diff --git a/.claude/skills/gcc-example/SKILL.md b/.claude/skills/gcc-example/SKILL.md
index 45ef8e2..3c8667a 100644
--- a/.claude/skills/gcc-example/SKILL.md
+++ b/.claude/skills/gcc-example/SKILL.md
@@ -11,23 +11,17 @@ The `examples/bsp/` BSP directory builds GCC and binutils from source using putu
 
 ```bash
 cd /path/to/pup/examples/bsp && \
-  make -f Makefile.pup SRCDIR=../../source-root
-```
-
-Or directly with putup:
-
-```bash
-cd /path/to/pup/examples/bsp && \
+  scripts/download-source.sh ../../source-root && \
   putup configure --config configs/x86_64-linux.config \
-    -C . -S ../../source-root -B ../../build-gcc
+    -C . -S ../../source-root -B ../../build-gcc && \
   putup -C . -S ../../source-root -B ../../build-gcc -j$(nproc)
 ```
 
 - `-C .` — config tree (BSP root: Tupfiles, tup.config, Tuprules.tup)
-- `-S ../source-root` — assembled source tree (gcc/ + binutils/ subdirs, read-only)
-- `-B ../build-gcc` — build output directory
+- `-S ../../source-root` — assembled source tree (gcc/ + binutils/ subdirs, read-only)
+- `-B ../../build-gcc` — build output directory
 
-After editing any `tup.config`, re-run `putup configure` to propagate to the build dir.
+After editing any `defaults.config`, re-run `putup configure` to propagate to the build dir.
 
 ## Architecture
 
@@ -166,7 +160,7 @@ The `tm-preds.h` is generated by `genpreds -h` and declares predicate functions
 
 Each library (gmp, mpfr, mpc, libiberty, libcpp, libdecnumber, libbacktrace) has:
 - Its own `Tuprules.tup` with `?=` defaults
-- Its own `tup.config` for library-specific config
+- Its own `defaults.config` for library-specific config (copied to `tup.config` by Tupfile rule)
 - Its own `Tupfile` for compilation rules
 
 The root `Tuprules.tup` sets prefixed DIR vars (`GMP_DIR = gmp`, etc.) that override the `?=` defaults when building as part of the larger project.
diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 26499fc..cd7a3de 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -185,10 +185,13 @@ jobs:
         run: chmod +x build/putup
       - name: Download source
         working-directory: examples/bsp
-        run: make -f Makefile.pup download-source SRCDIR=../../source-root
+        run: scripts/download-source.sh ../../source-root
+      - name: Configure
+        working-directory: examples/bsp
+        run: ../../build/putup configure --config configs/x86_64-linux.config -C . -S ../../source-root -B ../../build-gcc
       - name: Build
         working-directory: examples/bsp
-        run: make -f Makefile.pup PUTUP=../../build/putup SRCDIR=../../source-root
+        run: ../../build/putup -C . -S ../../source-root -B ../../build-gcc -j$(nproc)
       - name: Smoke test cc1
         run: |
           echo 'int main() { return 0; }' > /tmp/test.c
diff --git a/docs/plans/2026-03-09-eliminate-makefile-preprocessing-design.md b/docs/plans/2026-03-09-eliminate-makefile-preprocessing-design.md
new file mode 100644
index 0000000..f3bed5f
--- /dev/null
+++ b/docs/plans/2026-03-09-eliminate-makefile-preprocessing-design.md
@@ -0,0 +1,164 @@
+# Eliminate Makefile.pup Preprocessing
+
+## Problem
+
+The BSP example (`examples/bsp/`) uses `Makefile.pup` as a wrapper that runs
+preprocessing steps before invoking putup. Two targets mutate the config tree
+at `make` time:
+
+1. **`setup-host-configs`** — copies platform-specific `configs/host-darwin/*.config`
+   files over `defaults.config` files in the source tree. Mutates 9 config files
+   based on the `HOST` variable.
+
+2. **`resolve-mpn`** — runs `scripts/resolve-mpn.sh` to scan the GMP source tree
+   for assembly files, then writes `gcc/gmp/mpn/defaults.config` and appends
+   MPN-dependent variables to `gcc/gmp/defaults.config`.
+
+This violates putup's design: the build system should be the single source of
+truth for build logic. Preprocessing steps that mutate the config tree are
+invisible to putup's dependency tracking and make builds non-reproducible.
+
+## Design Goals
+
+- **Zero preprocessing**: `putup configure && putup` is the complete build command
+- **Config tree is immutable**: no step writes into the `-C` tree at build time
+- **Platform selection via `--config`**: the only knob for platform targeting
+- **Scoped config merging**: child `defaults.config` provides library defaults;
+  parent `--config` overrides on collision
+
+## Solution
+
+### 1. Eliminate `setup-host-configs`
+
+Different libraries need different values for the same `CONFIG_` key on darwin
+(e.g., `CONFIG_HAVE_ALLOCA_H` is `n` for binutils but `1` for gmp). This means
+per-library overrides cannot all live in the root platform config — scoped merge
+would force one value on all libraries.
+
+Instead, libraries with platform-specific configs get a `defaults-darwin.config`
+alongside their `defaults.config`. The Tupfile selects via `ifeq`:
+
+```tup
+ifeq (@(HOST_OS),darwin)
+: defaults-darwin.config |> ^ INSTALL %o^ cp %f %o |> tup.config
+else
+: defaults.config |> ^ INSTALL %o^ cp %f %o |> tup.config
+endif
+```
+
+Only 3 libraries have meaningful differences: **binutils**, **gcc**, **libcpp**.
+The other 6 libraries have identical configs across platforms (or the existing
+defaults.config already works on both).
+
+For **libbacktrace**, the only difference is `BACKTRACE_FORMAT` (elf vs macho).
+This is already in the root platform config (`x86_64-linux.config`), so the
+darwin platform config just needs `CONFIG_BACKTRACE_FORMAT=macho`.
+
+**Create**: `defaults-darwin.config` in binutils/, gcc/gcc/, gcc/libcpp/
+**Modify**: Those 3 Tupfiles to add `ifeq` selection
+**Modify**: `darwin-x86_64-linux.config` — add `CONFIG_BACKTRACE_FORMAT=macho`
+**Delete**: `examples/bsp/configs/host-darwin/` (entire directory, 9 files)
+
+### 2. Make `resolve-mpn` a config-generating Tupfile rule
+
+Add `CONFIG_MPN_CPU` to the platform configs (default: `generic`). The GMP mpn
+Tupfile reads `@(MPN_CPU)` and runs `resolve-mpn.sh` as a config-generating rule:
+
+```tup
+# In gcc/gmp/mpn/Tupfile:
+: |> ^ RESOLVE-MPN %o^ $(SCRIPTS)/resolve-mpn.sh @(MPN_CPU) $(GMP_SRC)/mpn > %o |> tup.config
+```
+
+For the parent-scope variables (`GMP_MPARAM`, `ASM_ENABLED`), the GMP Tupfile's
+config rule copies `defaults.config` and appends MPN-dependent lines computed
+from `@(MPN_CPU)`:
+
+```tup
+# In gcc/gmp/Tupfile:
+: defaults.config |> ^ GEN %o^ cp %f %o && \
+  cpu="@(MPN_CPU)"; \
+  if [ -z "$cpu" ] || [ "$cpu" = "generic" ]; then \
+    echo "CONFIG_GMP_MPARAM=mpn/generic/gmp-mparam.h" >> %o; \
+    echo "CONFIG_NO_ASM=1" >> %o; \
+  else \
+    arch="${cpu%%/*}"; \
+    if [ -f "mpn/$arch/gmp-mparam.h" ]; then \
+      echo "CONFIG_GMP_MPARAM=mpn/$arch/gmp-mparam.h" >> %o; \
+    else \
+      echo "CONFIG_GMP_MPARAM=mpn/generic/gmp-mparam.h" >> %o; \
+    fi; \
+    echo "CONFIG_ASM_ENABLED=y" >> %o; \
+  fi |> tup.config
+```
+
+**Move** `resolve-mpn.sh` stays in `scripts/` — it becomes a build-time tool
+invoked by the Tupfile rule rather than a Makefile preprocessing step.
+
+**Delete**: `examples/bsp/gcc/gmp/mpn/defaults.config` (fully generated)
+
+**Modify**: `examples/bsp/gcc/gmp/defaults.config` — remove the 3 MPN-dependent
+lines (`GMP_MPARAM`, `ASM_ENABLED`, `NO_ASM`), which are now generated.
+
+### 3. Remove `Makefile.pup`
+
+With preprocessing eliminated, the Makefile wrapper serves no purpose. The
+canonical build command becomes:
+
+```bash
+putup configure --config configs/x86_64-linux.config -C . -S ../../source-root -B ../../build-gcc
+putup -C . -S ../../source-root -B ../../build-gcc -j$(nproc)
+```
+
+Extract the download logic (`download-source`, `download-binutils`) into a
+standalone `scripts/download-source.sh` for users who need to fetch tarballs.
+
+**Delete**: `examples/bsp/Makefile.pup`
+
+**Create**: `examples/bsp/scripts/download-source.sh` (download logic only)
+
+### 4. Update CI
+
+Replace `make -f Makefile.pup configure` with direct putup invocations in
+`.github/workflows/ci.yml`. The `build-gcc-bsp` job becomes:
+
+```yaml
+- name: Download source
+  run: cd examples/bsp && scripts/download-source.sh
+
+- name: Configure
+  run: cd examples/bsp && putup configure --config configs/x86_64-linux.config -C . -S ../../source-root -B ../../build-gcc
+
+- name: Build
+  run: cd examples/bsp && putup -C . -S ../../source-root -B ../../build-gcc -j$(nproc)
+```
+
+## File Changes
+
+| # | File | Action |
+|---|------|--------|
+| 1 | `examples/bsp/Makefile.pup` | Delete |
+| 2 | `examples/bsp/scripts/resolve-mpn.sh` | Keep (becomes build-time tool) |
+| 3 | `examples/bsp/scripts/download-source.sh` | Create (extracted from Makefile.pup) |
+| 4 | `examples/bsp/configs/host-darwin/` (9 files) | Delete directory |
+| 5 | `examples/bsp/configs/darwin-x86_64-linux.config` | Modify — add backtrace override, update comments |
+| 6 | `examples/bsp/configs/x86_64-linux.config` | Modify — add `CONFIG_MPN_CPU=generic` |
+| 7 | `examples/bsp/binutils/defaults-darwin.config` | Create (from host-darwin/binutils.config) |
+| 8 | `examples/bsp/binutils/Tupfile` | Modify — add ifeq darwin selection |
+| 9 | `examples/bsp/gcc/gcc/defaults-darwin.config` | Create (from host-darwin/gcc.config) |
+| 10 | `examples/bsp/gcc/gcc/Tupfile` | Modify — add ifeq darwin selection |
+| 11 | `examples/bsp/gcc/libcpp/defaults-darwin.config` | Create (from host-darwin/libcpp.config) |
+| 12 | `examples/bsp/gcc/libcpp/Tupfile` | Modify — add ifeq darwin selection |
+| 13 | `examples/bsp/gcc/gmp/defaults.config` | Modify — remove MPN-dependent lines |
+| 14 | `examples/bsp/gcc/gmp/Tupfile` | Modify — config rule computes MPN vars |
+| 15 | `examples/bsp/gcc/gmp/mpn/defaults.config` | Delete |
+| 16 | `examples/bsp/gcc/gmp/mpn/Tupfile` | Modify — config rule runs resolve-mpn.sh |
+| 17 | `.github/workflows/ci.yml` | Modify — direct putup commands |
+| 18 | `docs/reference.md` | Modify — update Makefile.pup references |
+| 19 | `.claude/skills/gcc-example/SKILL.md` | Modify — update build commands |
+
+## Verification
+
+1. `putup configure --config configs/x86_64-linux.config -C . -S ../../source-root -B ../../build-gcc` succeeds
+2. `putup -C . -S ../../source-root -B ../../build-gcc` builds successfully
+3. CI `build-gcc-bsp` job passes
+4. Darwin cross-compile config contains all necessary overrides
diff --git a/docs/plans/2026-03-09-eliminate-makefile-preprocessing.md b/docs/plans/2026-03-09-eliminate-makefile-preprocessing.md
new file mode 100644
index 0000000..4053d09
--- /dev/null
+++ b/docs/plans/2026-03-09-eliminate-makefile-preprocessing.md
@@ -0,0 +1,460 @@
+# Eliminate Makefile.pup Preprocessing — Implementation Plan
+
+> **For Claude:** REQUIRED SUB-SKILL: Use superpowers:executing-plans to implement this plan task-by-task.
+
+**Goal:** Remove all preprocessing from the BSP example so `putup configure && putup` is the complete build — no Makefile wrapper, no external scripts mutating the config tree.
+
+**Architecture:** Two Makefile preprocessing steps (`setup-host-configs`, `resolve-mpn`) become Tupfile-driven. Darwin configs use per-library `ifeq` selection. MPN source resolution becomes a config-generating rule. `Makefile.pup` is deleted; download logic moves to a standalone script.
+
+**Tech Stack:** Tupfile rules, shell scripts, putup config-generating rules, CI YAML.
+
+**Design doc:** `docs/plans/2026-03-09-eliminate-makefile-preprocessing-design.md`
+
+---
+
+### Task 1: Eliminate setup-host-configs — per-library darwin configs
+
+The `setup-host-configs` Makefile target copies `configs/host-darwin/*.config` over each library's `defaults.config` when `HOST=darwin`. This mutates the config tree.
+
+**Replacement:** Libraries with platform-specific differences get a `defaults-darwin.config` file. The Tupfile uses `ifeq (@(HOST_OS),darwin)` to select which config to copy. `@(HOST_OS)` comes from the root platform config (e.g., `darwin-x86_64-linux.config` has `CONFIG_HOST_OS=darwin`).
+
+**Only 3 libraries need darwin variants** (the others have identical configs across platforms):
+- binutils (HAVE_ALLOCA_H, DECL_BASENAME, DECL_SBRK, DECL_ENVIRON differ)
+- gcc/gcc (MALLINFO, POSIX_FALLOCATE, GNU_AS/LD, asm features differ)
+- gcc/libcpp (*_unlocked functions, DECL_BASENAME, SIZEOF_DEV_T differ)
+
+For libbacktrace, the only darwin difference is `BACKTRACE_FORMAT=macho` — this goes in the root darwin platform config since no other library uses that key.
+
+**Files:**
+- Create: `examples/bsp/binutils/defaults-darwin.config`
+- Create: `examples/bsp/gcc/gcc/defaults-darwin.config`
+- Create: `examples/bsp/gcc/libcpp/defaults-darwin.config`
+- Modify: `examples/bsp/binutils/Tupfile:1-2`
+- Modify: `examples/bsp/gcc/gcc/Tupfile:1-2`
+- Modify: `examples/bsp/gcc/libcpp/Tupfile:1-2`
+- Modify: `examples/bsp/configs/darwin-x86_64-linux.config`
+- Delete: `examples/bsp/configs/host-darwin/` (entire directory)
+
+**Step 1: Create defaults-darwin.config for binutils**
+
+Copy `configs/host-darwin/binutils.config` to `binutils/defaults-darwin.config`:
+
+```bash
+cp examples/bsp/configs/host-darwin/binutils.config examples/bsp/binutils/defaults-darwin.config
+```
+
+**Step 2: Create defaults-darwin.config for gcc**
+
+```bash
+cp examples/bsp/configs/host-darwin/gcc.config examples/bsp/gcc/gcc/defaults-darwin.config
+```
+
+**Step 3: Create defaults-darwin.config for libcpp**
+
+```bash
+cp examples/bsp/configs/host-darwin/libcpp.config examples/bsp/gcc/libcpp/defaults-darwin.config
+```
+
+**Step 4: Update binutils/Tupfile — ifeq darwin selection**
+
+Replace line 2:
+```tup
+: defaults.config |> ^ INSTALL %o^ cp %f %o |> tup.config
+```
+
+With:
+```tup
+ifeq (@(HOST_OS),darwin)
+: defaults-darwin.config |> ^ INSTALL %o^ cp %f %o |> tup.config
+else
+: defaults.config |> ^ INSTALL %o^ cp %f %o |> tup.config
+endif
+```
+
+**Step 5: Update gcc/gcc/Tupfile — ifeq darwin selection**
+
+Same pattern as step 4: replace line 2 with the ifeq block.
+
+**Step 6: Update gcc/libcpp/Tupfile — ifeq darwin selection**
+
+Same pattern as step 4: replace line 2 with the ifeq block.
+
+**Step 7: Update darwin-x86_64-linux.config**
+
+Add libbacktrace override and update comments. The file should become:
+
+```
+# GCC Libraries - macOS host, x86-64 Linux target (cross-compiler)
+#
+# Usage:
+#   putup configure --config configs/darwin-x86_64-linux.config \
+#     -C . -S ../../source-root -B ../../build-gcc
+#
+# Builds cc1 on macOS (Apple Silicon) that generates x86_64 Linux code.
+# Host compiler is Apple Clang; target config reuses x86_64-pc-linux-gnu.
+
+# Toolchain (Apple Clang as host compiler)
+CONFIG_CC=clang
+CONFIG_CXX=clang++
+CONFIG_AR=ar
+CONFIG_HOSTCC=clang++
+
+# Platform selection
+CONFIG_TARGET=x86_64-pc-linux-gnu
+CONFIG_HOST_OS=darwin
+CONFIG_PLATFORM_LDFLAGS=-lm -lpthread -lz -liconv
+CONFIG_EXTRA_MODES_FILE=config/i386/i386-modes.def
+CONFIG_DECIMAL_FORMAT=bid
+
+# MPN CPU target (generic for cross-compile)
+CONFIG_MPN_CPU=generic
+
+# libbacktrace: Mach-O format on macOS
+CONFIG_BACKTRACE_FORMAT=macho
+```
+
+**Step 8: Delete host-darwin/ directory**
+
+```bash
+rm -rf examples/bsp/configs/host-darwin/
+```
+
+**Step 9: Commit**
+
+```bash
+git add examples/bsp/binutils/defaults-darwin.config \
+  examples/bsp/gcc/gcc/defaults-darwin.config \
+  examples/bsp/gcc/libcpp/defaults-darwin.config \
+  examples/bsp/binutils/Tupfile \
+  examples/bsp/gcc/gcc/Tupfile \
+  examples/bsp/gcc/libcpp/Tupfile \
+  examples/bsp/configs/darwin-x86_64-linux.config
+git rm -rf examples/bsp/configs/host-darwin/
+git commit -m "bsp: eliminate setup-host-configs — use ifeq for darwin config selection
+
+Libraries with darwin-specific configs (binutils, gcc, libcpp) get a
+defaults-darwin.config alongside their defaults.config. The Tupfile
+selects via ifeq (@(HOST_OS),darwin). No more Makefile preprocessing.
+
+Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>"
+```
+
+---
+
+### Task 2: Make resolve-mpn a config-generating Tupfile rule
+
+The `resolve-mpn` Makefile target runs `scripts/resolve-mpn.sh` as a preprocessing step to generate `gcc/gmp/mpn/defaults.config` and append MPN-dependent lines to `gcc/gmp/defaults.config`. This becomes Tupfile-driven.
+
+**Key insight:** During `putup configure`, only the root `tup.config` is active for `@()` variable resolution. So `CONFIG_MPN_CPU` must be in the root platform config for the config-generating rules to read it.
+
+**Files:**
+- Modify: `examples/bsp/configs/x86_64-linux.config`
+- Modify: `examples/bsp/gcc/gmp/defaults.config:100-101`
+- Modify: `examples/bsp/gcc/gmp/Tupfile:2`
+- Modify: `examples/bsp/gcc/gmp/mpn/Tupfile:1-2`
+- Delete: `examples/bsp/gcc/gmp/mpn/defaults.config`
+
+**Step 1: Add CONFIG_MPN_CPU to x86_64-linux.config**
+
+Append after the `CONFIG_BACKTRACE_ELF_SIZE=64` line:
+
+```
+# MPN CPU target (generic = pure C, x86_64 = arch asm, x86_64/core2 = CPU asm)
+CONFIG_MPN_CPU=generic
+```
+
+**Step 2: Remove MPN-dependent lines from gmp/defaults.config**
+
+Remove the last 2 lines of `examples/bsp/gcc/gmp/defaults.config`:
+
+```
+CONFIG_GMP_MPARAM=mpn/generic/gmp-mparam.h
+CONFIG_NO_ASM=1
+```
+
+These are now generated by the Tupfile config rule.
+
+**Step 3: Update gmp/Tupfile config rule**
+
+Replace line 2:
+```tup
+: defaults.config |> ^ INSTALL %o^ cp %f %o |> tup.config
+```
+
+With a rule that copies defaults.config and appends MPN-dependent variables:
+```tup
+: defaults.config |> ^ GEN %o^ cp %f %o && \
+  cpu="@(MPN_CPU)"; \
+  if [ -z "$cpu" ] || [ "$cpu" = "generic" ]; then \
+    echo "CONFIG_GMP_MPARAM=mpn/generic/gmp-mparam.h" >> %o; \
+    echo "CONFIG_NO_ASM=1" >> %o; \
+  else \
+    arch="${cpu%%/*}"; \
+    if [ -f "mpn/$arch/gmp-mparam.h" ]; then \
+      echo "CONFIG_GMP_MPARAM=mpn/$arch/gmp-mparam.h" >> %o; \
+    else \
+      echo "CONFIG_GMP_MPARAM=mpn/generic/gmp-mparam.h" >> %o; \
+    fi; \
+    echo "CONFIG_ASM_ENABLED=y" >> %o; \
+  fi |> tup.config
+```
+
+**Why this works:** `@(MPN_CPU)` is expanded by putup at parse time from the root config. The shell sees a literal string (e.g., `cpu="generic"` or `cpu="x86_64/core2"`). The `${cpu%%/*}` pattern strips the CPU subdir to get the arch. CWD at execution time is source-tree `gcc/gmp/`, so `mpn/$arch/gmp-mparam.h` resolves correctly.
+
+**Step 4: Update gmp/mpn/Tupfile config rule**
+
+Replace line 2:
+```tup
+: defaults.config |> ^ INSTALL %o^ cp %f %o |> tup.config
+```
+
+With:
+```tup
+: $(S)/scripts/resolve-mpn.sh |> ^ RESOLVE-MPN %o^ sh %f @(MPN_CPU) . > %o |> tup.config
+```
+
+**How this works:**
+- `$(S)` = BSP root (set in root `Tuprules.tup` as `$(TUP_CWD)`)
+- `$(S)/scripts/resolve-mpn.sh` references the config tree's script file
+- putup resolves the input to the correct cross-tree path (`%f` works via PR #11 fix)
+- `sh %f` runs the script regardless of execute permissions
+- `@(MPN_CPU)` is expanded at parse time from root config (e.g., `generic`)
+- `.` is the mpn source directory — CWD at execution time is source-tree `gcc/gmp/mpn/`
+- stdout redirects to `%o` which becomes `tup.config`
+
+**Step 5: Delete gmp/mpn/defaults.config**
+
+```bash
+rm examples/bsp/gcc/gmp/mpn/defaults.config
+```
+
+**Step 6: Commit**
+
+```bash
+git add examples/bsp/configs/x86_64-linux.config \
+  examples/bsp/gcc/gmp/defaults.config \
+  examples/bsp/gcc/gmp/Tupfile \
+  examples/bsp/gcc/gmp/mpn/Tupfile
+git rm examples/bsp/gcc/gmp/mpn/defaults.config
+git commit -m "bsp: make resolve-mpn a config-generating Tupfile rule
+
+MPN source resolution (generic C vs arch-specific asm) is now driven
+by @(MPN_CPU) in the platform config, read by Tupfile rules during
+putup configure. No more Makefile preprocessing step.
+
+Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>"
+```
+
+---
+
+### Task 3: Remove Makefile.pup — extract download script
+
+With both preprocessing steps eliminated, `Makefile.pup` has no purpose. Extract the download logic into a standalone shell script.
+
+**Files:**
+- Create: `examples/bsp/scripts/download-source.sh`
+- Delete: `examples/bsp/Makefile.pup`
+
+**Step 1: Create download-source.sh**
+
+```bash
+#!/bin/sh
+# download-source.sh — Download and assemble GCC + binutils source trees
+#
+# Usage: scripts/download-source.sh [SRCDIR]
+#
+# Environment:
+#   GCC_VERSION      GCC version to download (default: 15.2.0)
+#   BINUTILS_VERSION binutils version to download (default: 2.44)
+
+set -e
+
+GCC_VERSION=${GCC_VERSION:-15.2.0}
+BINUTILS_VERSION=${BINUTILS_VERSION:-2.44}
+SRCDIR=${1:-../../source-root}
+
+# Download binutils
+if [ ! -d "$SRCDIR/binutils" ]; then
+    echo "Downloading binutils $BINUTILS_VERSION..."
+    mkdir -p "$SRCDIR/binutils"
+    curl -L "https://ftp.gnu.org/gnu/binutils/binutils-$BINUTILS_VERSION.tar.xz" \
+        | tar xJ --strip-components=1 -C "$SRCDIR/binutils"
+else
+    echo "$SRCDIR/binutils already exists"
+fi
+
+# Download GCC + prerequisites
+if [ ! -d "$SRCDIR/gcc" ]; then
+    echo "Downloading GCC $GCC_VERSION..."
+    mkdir -p "$SRCDIR/gcc"
+    curl -L "https://ftp.gnu.org/gnu/gcc/gcc-$GCC_VERSION/gcc-$GCC_VERSION.tar.xz" \
+        | tar xJ --strip-components=1 -C "$SRCDIR/gcc"
+    cd "$SRCDIR/gcc" && ./contrib/download_prerequisites
+else
+    echo "$SRCDIR/gcc already exists"
+fi
+```
+
+Make executable:
+```bash
+chmod +x examples/bsp/scripts/download-source.sh
+```
+
+**Step 2: Delete Makefile.pup**
+
+```bash
+rm examples/bsp/Makefile.pup
+```
+
+**Step 3: Commit**
+
+```bash
+git add examples/bsp/scripts/download-source.sh
+git rm examples/bsp/Makefile.pup
+git commit -m "bsp: remove Makefile.pup, extract download script
+
+The canonical build command is now:
+  putup configure --config configs/x86_64-linux.config -C . -S ../src -B ../build
+  putup -C . -S ../src -B ../build -j\$(nproc)
+
+Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>"
+```
+
+---
+
+### Task 4: Update CI workflow
+
+Replace `make -f Makefile.pup` with direct putup invocations.
+
+**Files:**
+- Modify: `.github/workflows/ci.yml:173-196`
+
+**Step 1: Update build-gcc-bsp job**
+
+Replace the `Download source` and `Build` steps (lines 186-196):
+
+```yaml
+  build-gcc-bsp:
+    needs: build-linux
+    runs-on: ubuntu-latest
+    timeout-minutes: 20
+    steps:
+      - uses: actions/checkout@v4
+      - name: Download build artifacts
+        uses: actions/download-artifact@v4
+        with:
+          name: build-linux
+          path: build/
+      - name: Fix permissions
+        run: chmod +x build/putup
+      - name: Download source
+        working-directory: examples/bsp
+        run: scripts/download-source.sh ../../source-root
+      - name: Configure
+        working-directory: examples/bsp
+        run: ../../build/putup configure --config configs/x86_64-linux.config -C . -S ../../source-root -B ../../build-gcc
+      - name: Build
+        working-directory: examples/bsp
+        run: ../../build/putup -C . -S ../../source-root -B ../../build-gcc -j$(nproc)
+      - name: Smoke test cc1
+        run: |
+          echo 'int main() { return 0; }' > /tmp/test.c
+          build-gcc/gcc/gcc/cc1 /tmp/test.c -quiet -o /tmp/test.s
+          grep 'GCC.*15.2.0' /tmp/test.s
+```
+
+**Step 2: Commit**
+
+```bash
+git add .github/workflows/ci.yml
+git commit -m "ci: use direct putup commands for BSP build
+
+Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>"
+```
+
+---
+
+### Task 5: Update documentation and skill files
+
+**Files:**
+- Modify: `.claude/skills/gcc-example/SKILL.md:12-15`
+- Modify: `docs/reference.md` (grep for Makefile.pup references)
+
+**Step 1: Update SKILL.md build commands**
+
+Replace lines 12-15 (the `make -f Makefile.pup` section):
+
+```markdown
+## Build Command
+
+```bash
+cd /path/to/pup/examples/bsp && \
+  scripts/download-source.sh ../../source-root && \
+  putup configure --config configs/x86_64-linux.config \
+    -C . -S ../../source-root -B ../../build-gcc && \
+  putup -C . -S ../../source-root -B ../../build-gcc -j$(nproc)
+```
+
+Or step by step:
+
+```bash
+cd /path/to/pup/examples/bsp && \
+  putup configure --config configs/x86_64-linux.config \
+    -C . -S ../../source-root -B ../../build-gcc
+  putup -C . -S ../../source-root -B ../../build-gcc -j$(nproc)
+```
+```
+
+Remove the "Or directly with putup:" paragraph (that IS the primary command now).
+
+**Step 2: Update self-contained library convention section**
+
+In SKILL.md, update:
+```
+- Its own `tup.config` for library-specific config
+```
+to:
+```
+- Its own `defaults.config` for library-specific config (copied to `tup.config` by Tupfile rule)
+```
+
+**Step 3: Search reference.md for Makefile.pup references**
+
+Run: `grep -n Makefile.pup docs/reference.md`
+
+Update or remove any references. The main one is likely in the examples section.
+
+**Step 4: Commit**
+
+```bash
+git add .claude/skills/gcc-example/SKILL.md docs/reference.md
+git commit -m "docs: update BSP build commands — Makefile.pup removed
+
+Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>"
+```
+
+---
+
+### Task 6: Verify via CI
+
+**Step 1: Push branch and create PR**
+
+```bash
+git push -u origin HEAD
+gh pr create --title "bsp: eliminate Makefile.pup preprocessing" --body "..."
+```
+
+**Step 2: Monitor CI**
+
+The `build-gcc-bsp` job is the critical gate. It will:
+1. Download source tarballs via the new `download-source.sh`
+2. Run `putup configure` (which executes config-generating rules including resolve-mpn)
+3. Build the full GCC + binutils toolchain
+4. Smoke test cc1
+
+Expected: all green. If `build-gcc-bsp` fails, check:
+- `resolve-mpn.sh` path resolution in 3-tree mode (`%f` expansion)
+- `@(MPN_CPU)` availability during configure (must be in root config)
+- ifeq `@(HOST_OS)` resolution (must be in root config — linux config doesn't set it, so the `else` branch fires, which is correct)
+
+**Important:** `x86_64-linux.config` does NOT set `CONFIG_HOST_OS`. This means `@(HOST_OS)` is empty during configure for linux builds. The `ifeq (@(HOST_OS),darwin)` will NOT match, so the `else` branch (linux defaults.config) is used. This is the correct behavior.
diff --git a/examples/bsp/Makefile.pup b/examples/bsp/Makefile.pup
deleted file mode 100644
index 68b76a8..0000000
--- a/examples/bsp/Makefile.pup
+++ /dev/null
@@ -1,116 +0,0 @@
-# Makefile.pup - Build GCC cross-compiler toolchain with putup
-#
-# Usage:
-#   make -f Makefile.pup                # Build toolchain (libraries + compilers + tools)
-#   make -f Makefile.pup clean          # Clean build artifacts
-#   make -f Makefile.pup distclean      # Full clean
-#
-# Assembly support:
-#   make -f Makefile.pup MPN_CPU=x86_64           # x86-64 arch-level assembly
-#   make -f Makefile.pup MPN_CPU=x86_64/core2     # CPU-specific assembly
-#   make -f Makefile.pup MPN_CPU=generic           # Pure C (default)
-#
-# Cross-compiler (macOS host → x86-64 Linux target):
-#   make -f Makefile.pup PLATFORM=darwin-x86_64-linux HOST=darwin
-#
-# 3-tree layout:
-#   -C .             BSP root (examples/bsp/)
-#   -S $(SRCDIR)     Assembled source tree (gcc/, binutils/ subdirs)
-#   -B $(BUILD)      Build output directory
-
-.PHONY: all build configure resolve-mpn setup-host-configs \
-       clean distclean download-source download-binutils
-
-PLATFORM ?= x86_64-linux
-HOST ?= linux
-SRCDIR ?= ../../source-root
-BUILD ?= ../../build-gcc
-MPN_CPU ?= generic
-PUTUP ?= putup
-TREES = -C . -S $(SRCDIR) -B $(BUILD)
-
-all: build
-
-build: resolve-mpn setup-host-configs
-	$(PUTUP) configure --config configs/$(PLATFORM).config $(TREES)
-	$(PUTUP) $(TREES) -j$$(nproc)
-
-configure: resolve-mpn setup-host-configs
-	$(PUTUP) configure --config configs/$(PLATFORM).config $(TREES)
-
-# Copy host-specific library configs when building for non-Linux hosts.
-# Linux configs are the in-tree defaults; other hosts overlay from configs/host-$(HOST)/.
-# To restore Linux defaults: git checkout -- gcc/*/defaults.config binutils/defaults.config
-setup-host-configs:
-	@if [ "$(HOST)" != "linux" ] && [ -d configs/host-$(HOST) ]; then \
-		for cfg in configs/host-$(HOST)/*.config; do \
-			lib=$$(basename "$$cfg" .config); \
-			if [ -f "$$lib/defaults.config" ]; then \
-				echo "  COPY $$lib/defaults.config (host-$(HOST))"; \
-				cp "$$cfg" "$$lib/defaults.config"; \
-			elif [ -f "gcc/$$lib/defaults.config" ]; then \
-				echo "  COPY gcc/$$lib/defaults.config (host-$(HOST))"; \
-				cp "$$cfg" "gcc/$$lib/defaults.config"; \
-			fi; \
-		done; \
-	fi
-
-# Resolve mpn sources for the selected CPU target.
-# - gcc/gmp/mpn/defaults.config: source lists and per-function toggles (child scope)
-# - gcc/gmp/defaults.config: GMP_MPARAM and ASM_ENABLED (parent scope, visible to gmp/Tupfile)
-resolve-mpn:
-	@scripts/resolve-mpn.sh $(MPN_CPU) $(SRCDIR)/gcc/gmp/mpn > gcc/gmp/mpn/defaults.config
-	@grep -v '^CONFIG_GMP_MPARAM=\|^CONFIG_ASM_ENABLED=\|^CONFIG_NO_ASM=' gcc/gmp/defaults.config > gcc/gmp/defaults.config.tmp && mv gcc/gmp/defaults.config.tmp gcc/gmp/defaults.config
-	@if [ "$(MPN_CPU)" != "generic" ]; then \
-		arch=$${MPN_CPU%%/*}; \
-		if [ -f "$(SRCDIR)/gcc/gmp/mpn/$$arch/gmp-mparam.h" ]; then \
-			echo "CONFIG_GMP_MPARAM=mpn/$$arch/gmp-mparam.h" >> gcc/gmp/defaults.config; \
-		else \
-			echo "CONFIG_GMP_MPARAM=mpn/generic/gmp-mparam.h" >> gcc/gmp/defaults.config; \
-		fi; \
-		echo "CONFIG_ASM_ENABLED=y" >> gcc/gmp/defaults.config; \
-	else \
-		echo "CONFIG_GMP_MPARAM=mpn/generic/gmp-mparam.h" >> gcc/gmp/defaults.config; \
-		echo "CONFIG_NO_ASM=1" >> gcc/gmp/defaults.config; \
-	fi
-
-# Multi-variant: build for multiple platforms in parallel
-#   make -f Makefile.pup multi SRCDIR=../../source-root
-PLATFORMS ?= x86_64-linux aarch64-linux
-multi:
-	@for p in $(PLATFORMS); do \
-		$(PUTUP) configure --config configs/$$p.config -C . -S $(SRCDIR) -B $(BUILD)-$$p; \
-	done
-	$(PUTUP) $(foreach p,$(PLATFORMS),-B $(BUILD)-$(p)) -C . -S $(SRCDIR) -j$$(nproc)
-
-# Download and assemble source tree.
-# Source assembly: extract each tarball, rename into $(SRCDIR)/.
-GCC_VERSION ?= 15.2.0
-BINUTILS_VERSION ?= 2.44
-
-download-source: download-binutils
-	@if [ ! -d "$(SRCDIR)/gcc" ]; then \
-		echo "Downloading GCC $(GCC_VERSION)..."; \
-		mkdir -p $(SRCDIR)/gcc; \
-		curl -L "https://ftp.gnu.org/gnu/gcc/gcc-$(GCC_VERSION)/gcc-$(GCC_VERSION).tar.xz" \
-		  | tar xJ --strip-components=1 -C $(SRCDIR)/gcc; \
-		cd $(SRCDIR)/gcc && ./contrib/download_prerequisites; \
-	else \
-		echo "$(SRCDIR)/gcc already exists"; \
-	fi
-
-download-binutils:
-	@if [ ! -d "$(SRCDIR)/binutils" ]; then \
-		echo "Downloading binutils $(BINUTILS_VERSION)..."; \
-		mkdir -p $(SRCDIR)/binutils; \
-		curl -L "https://ftp.gnu.org/gnu/binutils/binutils-$(BINUTILS_VERSION).tar.xz" \
-		  | tar xJ --strip-components=1 -C $(SRCDIR)/binutils; \
-	else \
-		echo "$(SRCDIR)/binutils already exists"; \
-	fi
-
-clean:
-	$(PUTUP) clean $(TREES)
-
-distclean:
-	$(PUTUP) distclean $(TREES)
diff --git a/examples/bsp/README.md b/examples/bsp/README.md
index c9840e8..f8f3ab3 100644
--- a/examples/bsp/README.md
+++ b/examples/bsp/README.md
@@ -54,16 +54,18 @@ examples/bsp/       (config tree)     source-root/        (source tree)
 ```bash
 cd examples/bsp
 
-# Assemble source tree (or use: make -f Makefile.pup download-source)
-# ... see above ...
+# Download source (or assemble manually — see above)
+scripts/download-source.sh ../../source-root
 
-# Build (macOS host → x86-64 Linux target)
-make -f Makefile.pup SRCDIR=../../source-root PLATFORM=darwin-x86_64-linux HOST=darwin
+# Configure + build (native x86-64 Linux)
+putup configure --config configs/x86_64-linux.config \
+    -C . -S ../../source-root -B ../../build-gcc
+putup -C . -S ../../source-root -B ../../build-gcc -j$(nproc)
 
-# Or directly with putup:
+# Cross-compile (macOS host → x86-64 Linux target)
 putup configure --config configs/darwin-x86_64-linux.config \
     -C . -S ../../source-root -B ../../build-gcc
-putup -C . -S ../../source-root -B ../../build-gcc -j8
+putup -C . -S ../../source-root -B ../../build-gcc -j$(nproc)
 ```
 
 ## Scoped Builds
@@ -93,29 +95,30 @@ putup configure -S /path/to/gcc-15.2.0 -B /path/to/build
 putup -S /path/to/gcc-15.2.0 -B /path/to/build
 ```
 
-## Makefile Targets
+## Build Commands
+
+| Command | Description |
+|---------|-------------|
+| `scripts/download-source.sh [SRCDIR]` | Download GCC + binutils tarballs |
+| `putup configure --config configs/<platform>.config -C . -S <src> -B <build>` | Configure |
+| `putup -C . -S <src> -B <build> -j$(nproc)` | Build |
+| `putup clean -C . -S <src> -B <build>` | Clean build artifacts |
+| `putup distclean -C . -S <src> -B <build>` | Full clean (remove build directory) |
+
+**Platform configs:** `configs/x86_64-linux.config`, `configs/darwin-x86_64-linux.config`
 
-| Target | Description |
-|--------|-------------|
-| `make -f Makefile.pup` | Full build (resolve-mpn + configure + build) |
-| `make -f Makefile.pup configure` | Configure only (no build) |
-| `make -f Makefile.pup download-source` | Download GCC + binutils tarballs into `SRCDIR` |
-| `make -f Makefile.pup clean` | Clean build artifacts |
-| `make -f Makefile.pup distclean` | Full clean (remove build directory) |
-| `make -f Makefile.pup multi` | Multi-variant parallel build for `PLATFORMS` |
+**Environment variables for `download-source.sh`:**
+
+| Variable | Default | Description |
+|----------|---------|-------------|
+| `GCC_VERSION` | `15.2.0` | GCC tarball version |
+| `BINUTILS_VERSION` | `2.44` | binutils tarball version |
 
-**Variables:**
+**Config variables (in platform config):**
 
 | Variable | Default | Description |
 |----------|---------|-------------|
-| `PLATFORM` | `x86_64-linux` | Config file selector (`configs/$(PLATFORM).config`) |
-| `HOST` | `linux` | Host OS (overlays `configs/host-$(HOST)/` if not linux) |
-| `SRCDIR` | `../../source-root` | Assembled source tree path |
-| `BUILD` | `../../build-gcc` | Build output directory |
-| `MPN_CPU` | `generic` | GMP assembly target (`generic`, `x86_64`, `x86_64/core2`) |
-| `PUTUP` | `putup` | Path to putup binary (override for CI) |
-| `GCC_VERSION` | `15.2.0` | GCC tarball version for `download-source` |
-| `BINUTILS_VERSION` | `2.44` | binutils tarball version for `download-source` |
+| `CONFIG_MPN_CPU` | `generic` | GMP assembly target (`generic`, `x86_64`, `x86_64/core2`) |
 
 ## Architecture
 
diff --git a/examples/bsp/binutils/Tupfile b/examples/bsp/binutils/Tupfile
index a1ec142..5218029 100644
--- a/examples/bsp/binutils/Tupfile
+++ b/examples/bsp/binutils/Tupfile
@@ -1,5 +1,9 @@
 include_rules
+ifeq (@(HOST_OS),darwin)
+: defaults-darwin.config |> ^ INSTALL %o^ cp %f %o |> tup.config
+else
 : defaults.config |> ^ INSTALL %o^ cp %f %o |> tup.config
+endif
 
 # ============================================================
 # binutils — cross-assembler (as) + cross-archiver (ar)
diff --git a/examples/bsp/configs/host-darwin/binutils.config b/examples/bsp/binutils/defaults-darwin.config
similarity index 100%
rename from examples/bsp/configs/host-darwin/binutils.config
rename to examples/bsp/binutils/defaults-darwin.config
diff --git a/examples/bsp/configs/darwin-x86_64-linux.config b/examples/bsp/configs/darwin-x86_64-linux.config
index bf3c1c4..9d751a0 100644
--- a/examples/bsp/configs/darwin-x86_64-linux.config
+++ b/examples/bsp/configs/darwin-x86_64-linux.config
@@ -1,7 +1,8 @@
 # GCC Libraries - macOS host, x86-64 Linux target (cross-compiler)
 #
 # Usage:
-#   make -f Makefile.pup PLATFORM=darwin-x86_64-linux HOST=darwin
+#   putup configure --config configs/darwin-x86_64-linux.config \
+#     -C . -S ../../source-root -B ../../build-gcc
 #
 # Builds cc1 on macOS (Apple Silicon) that generates x86_64 Linux code.
 # Host compiler is Apple Clang; target config reuses x86_64-pc-linux-gnu.
@@ -18,3 +19,9 @@ CONFIG_HOST_OS=darwin
 CONFIG_PLATFORM_LDFLAGS=-lm -lpthread -lz -liconv
 CONFIG_EXTRA_MODES_FILE=config/i386/i386-modes.def
 CONFIG_DECIMAL_FORMAT=bid
+
+# MPN CPU target (generic for cross-compile)
+CONFIG_MPN_CPU=generic
+
+# libbacktrace: Mach-O format on macOS
+CONFIG_BACKTRACE_FORMAT=macho
diff --git a/examples/bsp/configs/host-darwin/gmp.config b/examples/bsp/configs/host-darwin/gmp.config
deleted file mode 100644
index a280c8c..0000000
--- a/examples/bsp/configs/host-darwin/gmp.config
+++ /dev/null
@@ -1,101 +0,0 @@
-# GMP 6.2.1 - macOS (aarch64-apple-darwin) config.h defines
-#
-# Key differences from Linux: no sys_sysinfo, no hidden_alias (macOS linker),
-# no obstack_vprintf, host CPU is aarch64 not x86_64.
-
-# Architecture
-CONFIG_LIMB_BITS=64
-CONFIG_NAIL_BITS=0
-
-# MPN CPU target
-CONFIG_MPN_CPU=generic
-
-# Feature detection
-CONFIG_HAVE_ALARM=1
-CONFIG_HAVE_ALLOCA=1
-CONFIG_HAVE_ALLOCA_H=1
-CONFIG_HAVE_ATTRIBUTE_CONST=1
-CONFIG_HAVE_ATTRIBUTE_MALLOC=1
-CONFIG_HAVE_ATTRIBUTE_MODE=1
-CONFIG_HAVE_ATTRIBUTE_NORETURN=1
-CONFIG_HAVE_CLOCK=1
-CONFIG_HAVE_CLOCK_GETTIME=1
-CONFIG_HAVE_DECL_FGETC=1
-CONFIG_HAVE_DECL_FSCANF=1
-CONFIG_HAVE_DECL_OPTARG=1
-CONFIG_HAVE_DECL_SYS_ERRLIST=0
-CONFIG_HAVE_DECL_SYS_NERR=0
-CONFIG_HAVE_DECL_UNGETC=1
-CONFIG_HAVE_DECL_VFPRINTF=1
-CONFIG_HAVE_DLFCN_H=1
-CONFIG_HAVE_DOUBLE_IEEE_LITTLE_ENDIAN=1
-CONFIG_HAVE_FCNTL_H=1
-CONFIG_HAVE_FLOAT_H=1
-CONFIG_HAVE_GETPAGESIZE=1
-CONFIG_HAVE_GETRUSAGE=1
-CONFIG_HAVE_GETTIMEOFDAY=1
-CONFIG_HAVE_INTMAX_T=1
-CONFIG_HAVE_INTPTR_T=1
-CONFIG_HAVE_INTTYPES_H=1
-CONFIG_HAVE_LANGINFO_H=1
-CONFIG_HAVE_LIMB_LITTLE_ENDIAN=1
-CONFIG_HAVE_LOCALECONV=1
-CONFIG_HAVE_LOCALE_H=1
-CONFIG_HAVE_LONG_DOUBLE=1
-CONFIG_HAVE_LONG_LONG=1
-CONFIG_HAVE_MEMORY_H=1
-CONFIG_HAVE_MEMSET=1
-CONFIG_HAVE_MMAP=1
-CONFIG_HAVE_MPROTECT=1
-CONFIG_HAVE_NL_LANGINFO=1
-CONFIG_HAVE_NL_TYPES_H=1
-CONFIG_HAVE_POPEN=1
-CONFIG_HAVE_PTRDIFF_T=1
-CONFIG_HAVE_RAISE=1
-CONFIG_HAVE_SIGACTION=1
-CONFIG_HAVE_SIGALTSTACK=1
-CONFIG_HAVE_SIGNAL_H=1
-CONFIG_HAVE_STACK_T=1
-CONFIG_HAVE_STDINT_H=1
-CONFIG_HAVE_STDLIB_H=1
-CONFIG_HAVE_STRCHR=1
-CONFIG_HAVE_STRERROR=1
-CONFIG_HAVE_STRING_H=1
-CONFIG_HAVE_STRINGS_H=1
-CONFIG_HAVE_STRNLEN=1
-CONFIG_HAVE_STRTOL=1
-CONFIG_HAVE_STRTOUL=1
-CONFIG_HAVE_SYSCONF=1
-CONFIG_HAVE_SYS_MMAN_H=1
-CONFIG_HAVE_SYS_PARAM_H=1
-CONFIG_HAVE_SYS_RESOURCE_H=1
-CONFIG_HAVE_SYS_STAT_H=1
-CONFIG_HAVE_SYS_TIME_H=1
-CONFIG_HAVE_SYS_TIMES_H=1
-CONFIG_HAVE_SYS_TYPES_H=1
-CONFIG_HAVE_TIMES=1
-CONFIG_HAVE_UINT_LEAST32_T=1
-CONFIG_HAVE_UNISTD_H=1
-CONFIG_HAVE_VSNPRINTF=1
-CONFIG_LSYM_PREFIX="_"
-CONFIG_RETSIGTYPE=void
-CONFIG_SIZEOF_UNSIGNED=4
-CONFIG_SIZEOF_UNSIGNED_SHORT=2
-CONFIG_SIZEOF_VOID_P=8
-CONFIG_TIME_WITH_SYS_TIME=1
-CONFIG_TUNE_SQR_TOOM2_MAX=SQR_TOOM2_MAX_GENERIC
-CONFIG_PACKAGE="gmp"
-CONFIG_PACKAGE_BUGREPORT="gmp-bugs@gmp.org"
-CONFIG_PACKAGE_NAME="GNU MP"
-CONFIG_PACKAGE_STRING="GNU MP 6.2.1"
-CONFIG_PACKAGE_TARNAME="gmp"
-CONFIG_PACKAGE_VERSION="6.2.1"
-CONFIG_SIZEOF_MP_LIMB_T=8
-CONFIG_SIZEOF_UNSIGNED_LONG=8
-CONFIG_SIZEOF_UNSIGNED_LONG_LONG=8
-CONFIG_STDC_HEADERS=1
-CONFIG_VERSION="6.2.1"
-CONFIG_WANT_FFT=1
-CONFIG_WANT_TMP_ALLOCA=1
-CONFIG_GMP_MPARAM=mpn/generic/gmp-mparam.h
-CONFIG_NO_ASM=1
diff --git a/examples/bsp/configs/host-darwin/libbacktrace.config b/examples/bsp/configs/host-darwin/libbacktrace.config
deleted file mode 100644
index 645b8cb..0000000
--- a/examples/bsp/configs/host-darwin/libbacktrace.config
+++ /dev/null
@@ -1,48 +0,0 @@
-# libbacktrace - macOS (aarch64-apple-darwin) config.h defines
-#
-# Key differences from Linux: Mach-O format (not ELF), no dl_iterate_phdr,
-# no link.h, no _GNU_SOURCE.
-
-# Object format: macho (macOS) instead of elf (Linux)
-CONFIG_BACKTRACE_FORMAT=macho
-
-# Atomics
-CONFIG_HAVE_ATOMIC_FUNCTIONS=1
-CONFIG_HAVE_SYNC_FUNCTIONS=1
-
-# Functions
-CONFIG_HAVE_CLOCK_GETTIME=1
-CONFIG_HAVE_FCNTL=1
-CONFIG_HAVE_LSTAT=1
-CONFIG_HAVE_READLINK=1
-
-# Declarations
-CONFIG_HAVE_DECL_GETPAGESIZE=1
-CONFIG_HAVE_DECL_STRNLEN=1
-CONFIG_HAVE_DECL__PGMPTR=0
-
-# Headers
-CONFIG_HAVE_DLFCN_H=1
-CONFIG_HAVE_INTTYPES_H=1
-CONFIG_HAVE_MEMORY_H=1
-CONFIG_HAVE_STDINT_H=1
-CONFIG_HAVE_STDLIB_H=1
-CONFIG_HAVE_STRINGS_H=1
-CONFIG_HAVE_STRING_H=1
-CONFIG_HAVE_SYS_MMAN_H=1
-CONFIG_HAVE_SYS_STAT_H=1
-CONFIG_HAVE_SYS_TYPES_H=1
-CONFIG_HAVE_UNISTD_H=1
-
-# Compression (optional, can link -lz)
-CONFIG_HAVE_ZLIB=1
-
-# Sizes
-CONFIG_SIZEOF_CHAR=1
-CONFIG_SIZEOF_SHORT=2
-CONFIG_SIZEOF_INT=4
-CONFIG_SIZEOF_LONG=8
-CONFIG_SIZEOF_VOID_P=8
-
-# Misc
-CONFIG_STDC_HEADERS=1
diff --git a/examples/bsp/configs/host-darwin/libdecnumber.config b/examples/bsp/configs/host-darwin/libdecnumber.config
deleted file mode 100644
index de25363..0000000
--- a/examples/bsp/configs/host-darwin/libdecnumber.config
+++ /dev/null
@@ -1,7 +0,0 @@
-# libdecnumber - macOS (aarch64-apple-darwin) config.h defines
-#
-# Identical to Linux — libdecnumber only needs endianness and standard headers.
-
-CONFIG_STDC_HEADERS=1
-CONFIG_HAVE_STRING_H=1
-CONFIG_HAVE_STDLIB_H=1
diff --git a/examples/bsp/configs/host-darwin/libiberty.config b/examples/bsp/configs/host-darwin/libiberty.config
deleted file mode 100644
index 4fb2d0a..0000000
--- a/examples/bsp/configs/host-darwin/libiberty.config
+++ /dev/null
@@ -1,132 +0,0 @@
-# libiberty - macOS (aarch64-apple-darwin) config.h defines
-#
-# Key differences from Linux: no canonicalize_file_name, no on_exit,
-# no pipe2, no sys_prctl_h, no sys_sysinfo_h, no __fsetlocking,
-# no stdio_ext.h, no sigsetmask, no strverscmp.
-
-# Headers
-CONFIG_HAVE_ALLOCA_H=1
-CONFIG_HAVE_FCNTL_H=1
-CONFIG_HAVE_INTTYPES_H=1
-CONFIG_HAVE_LIMITS_H=1
-CONFIG_HAVE_MEMORY_H=1
-CONFIG_HAVE_SPAWN_H=1
-CONFIG_HAVE_STDINT_H=1
-CONFIG_HAVE_STDLIB_H=1
-CONFIG_HAVE_STRINGS_H=1
-CONFIG_HAVE_STRING_H=1
-CONFIG_HAVE_SYS_FILE_H=1
-CONFIG_HAVE_SYS_MMAN_H=1
-CONFIG_HAVE_SYS_PARAM_H=1
-CONFIG_HAVE_SYS_RESOURCE_H=1
-CONFIG_HAVE_SYS_STAT_H=1
-CONFIG_HAVE_SYS_TIME_H=1
-CONFIG_HAVE_SYS_TYPES_H=1
-CONFIG_HAVE_SYS_WAIT_H=1
-CONFIG_HAVE_TIME_H=1
-CONFIG_HAVE_UNISTD_H=1
-
-# Functions
-CONFIG_HAVE_ASPRINTF=1
-CONFIG_HAVE_ATEXIT=1
-CONFIG_HAVE_BASENAME=1
-CONFIG_HAVE_BCMP=1
-CONFIG_HAVE_BCOPY=1
-CONFIG_HAVE_BSEARCH=1
-CONFIG_HAVE_BZERO=1
-CONFIG_HAVE_CALLOC=1
-CONFIG_HAVE_CLOCK=1
-CONFIG_HAVE_FFS=1
-CONFIG_HAVE_FORK=1
-CONFIG_HAVE_GETCWD=1
-CONFIG_HAVE_GETPAGESIZE=1
-CONFIG_HAVE_GETRLIMIT=1
-CONFIG_HAVE_GETRUSAGE=1
-CONFIG_HAVE_GETTIMEOFDAY=1
-CONFIG_HAVE_INDEX=1
-CONFIG_HAVE_INSQUE=1
-CONFIG_HAVE_MEMCHR=1
-CONFIG_HAVE_MEMCMP=1
-CONFIG_HAVE_MEMCPY=1
-CONFIG_HAVE_MEMMEM=1
-CONFIG_HAVE_MEMMOVE=1
-CONFIG_HAVE_MEMSET=1
-CONFIG_HAVE_MKSTEMPS=1
-CONFIG_HAVE_MMAP=1
-CONFIG_HAVE_POSIX_SPAWN=1
-CONFIG_HAVE_POSIX_SPAWNP=1
-CONFIG_HAVE_PSIGNAL=1
-CONFIG_HAVE_PUTENV=1
-CONFIG_HAVE_RANDOM=1
-CONFIG_HAVE_REALPATH=1
-CONFIG_HAVE_RENAME=1
-CONFIG_HAVE_RINDEX=1
-CONFIG_HAVE_SBRK=1
-CONFIG_HAVE_SETENV=1
-CONFIG_HAVE_SETRLIMIT=1
-CONFIG_HAVE_SNPRINTF=1
-CONFIG_HAVE_STPCPY=1
-CONFIG_HAVE_STPNCPY=1
-CONFIG_HAVE_STRCASECMP=1
-CONFIG_HAVE_STRCHR=1
-CONFIG_HAVE_STRDUP=1
-CONFIG_HAVE_STRERROR=1
-CONFIG_HAVE_STRNCASECMP=1
-CONFIG_HAVE_STRNDUP=1
-CONFIG_HAVE_STRNLEN=1
-CONFIG_HAVE_STRRCHR=1
-CONFIG_HAVE_STRSIGNAL=1
-CONFIG_HAVE_STRSTR=1
-CONFIG_HAVE_STRTOD=1
-CONFIG_HAVE_STRTOL=1
-CONFIG_HAVE_STRTOLL=1
-CONFIG_HAVE_STRTOUL=1
-CONFIG_HAVE_STRTOULL=1
-CONFIG_HAVE_SYSCONF=1
-CONFIG_HAVE_TIMES=1
-CONFIG_HAVE_TMPNAM=1
-CONFIG_HAVE_VASPRINTF=1
-CONFIG_HAVE_VFORK=1
-CONFIG_HAVE_VFPRINTF=1
-CONFIG_HAVE_VPRINTF=1
-CONFIG_HAVE_VSPRINTF=1
-CONFIG_HAVE_WAIT3=1
-CONFIG_HAVE_WAIT4=1
-CONFIG_HAVE_WAITPID=1
-CONFIG_HAVE_WORKING_FORK=1
-CONFIG_HAVE_WORKING_VFORK=1
-
-# Types
-CONFIG_HAVE_INTPTR_T=1
-CONFIG_HAVE_UINTPTR_T=1
-CONFIG_HAVE_LONG_LONG=1
-
-# Declarations
-CONFIG_HAVE_DECL_ASPRINTF=1
-CONFIG_HAVE_DECL_BASENAME=0
-CONFIG_HAVE_DECL_CALLOC=1
-CONFIG_HAVE_DECL_FFS=1
-CONFIG_HAVE_DECL_GETENV=1
-CONFIG_HAVE_DECL_GETOPT=1
-CONFIG_HAVE_DECL_MALLOC=1
-CONFIG_HAVE_DECL_REALLOC=1
-CONFIG_HAVE_DECL_SBRK=0
-CONFIG_HAVE_DECL_SNPRINTF=1
-CONFIG_HAVE_DECL_STRNLEN=1
-CONFIG_HAVE_DECL_STRTOL=1
-CONFIG_HAVE_DECL_STRTOLL=1
-CONFIG_HAVE_DECL_STRTOUL=1
-CONFIG_HAVE_DECL_STRTOULL=1
-CONFIG_HAVE_DECL_VASPRINTF=1
-CONFIG_HAVE_DECL_VSNPRINTF=1
-
-# Sizes
-CONFIG_SIZEOF_INT=4
-CONFIG_SIZEOF_LONG=8
-CONFIG_SIZEOF_LONG_LONG=8
-CONFIG_SIZEOF_SIZE_T=8
-
-# Misc
-CONFIG_STDC_HEADERS=1
-CONFIG_TIME_WITH_SYS_TIME=1
-CONFIG_UNSIGNED_64BIT_TYPE=unsigned long
diff --git a/examples/bsp/configs/host-darwin/mpc.config b/examples/bsp/configs/host-darwin/mpc.config
deleted file mode 100644
index d4568ae..0000000
--- a/examples/bsp/configs/host-darwin/mpc.config
+++ /dev/null
@@ -1,24 +0,0 @@
-# MPC 1.2.1 - macOS (aarch64-apple-darwin) config.h defines
-#
-# Identical to Linux — MPC is pure C with no platform-specific features.
-
-CONFIG_HAVE_INTTYPES_H=1
-CONFIG_HAVE_LOCALE_H=1
-CONFIG_HAVE_LOCALECONV=1
-CONFIG_HAVE_MEMORY_H=1
-CONFIG_HAVE_SETLOCALE=1
-CONFIG_HAVE_STDINT_H=1
-CONFIG_HAVE_STDLIB_H=1
-CONFIG_HAVE_STRING_H=1
-CONFIG_HAVE_STRINGS_H=1
-CONFIG_HAVE_SYS_STAT_H=1
-CONFIG_HAVE_SYS_TIME_H=1
-CONFIG_HAVE_SYS_TYPES_H=1
-CONFIG_HAVE_UNISTD_H=1
-CONFIG_PACKAGE="mpc"
-CONFIG_PACKAGE_NAME="mpc"
-CONFIG_PACKAGE_STRING="mpc 1.2.1"
-CONFIG_PACKAGE_TARNAME="mpc"
-CONFIG_PACKAGE_VERSION="1.2.1"
-CONFIG_STDC_HEADERS=1
-CONFIG_VERSION="1.2.1"
diff --git a/examples/bsp/configs/host-darwin/mpfr.config b/examples/bsp/configs/host-darwin/mpfr.config
deleted file mode 100644
index f92072b..0000000
--- a/examples/bsp/configs/host-darwin/mpfr.config
+++ /dev/null
@@ -1,31 +0,0 @@
-# MPFR 4.1.0 - macOS (aarch64-apple-darwin) config.h defines
-#
-# Nearly identical to Linux. Only endianness representation differs slightly.
-
-CONFIG_HAVE_ALLOCA=1
-CONFIG_HAVE_ALLOCA_H=1
-CONFIG_HAVE_INTTYPES_H=1
-CONFIG_HAVE_LOCALE_H=1
-CONFIG_HAVE_LOCALECONV=1
-CONFIG_HAVE_LONG_LONG=1
-CONFIG_HAVE_MEMORY_H=1
-CONFIG_HAVE_SETLOCALE=1
-CONFIG_HAVE_STDARG=1
-CONFIG_HAVE_STDINT_H=1
-CONFIG_HAVE_STDLIB_H=1
-CONFIG_HAVE_STRING_H=1
-CONFIG_HAVE_STRINGS_H=1
-CONFIG_HAVE_SYS_STAT_H=1
-CONFIG_HAVE_SYS_TIME_H=1
-CONFIG_HAVE_SYS_TYPES_H=1
-CONFIG_HAVE_UNISTD_H=1
-CONFIG_HAVE_VA_COPY=1
-CONFIG_HAVE_WCHAR_H=1
-CONFIG_HAVE_LITTLE_ENDIAN=1
-CONFIG_PACKAGE="mpfr"
-CONFIG_PACKAGE_NAME="MPFR"
-CONFIG_PACKAGE_STRING="MPFR 4.1.0"
-CONFIG_PACKAGE_TARNAME="mpfr"
-CONFIG_PACKAGE_VERSION="4.1.0"
-CONFIG_STDC_HEADERS=1
-CONFIG_VERSION="4.1.0"
diff --git a/examples/bsp/configs/x86_64-linux.config b/examples/bsp/configs/x86_64-linux.config
index 7433a5b..72e9a96 100644
--- a/examples/bsp/configs/x86_64-linux.config
+++ b/examples/bsp/configs/x86_64-linux.config
@@ -1,10 +1,12 @@
 # GCC Libraries - x86-64 Linux toolchain (native)
 #
 # Usage:
-#   putup configure --config configs/x86_64-linux.config -S ../gcc-15.2.0 -B ../build-gcc
+#   putup configure --config configs/x86_64-linux.config \
+#     -C . -S ../../source-root -B ../../build-gcc
 #
-# Per-library configs live in gmp/tup.config, mpfr/tup.config, mpc/tup.config.
-# Scoped config merging combines these with the toolchain vars below.
+# Per-library configs live in defaults.config files, copied to tup.config by
+# Tupfile rules during configure. Scoped config merging combines these with
+# the toolchain vars below.
 
 # Toolchain
 CONFIG_CC=gcc
@@ -21,3 +23,6 @@ CONFIG_EXTRA_MODES_FILE=config/i386/i386-modes.def
 # libbacktrace: ELF format on Linux, 64-bit
 CONFIG_BACKTRACE_FORMAT=elf
 CONFIG_BACKTRACE_ELF_SIZE=64
+
+# MPN CPU target (generic = pure C, x86_64 = arch asm, x86_64/core2 = CPU asm)
+CONFIG_MPN_CPU=generic
diff --git a/examples/bsp/gcc/gcc/Tupfile b/examples/bsp/gcc/gcc/Tupfile
index d4bdcc6..10206e7 100644
--- a/examples/bsp/gcc/gcc/Tupfile
+++ b/examples/bsp/gcc/gcc/Tupfile
@@ -1,5 +1,9 @@
 include_rules
+ifeq (@(HOST_OS),darwin)
+: defaults-darwin.config |> ^ INSTALL %o^ cp %f %o |> tup.config
+else
 : defaults.config |> ^ INSTALL %o^ cp %f %o |> tup.config
+endif
 
 # ============================================================
 # GCC 15.2.0 — C compiler backend
diff --git a/examples/bsp/configs/host-darwin/gcc.config b/examples/bsp/gcc/gcc/defaults-darwin.config
similarity index 100%
rename from examples/bsp/configs/host-darwin/gcc.config
rename to examples/bsp/gcc/gcc/defaults-darwin.config
diff --git a/examples/bsp/gcc/gmp/Tupfile b/examples/bsp/gcc/gmp/Tupfile
index 8debbce..02b5e18 100644
--- a/examples/bsp/gcc/gmp/Tupfile
+++ b/examples/bsp/gcc/gmp/Tupfile
@@ -1,5 +1,18 @@
 include_rules
-: defaults.config |> ^ INSTALL %o^ cp %f %o |> tup.config
+: defaults.config |> ^ GEN %o^ cp %f %o && \
+  cpu="@(MPN_CPU)"; \
+  if [ -z "$cpu" ] || [ "$cpu" = "generic" ]; then \
+    echo "CONFIG_GMP_MPARAM=mpn/generic/gmp-mparam.h" >> %o; \
+    echo "CONFIG_NO_ASM=1" >> %o; \
+  else \
+    arch="${cpu%%/*}"; \
+    if [ -f "mpn/$arch/gmp-mparam.h" ]; then \
+      echo "CONFIG_GMP_MPARAM=mpn/$arch/gmp-mparam.h" >> %o; \
+    else \
+      echo "CONFIG_GMP_MPARAM=mpn/generic/gmp-mparam.h" >> %o; \
+    fi; \
+    echo "CONFIG_ASM_ENABLED=y" >> %o; \
+  fi |> tup.config
 
 # ============================================================
 # GMP 6.2.1
diff --git a/examples/bsp/gcc/gmp/defaults.config b/examples/bsp/gcc/gmp/defaults.config
index a280c8c..0a24f8c 100644
--- a/examples/bsp/gcc/gmp/defaults.config
+++ b/examples/bsp/gcc/gmp/defaults.config
@@ -97,5 +97,3 @@ CONFIG_STDC_HEADERS=1
 CONFIG_VERSION="6.2.1"
 CONFIG_WANT_FFT=1
 CONFIG_WANT_TMP_ALLOCA=1
-CONFIG_GMP_MPARAM=mpn/generic/gmp-mparam.h
-CONFIG_NO_ASM=1
diff --git a/examples/bsp/gcc/gmp/mpn/Tupfile b/examples/bsp/gcc/gmp/mpn/Tupfile
index 9ab015d..8049473 100644
--- a/examples/bsp/gcc/gmp/mpn/Tupfile
+++ b/examples/bsp/gcc/gmp/mpn/Tupfile
@@ -1,5 +1,5 @@
 include_rules
-: defaults.config |> ^ INSTALL %o^ cp %f %o |> tup.config
+: $(S)/scripts/resolve-mpn.sh |> ^ RESOLVE-MPN %o^ sh %f @(MPN_CPU) . > %o |> tup.config
 
 # MPN — low-level multiprecision arithmetic
 #
diff --git a/examples/bsp/gcc/gmp/mpn/defaults.config b/examples/bsp/gcc/gmp/mpn/defaults.config
deleted file mode 100644
index d8a5628..0000000
--- a/examples/bsp/gcc/gmp/mpn/defaults.config
+++ /dev/null
@@ -1,20 +0,0 @@
-CONFIG_MPN_ARCH=generic
-CONFIG_MPN_SUBDIR=generic
-CONFIG_C_SOURCES=generic/add_1.c generic/add.c generic/add_err1_n.c generic/add_err2_n.c generic/add_err3_n.c generic/addmul_1.c generic/add_n.c generic/add_n_sub_n.c generic/bdiv_dbm1c.c generic/bdiv_q_1.c generic/bdiv_q.c generic/bdiv_qr.c generic/binvert.c generic/broot.c generic/brootinv.c generic/bsqrt.c generic/bsqrtinv.c generic/cmp.c generic/cnd_add_n.c generic/cnd_sub_n.c generic/cnd_swap.c generic/comb_tables.c generic/com.c generic/compute_powtab.c generic/copyd.c generic/copyi.c generic/dcpi1_bdiv_q.c generic/dcpi1_bdiv_qr.c generic/dcpi1_divappr_q.c generic/dcpi1_div_q.c generic/dcpi1_div_qr.c generic/dive_1.c generic/diveby3.c generic/divexact.c generic/divis.c generic/div_q.c generic/div_qr_1.c generic/div_qr_1n_pi1.c generic/div_qr_1n_pi2.c generic/div_qr_1u_pi2.c generic/div_qr_2.c generic/div_qr_2n_pi1.c generic/div_qr_2u_pi1.c generic/divrem_1.c generic/divrem_2.c generic/divrem.c generic/dump.c generic/fib2m.c generic/fib2_ui.c generic/gcd_11.c generic/gcd_1.c generic/gcd_22.c generic/gcd.c generic/gcdext_1.c generic/gcdext.c generic/gcdext_lehmer.c generic/gcd_subdiv_step.c generic/get_d.c generic/get_str.c generic/hgcd2.c generic/hgcd2_jacobi.c generic/hgcd_appr.c generic/hgcd.c generic/hgcd_jacobi.c generic/hgcd_matrix.c generic/hgcd_reduce.c generic/hgcd_step.c generic/invertappr.c generic/invert.c generic/jacbase.c generic/jacobi_2.c generic/jacobi.c generic/lshift.c generic/lshiftc.c generic/matrix22_mul1_inverse_vector.c generic/matrix22_mul.c generic/mod_1_1.c generic/mod_1_2.c generic/mod_1_3.c generic/mod_1_4.c generic/mod_1.c generic/mod_34lsub1.c generic/mode1o.c generic/mu_bdiv_q.c generic/mu_bdiv_qr.c generic/mu_divappr_q.c generic/mu_div_q.c generic/mu_div_qr.c generic/mul_1.c generic/mul_basecase.c generic/mul.c generic/mul_fft.c generic/mullo_basecase.c generic/mullo_n.c generic/mulmid_basecase.c generic/mulmid.c generic/mulmid_n.c generic/mulmod_bnm1.c generic/mul_n.c generic/neg.c generic/nussbaumer_mul.c generic/perfpow.c generic/perfsqr.c generic/pow_1.c generic/powlo.c generic/powm.c generic/pre_divrem_1.c generic/pre_mod_1.c generic/random2.c generic/random.c generic/redc_1.c generic/redc_2.c generic/redc_n.c generic/remove.c generic/rootrem.c generic/rshift.c generic/sbpi1_bdiv_q.c generic/sbpi1_bdiv_qr.c generic/sbpi1_bdiv_r.c generic/sbpi1_divappr_q.c generic/sbpi1_div_q.c generic/sbpi1_div_qr.c generic/scan0.c generic/scan1.c generic/sec_invert.c generic/sec_mul.c generic/sec_powm.c generic/sec_sqr.c generic/sec_tabselect.c generic/set_str.c generic/sizeinbase.c generic/sqr_basecase.c generic/sqr.c generic/sqrlo_basecase.c generic/sqrlo.c generic/sqrmod_bnm1.c generic/sqrtrem.c generic/strongfibo.c generic/sub_1.c generic/sub.c generic/sub_err1_n.c generic/sub_err2_n.c generic/sub_err3_n.c generic/submul_1.c generic/sub_n.c generic/tdiv_qr.c generic/toom22_mul.c generic/toom2_sqr.c generic/toom32_mul.c generic/toom33_mul.c generic/toom3_sqr.c generic/toom42_mul.c generic/toom42_mulmid.c generic/toom43_mul.c generic/toom44_mul.c generic/toom4_sqr.c generic/toom52_mul.c generic/toom53_mul.c generic/toom54_mul.c generic/toom62_mul.c generic/toom63_mul.c generic/toom6h_mul.c generic/toom6_sqr.c generic/toom8h_mul.c generic/toom8_sqr.c generic/toom_couple_handling.c generic/toom_eval_dgr3_pm1.c generic/toom_eval_dgr3_pm2.c generic/toom_eval_pm1.c generic/toom_eval_pm2.c generic/toom_eval_pm2exp.c generic/toom_eval_pm2rexp.c generic/toom_interpolate_12pts.c generic/toom_interpolate_16pts.c generic/toom_interpolate_5pts.c generic/toom_interpolate_6pts.c generic/toom_interpolate_7pts.c generic/toom_interpolate_8pts.c generic/trialdiv.c generic/zero.c generic/zero_p.c
-CONFIG_ASM_SOURCES=
-CONFIG_C_and_n=y
-CONFIG_C_andn_n=y
-CONFIG_C_ior_n=y
-CONFIG_C_iorn_n=y
-CONFIG_C_nand_n=y
-CONFIG_C_nior_n=y
-CONFIG_C_xnor_n=y
-CONFIG_C_xor_n=y
-CONFIG_C_popcount=y
-CONFIG_C_hamdist=y
-CONFIG_C_sec_add_1=y
-CONFIG_C_sec_sub_1=y
-CONFIG_C_sec_div_qr=y
-CONFIG_C_sec_div_r=y
-CONFIG_C_sec_pi1_div_qr=y
-CONFIG_C_sec_pi1_div_r=y
diff --git a/examples/bsp/gcc/libcpp/Tupfile b/examples/bsp/gcc/libcpp/Tupfile
index 85e70c9..d74ecaa 100644
--- a/examples/bsp/gcc/libcpp/Tupfile
+++ b/examples/bsp/gcc/libcpp/Tupfile
@@ -1,5 +1,9 @@
 include_rules
+ifeq (@(HOST_OS),darwin)
+: defaults-darwin.config |> ^ INSTALL %o^ cp %f %o |> tup.config
+else
 : defaults.config |> ^ INSTALL %o^ cp %f %o |> tup.config
+endif
 
 # ============================================================
 # libcpp - C preprocessor library (GCC 15.2.0)
diff --git a/examples/bsp/configs/host-darwin/libcpp.config b/examples/bsp/gcc/libcpp/defaults-darwin.config
similarity index 100%
rename from examples/bsp/configs/host-darwin/libcpp.config
rename to examples/bsp/gcc/libcpp/defaults-darwin.config
diff --git a/examples/bsp/scripts/download-source.sh b/examples/bsp/scripts/download-source.sh
new file mode 100755
index 0000000..6d2bc40
--- /dev/null
+++ b/examples/bsp/scripts/download-source.sh
@@ -0,0 +1,35 @@
+#!/bin/sh
+# download-source.sh — Download and assemble GCC + binutils source trees
+#
+# Usage: scripts/download-source.sh [SRCDIR]
+#
+# Environment:
+#   GCC_VERSION      GCC version to download (default: 15.2.0)
+#   BINUTILS_VERSION binutils version to download (default: 2.44)
+
+set -e
+
+GCC_VERSION=${GCC_VERSION:-15.2.0}
+BINUTILS_VERSION=${BINUTILS_VERSION:-2.44}
+SRCDIR=${1:-../../source-root}
+
+# Download binutils
+if [ ! -d "$SRCDIR/binutils" ]; then
+    echo "Downloading binutils $BINUTILS_VERSION..."
+    mkdir -p "$SRCDIR/binutils"
+    curl -L "https://ftp.gnu.org/gnu/binutils/binutils-$BINUTILS_VERSION.tar.xz" \
+        | tar xJ --strip-components=1 -C "$SRCDIR/binutils"
+else
+    echo "$SRCDIR/binutils already exists"
+fi
+
+# Download GCC + prerequisites
+if [ ! -d "$SRCDIR/gcc" ]; then
+    echo "Downloading GCC $GCC_VERSION..."
+    mkdir -p "$SRCDIR/gcc"
+    curl -L "https://ftp.gnu.org/gnu/gcc/gcc-$GCC_VERSION/gcc-$GCC_VERSION.tar.xz" \
+        | tar xJ --strip-components=1 -C "$SRCDIR/gcc"
+    cd "$SRCDIR/gcc" && ./contrib/download_prerequisites
+else
+    echo "$SRCDIR/gcc already exists"
+fi