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# 1.4.3.1 Bootloader linker configurations for CORTEX-M based MCUs

Linker configurations for the UART, I2C and CAN Bootloaders

- Bootloader library uses a custom linker script which is generated through MCC

- The values populated in the linker script are based on the Bootloader component MCC configurations

- Bootloader is configured to run from RAM in this linker script to achieve simultaneous Flash memory write and reception of the next block of data.

- Note: The below sections provides overview of changes done to bootloader linker scripts when compared to default linker script. The <bootloader_start> address and <bootloader_length> may vary based on the specific device used

    #define ROM_START <bootloader_start>

    /* Bootloader size is calculated with below criteria with optimization level -O2
     * bootloader size = Minimum Flash Erase Size Or actual bootloader ELF size
                         (Rounded of to nearest erase boundary) whichever is
                         greater.
     */
    #define ROM_SIZE  <bootloader_length>

    /* Bootloader Trigger pattern needs to be stored in starting <trigger_len> Bytes
     * of Ram by the application if it wants to run bootloader at startup without any
     * external trigger.
     * Example:
     *     ram[0] = 0x5048434D;
     *     ram[1] = 0x5048434D;
     *     ....
     *     ram[n] = 0x5048434D;
     */
    #define RAM_START (<ram_start> + <trigger_len>)

    #define RAM_SIZE  (<ram_length> - trigger_len)

    MEMORY
    {
      rom (rx) : ORIGIN = ROM_START, LENGTH = ROM_SIZE
      ram (rwx) : ORIGIN = RAM_START, LENGTH = RAM_SIZE
    }

    SECTIONS
    {
        /*
         * Configure to place the vector table in Flash but to be run from RAM
         */
        .vectors :
        {
            . = ALIGN(4);
            _sfixed = .;
            KEEP(*(.vectors .vectors.*))
        } > ram AT > rom

        .text :
        {
            . = ALIGN(4);
            ....
            ....
            . = ALIGN(4);
            _efixed = .;            /* End of text section */
        } > rom

        ....
        ....

        . = ALIGN(4);
        _etext = .;

        /* Locate text/rodata in special data section to be copied to RAM
           in startup sequence. */
        .data :
        {
            . = ALIGN(4);
            __data_start__ = .;
            _sdata = .;
            *(.dinit)
            *(.text)
            *(.text.*)
            *(.rodata)
            *(.rodata.*)
            . = ALIGN(4);
            __data_end__ = .;
            _edata = .;
        } > ram AT > rom

        ....
    }

Custom startup file for UART, I2C and CAN Bootloaders

- To reduce the size of the binary these bootloaders make use of custom startup file which is generated by MCC

- This startup file places only the Reset handler in vector table instead of populating all the device vectors there by reducing the size of final .vector section

- This startup file also copies the entire bootloader code placed in .data section above from flash to RAM as it is built to run from RAM

    /* Declaration of Reset handler (may be custom) */
    void __attribute__((noinline)) Reset_Handler(void);

    __attribute__ ((used, section(".vectors")))
    void (* const vectors[])(void) =
    {
      &_ram_end_,
      Reset_Handler,
    };

    ...
    ...

    /* Linker-defined symbols for data initialization. */
    extern uint32_t _sdata, _edata, _etext;
    extern uint32_t _sbss, _ebss;

    void __attribute__((noinline, section(".romfunc.Reset_Handler"))) Reset_Handler(void)
    {
        uint32_t *pSrc, *pDst;;

        pSrc = (uint32_t *) &_etext; /* flash functions start after .text */
        pDst = (uint32_t *) &_sdata;  /* boundaries of .data area to init */

        /* Copy code from flash to RAM using .data section */
        while (pDst < &_edata)
            *pDst++ = *pSrc++;

        /* Init .bss */
        pDst = &_sbss;
        while (pDst < &_ebss)
          *pDst++ = 0;

    ....
    ....
    }

MPLAB X Setting for UART, I2C and CAN Bootloaders

- Below MPLAB X option is enabled by MCC for these bootloaders to remove the XC32 crt0 startup code

- By disabling this crt0 startup code we further reduce the size as it removes the default initialization mechanism code by XC32.