ARMv7 Baremetal Bootloader

A minimal, single-stage bootloader for the AM335x (BeagleBone Black) written from scratch in C and ARM assembly. No U-Boot, no external dependencies — just bare metal.

What it does

ROM (chip) ──► Bootloader (SRAM 110KB) ──► Kernel (DDR @ 0x80000000)

The bootloader runs entirely from the AM335x internal SRAM and performs:

• Watchdog timer disable
• Clock configuration (DDR PLL @ 400MHz)
• UART0 initialization (115200 8N1)
• DDR3 memory init (256MB) with self-test
• MMC/SD card initialization and raw sector read
• Kernel image loading and validation
• Kernel handoff with boot parameters
• Full exception handling (Data Abort, Prefetch Abort, Undefined Instruction)

Project Structure

├── src/
│   ├── arch/arm/start.S   # Entry point, vector table, CPU mode setup
│   ├── main.c             # Boot flow orchestration
│   ├── clock.c            # PLL and clock configuration
│   ├── ddr.c              # DDR3 controller initialization
│   ├── mmc.c              # MMC/SD card driver
│   ├── uart.c             # UART serial driver
│   └── utils.c            # Utility functions (delay, memcpy, memset)
├── include/
│   ├── am335x.h           # AM335x register definitions
│   ├── boot.h             # Function prototypes
│   └── types.h            # Type definitions
├── linker/
│   └── bootloader.ld      # Linker script (SRAM @ 0x402F0400)
├── tools/
│   └── gp_header.py       # GP header generator for AM335x ROM
└── Makefile

Build

Environment

• OS: Ubuntu 24.04+ (or equivalent Linux distro)
• Python: 3.x
• Toolchain: arm-none-eabi-gcc, arm-none-eabi-binutils

Install dependencies on Ubuntu:

sudo apt update
sudo apt install -y gcc-arm-none-eabi binutils-arm-none-eabi python3

Verify toolchain:

arm-none-eabi-gcc --version
arm-none-eabi-objcopy --version
python3 --version

make          # Build → outputs MLO
make clean    # Clean build artifacts
make size     # Show binary size

Deploy to SD Card

# Format SD card with FAT32 partition (must be bootable)
# Copy MLO to the root of the FAT32 partition
cp build/MLO /media/sdcard/

The AM335x ROM looks for a file named MLO on the first FAT32 partition of the SD card.

How it works

1. ROM bootloader (built into AM335x silicon) initializes minimal hardware, finds MLO on SD card, loads it into SRAM at 0x402F0400
2. This bootloader takes over: sets up vector table, exception stacks, disables watchdog, inits clocks/DDR/UART/MMC
3. Reads kernel image from SD card raw sectors into DDR at 0x80000000
4. Validates kernel image header and jumps to execution address

Target Hardware

• SoC: Texas Instruments AM335x (ARM Cortex-A8)
• Board: BeagleBone Black
• RAM: 256MB DDR3 @ 400MHz
• Boot media: microSD card