)
+          fi
+        done
+
+        mkdir -p $out/bin
+        $ld $ldflags -o $out/bin/$name ''${ARGS[@]}
+      '';
+    };
+
+  set = deps:
+    pkgs.symlinkJoin {
+      name = "";
+      paths = autowrap_deps deps;
+    };
+in {
+  inherit asm include object static binary set;
+}
diff --git a/rules/default.nix b/rules/default.nix
new file mode 100644
index 00000000..48982fe1
--- /dev/null
+++ b/rules/default.nix
@@ -0,0 +1,3 @@
+{pkgs}: {
+  cc = pkgs.callPackage ./cc.nix {};
+}
diff --git a/sw/c/common/default.nix b/sw/c/common/default.nix
new file mode 100644
index 00000000..75548ac2
--- /dev/null
+++ b/sw/c/common/default.nix
@@ -0,0 +1,64 @@
+{
+  rules,
+  pkgs,
+  lowrisc-toolchain-gcc-rv32imcb,
+}: let
+  toolchain = rec {
+    cc = "${lowrisc-toolchain-gcc-rv32imcb}/bin/riscv32-unknown-elf-gcc";
+    cflags = ["-march=rv32imc" "-mabi=ilp32" "-mcmodel=medany" "-Wall" "-fvisibility=hidden" "-ffreestanding"];
+    ld = cc;
+    ldflags = ["-nostartfiles" "-T" "${../../common/link.ld}"];
+    asm = cc;
+    asmflags = ["-march=rv32imc"];
+  };
+
+  rules_cc = rules.cc.override {
+    inherit toolchain;
+  };
+
+  passthru = with rules_cc; rec {
+    inherit toolchain rules_cc;
+
+    crt0 = asm {
+      src = ./crt0.S;
+      deps = [./demo_system_regs.h];
+    };
+
+    gpio = object {
+      src = ./gpio.c;
+      deps = [./gpio.h ./uart.h ./dev_access.h ./demo_system.h ./demo_system_regs.h];
+      extra-cflags = ["-O3"];
+    };
+
+    uart = object {
+      src = ./uart.c;
+      deps = [./gpio.h ./uart.h ./dev_access.h ./demo_system.h ./demo_system_regs.h];
+    };
+
+    pwm = object {
+      src = ./pwm.c;
+      deps = [./gpio.h ./uart.h ./pwm.h ./dev_access.h ./demo_system.h ./demo_system_regs.h];
+    };
+
+    spi = object {
+      src = ./spi.c;
+      deps = [./gpio.h ./uart.h ./spi.h ./dev_access.h ./demo_system.h ./demo_system_regs.h];
+    };
+
+    timer = object {
+      src = ./timer.c;
+      deps = [./gpio.h ./uart.h ./timer.h ./dev_access.h ./demo_system.h ./demo_system_regs.h];
+    };
+
+    demo_system = object {
+      src = ./demo_system.c;
+      deps = [./gpio.h ./uart.h ./dev_access.h ./demo_system.h ./demo_system_regs.h];
+    };
+
+    common = static {
+      name = "common.a";
+      deps = [crt0 gpio uart pwm spi timer ./demo_system.c] ++ [./gpio.h ./uart.h ./timer.h ./spi.h ./pwm.h ./dev_access.h ./demo_system.h ./demo_system_regs.h];
+    };
+  };
+in
+  passthru.common // passthru
diff --git a/sw/c/demo/default.nix b/sw/c/demo/default.nix
new file mode 100644
index 00000000..1d8a2158
--- /dev/null
+++ b/sw/c/demo/default.nix
@@ -0,0 +1,11 @@
+{
+  rules,
+  common,
+  lowrisc-toolchain-gcc-rv32imcb,
+}:
+with common.rules_cc; {
+  hello_world = binary {
+    name = "hello_world";
+    deps = [hello_world/main.c common];
+  };
+}