Build System

This document describes the internal workings of the F9 microkernel build system.

Directory Structure

The build entry point is the top-level Makefile. The build system performs the following tasks:

• Reads platform-related information to set include paths and platform-specific source files
• Sets the output directory
• Determines which source files to compile
• Sets toolchain compile flags
• Loads F9 microkernel configurations from .config (generated via make config)
• Includes build.mk files from subdirectories to determine compilation targets
• Builds and optionally flashes the image

The source tree layout:

$topdir/
├── board/
│   └── $(BOARD)/
├── kernel/
│   └── lib/
├── loader/
│   └── elf/
├── platform/
│   └── $(CHIP)/
└── user/

Configuration Process

F9 uses Kconfiglib, a Python-based Kconfig implementation, for build configuration. Kconfiglib provides the same functionality as the Linux kernel's Kconfig tools but with no compilation step and better portability.

Kconfiglib Tools

Tool	Purpose
menuconfig.py	Interactive configuration menu
genconfig.py	Generate include/autoconf.h from .config
defconfig.py	Apply default configuration
oldconfig.py	Update .config with new options
savedefconfig.py	Save minimal configuration

Configuration Sequence

Running make config (or make menuconfig) triggers:

1. Kconfiglib is cloned from the repository if not present in tools/kconfig/
2. menuconfig.py runs with the root Kconfig file
3. User selections are saved to .config
4. genconfig.py generates include/autoconf.h with CONFIG_* macros

Board-Specific Defaults

Each board has a default configuration in board/$(BOARD)/defconfig:

make discoveryf4_defconfig    # Apply STM32F4-Discovery defaults
make discoveryf429_defconfig  # Apply STM32F429I-Discovery defaults

Build System Components

The mk/ directory contains the core build logic:

mk/
├── config.mk
├── generic.mk
├── loader.mk
├── rules/
│   └── symmap.mk
├── target.mk
└── toolchain.mk

config.mk

Legacy configuration helper. Configuration is now handled by Kconfiglib in tools/kconfig/. This file remains for compatibility notes.

generic.mk

The primary build orchestrator that handles:

• Configuration generation
• Source file compilation
• Image building (when CONFIG_LOADER is disabled)
• Image flashing
• Build rules for different file types (.c, .s, .bin)
• Clean targets

loader.mk

Active only when CONFIG_LOADER is enabled. Handles image building for configurations that use a separate loader stage.

target.mk

Entry point for make all. Selects the appropriate linker script based on whether CONFIG_LOADER is set.

toolchain.mk

Configures the cross-compilation toolchain and sets compiler flags for the target architecture.

rules/symmap.mk

Generates symbol map code for the KDB sampling command when CONFIG_SYMMAP is enabled. This allows runtime symbol resolution for debugging purposes.

Build Options

Verbose Output

Enable detailed build output:

make V=1

Platform Selection

Configure the target board in the top-level Makefile:

• BOARD: Target board identifier (e.g., discoveryf4, discoveryf429)
• PROJECT: Project-specific configuration

Toolchain Configuration

Modify mk/toolchain.mk to adjust:

• Cross-compiler prefix
• Compiler flags
• Linker flags
• Architecture-specific options

Output Artifacts

Build outputs are placed in build/$(BOARD)/:

• f9.elf: ELF executable with debug symbols
• f9.bin: Raw binary for flashing
• Object files and dependency files