ARM bare metal binary linking

Question

I have an ARM board with ROM at 0x80000000 and RAM at 0x20000000. Board starts execution of raw binary code at 0x80000000.

I managed to get a simple ARM assembler program running on it, but I have to use C instead of ASM. I know I will need to use some kind of linker script, and then manually copy .data section to RAM and clear .bss, setup stack etc. but I haven't found a reliable solution how to do this yet, especially the linker script (it's very messy in my opinion).

Also, I can't get the linker to output a raw binary instead of ELF, but it's not a big deal as I can use objcopy later.

Thanks in advance.

Answer 1

This example is for STM32F051 MCU:

Very simple application as "blinky" (without delays):

// define used registers
#define RCC_AHB1 *(volatile unsigned int *)(0x40021014)
#define GPIOC_MODER *(volatile unsigned int *)(0x48000800)
#define GPIOC_BSRR *(volatile unsigned int *)(0x48000818)

// main program
void mainApp() {
    RCC_AHB1 = 1 << 19;  // enable clock for GPIOC
    GPIOC_MODER = 1 << (9 * 2);  // set output on GPIOC.P9
    while (1) {
        GPIOC_BSRR = 1 << 9;  // set output on GPIOC.P9
        GPIOC_BSRR = 1 << (9 + 16);  // clear output on GPIOC.P9
    }
}

// variables for testing memory initialisation
int x = 10;
int y = 0;
int z;

Here is also very simple startup file written in C (also will work in C++), which start application mainApp and initialise static variables .data initialised from ROM and .bss only set to zero, there are variables initialised to zero and uninitialised variables.

extern void mainApp();

// external variables defined in linker script
// address in FLASH where are stored initial data for .data section
extern unsigned int _data_load;
// defines start and end of .data section in RAM
extern unsigned int _data_start;
extern unsigned int _data_end;
// defines start and end of .bss section in RAM
extern unsigned int _bss_start;
extern unsigned int _bss_end;

void resetHandler() {
    unsigned int *src, *dst;

    // copy .data area
    src = &_data_load;
    dst = &_data_start;
    while (dst < &_data_end) {
        *dst++ = *src++;
    }

    // clear .bss area
    dst = &_bss_start;
    while (dst < &_bss_end) {
        *dst++ = 0;
    }

    mainApp();

    while(1);
}

// _stacktop is defined in linker script
extern unsigned int _stacktop;

// vector table, will be placed on begin of FLASH memory, is defined in linker script
// only reset vector defined, need add other used vectors (especially NMI)
__attribute__((section(".vectors"), used)) void *isr_vectors[] = {
    &_stacktop,  // first vector is not vector but initial stack position
    (void *)resetHandler,  // vector which is called after MCU start
};

And finaly linker script which define location and sizes of memories, sections for vector, program (text), data (.data and .bss) a stacktop

MEMORY {
    FLASH(rx) : ORIGIN = 0x08000000, LENGTH = 64K
    SRAM(rwx) : ORIGIN = 0x20000000, LENGTH = 8K
}

SECTIONS {
    . = ORIGIN(FLASH);
    .text : {
        *(.vectors)
        *(.text)
    } >FLASH

    . = ORIGIN(SRAM);
    .data ALIGN(4) : {
        _data_start = .;
        *(.data)
        . = ALIGN(4);
        _data_end = .;
    } >SRAM AT >FLASH

    .bss ALIGN(4) (NOLOAD) : {
        _bss_start = .;
        *(.bss)
        . = ALIGN(4);
        _bss_end = .;
    } >SRAM

    _stacktop = ORIGIN(SRAM) + LENGTH(SRAM);
    _data_load = LOADADDR(.data);
}

Build this with these command (build and link at once):

$ arm-none-eabi-gcc -mcpu=cortex-m0 -mthumb -nostartfiles main.c startup.c -T stm32f051x8.ld -o main.elf

In symbol table is possible to see where are stored data:

$ arm-none-eabi-nm -C -l -n -S main.elf
08000000 00000008 T isr_vectors
08000008 00000034 T mainApp
0800003c 0000005c T resetHandler
08000098 A _data_load
20000000 D _data_start
20000000 00000004 D x
20000004 D _data_end
20000004 B _bss_start
20000004 00000004 B y
20000008 B _bss_end
20000008 00000004 B z
20002000 A _stacktop

Also you can look into listing:

arm-none-eabi-objdump -S main.elf

Raw binary:

arm-none-eabi-objcopy -O binary main.elf main.bin