Embedded Bootloaders

Purpose

Guide agents through embedded bootloader fundamentals: vector table relocation, safe handoff from bootloader to application, flash partitioning, and basic firmware-update patterns (UART, USB DFU, or custom protocol) on Cortex-M and similar MCUs.

When to Use

• Application must run at non-zero flash offset (e.g. 0x08010000)
• Implementing OTA or USB DFU without vendor HAL
• Debugging "app works when flashed alone but not via bootloader"
• Integrating with skills/baremetal/baremetal-startup and skills/baremetal/stm32-baremetal

Workflow

1. Memory layout (typical STM32)

Linker script for app must set FLASH ORIGIN to app base; vector table must live at app base.

2. Valid application image check

Before jump, verify:

App vector[0] (initial SP) points into RAM region
App vector[1] (Reset) points into flash region and has Thumb bit set (LSB=1)
Optional: CRC or magic word in app metadata section

#define APP_BASE  0x08010000U

static int app_valid(uint32_t base)
{
    uint32_t sp = *(uint32_t *)base;
    uint32_t reset = *(uint32_t *)(base + 4);
    if (sp < SRAM_BASE || sp > SRAM_END)
        return 0;
    if ((reset & 1U) == 0U)
        return 0;
    if (reset < base || reset > FLASH_END)
        return 0;
    return 1;
}

3. Cortex-M handoff sequence

typedef void (*app_entry_t)(void);

void jump_to_app(uint32_t app_base)
{
    uint32_t sp    = *(uint32_t *)app_base;
    uint32_t reset = *(uint32_t *)(app_base + 4);

    /* Disable interrupts and de-init peripherals/boot-owned hardware */
    __disable_irq();
    SysTick->CTRL = 0;
    for (int i = 0; i < 8; i++) {
        NVIC->ICER[i] = 0xFFFFFFFFU;
        NVIC->ICPR[i] = 0xFFFFFFFFU;
    }

    SCB->VTOR = app_base;
    __set_MSP(sp);
    __DSB();
    __ISB();

    app_entry_t entry = (app_entry_t)reset;
    entry(); /* does not return */
}

Application must set SCB->VTOR = APP_BASE early in Reset_Handler if startup assumes relocatable vector table.

4. Bootloader responsibilities

Power-on
├── Init minimal clock + UART/USB for update
├── Check update flag in RTC backup / GPIO strap
├── If update requested → receive image, verify, program flash
└── Else if valid app → jump_to_app()
    └── Else stay in bootloader shell

5. STM32 system memory DFU (factory ROM)

STM32 chips expose USB DFU in system memory when BOOT0=1. Custom bootloaders are separate — do not confuse ROM DFU with user flash BL.

6. Update safety

• Write to scratch sector, verify CRC, then swap metadata pointer (A/B)
• Never erase the only valid image without recovery path
• Reset watchdog only after verified commit

7. Agent usage

/bootloaders-embedded Write STM32F4 jump-to-app at 0x08010000 with VTOR setup

Common Problems

Related Skills

• skills/baremetal/baremetal-startup — vector table and Reset_Handler
• skills/baremetal/stm32-baremetal — flash map and CMSIS
• skills/baremetal/interrupts-and-exceptions-baremetal — NVIC disable pattern
• skills/embedded/linker-scripts — VMA/LMA for split images
• skills/embedded/openocd-jtag — recover bricked flash