Hands-On: Basic Bootloader Skeleton

/* PBL: minimal C main — runs from flash 0x08000000 */
/* Application: must be at 0x08010000 (separate compile) */
#include 

/* Boot flags in RAM (zero-initialised at startup) */
/* In production: stored in dedicated flash sector */
static volatile uint32_t g_reprog_flag;
#define REPROG_MAGIC  0xA5A5A5A5u

/* Application header at app base address */
typedef struct {
    uint32_t magic;       /* 0xDEADC0DE */
    uint32_t length;      /* bytes of application code */
    uint32_t crc32;       /* CRC-32 of [APP_BASE + 256 ... APP_BASE + length] */
    uint8_t  pad[244];    /* align to 256 bytes */
} AppHdr_t;

#define APP_BASE  0x08010000u
#define APP_MAGIC 0xDEADC0DEu

static uint32_t crc32_simple(const uint8_t *data, uint32_t len)
{
    uint32_t crc = 0xFFFFFFFFu;
    for (uint32_t i = 0; i < len; i++) {
        crc ^= data[i];
        for (int j = 0; j < 8; j++) {
            crc = (crc & 1u) ? ((crc >> 1) ^ 0xEDB88320u) : (crc >> 1);
        }
    }
    return ~crc;
}

static int app_is_valid(void)
{
    const AppHdr_t *hdr = (const AppHdr_t *)APP_BASE;
    if (hdr->magic != APP_MAGIC || hdr->length == 0 || hdr->length > 512*1024u)
        return 0;
    uint32_t crc = crc32_simple((const uint8_t *)(APP_BASE + 256u), hdr->length);
    return (crc == hdr->crc32);
}

int main(void)
{
    /* Reprogramming requested? */
    if (g_reprog_flag == REPROG_MAGIC) {
        g_reprog_flag = 0u;
        /* In full implementation: init UART/CAN, wait for UDS */
        while (1) { /* stub: hang in reprog mode */ }
    }

    /* Validate and jump to application */
    if (app_is_valid()) {
        __disable_irq();
        /* Cortex-M VTOR relocation */
        *(volatile uint32_t *)0xE000ED08u = APP_BASE;
        const uint32_t *vec  = (const uint32_t *)APP_BASE;
        __asm__ volatile (
            "msr msp, %0
"   /* set main stack pointer */
            "bx  %1
"        /* branch to app reset handler */
            :: "r"(vec[0]), "r"(vec[1])
        );
    }

    /* No valid app: hang (production: enter reprog mode) */
    while (1) {}
}

/* Linker script for PBL at 0x08000000; App at 0x08010000 */
/* Build PBL with -T pbl.ld; build App with -T app.ld (same format with APP_BASE) */

MEMORY {
    FLASH_PBL (rx)  : ORIGIN = 0x08000000, LENGTH = 64K   /* PBL */
    FLASH_APP (rx)  : ORIGIN = 0x08010000, LENGTH = 448K  /* Application */
    RAM       (rwx) : ORIGIN = 0x20000000, LENGTH = 128K  /* SRAM */
    RAMCODE   (rwx) : ORIGIN = 0x20010000, LENGTH = 32K   /* Flash driver RAM */
}

SECTIONS {
    /* PBL vector table at absolute start of PBL flash */
    .isr_vector : {
        KEEP(*(.isr_vector))
    } > FLASH_PBL

    .text : {
        *(.text*)
        *(.rodata*)
    } > FLASH_PBL

    /* Flash driver: copied to RAM at startup */
    .ramcode : {
        __ramcode_start__ = .;
        *(.ramcode*)
        __ramcode_end__ = .;
    } > RAMCODE AT > FLASH_PBL  /* LMA in flash; VMA in RAM */

    .data : {
        *(.data*)
    } > RAM AT > FLASH_PBL

    .bss : {
        *(.bss*)
        *(COMMON)
    } > RAM
}