S3c2440 code relocation details 3---analysis of link script

Linker script syntax

SECTIONS {

...

secname start BLOCK(align) (NOLOAD) : AT ( ldadr )

  { contents } >region :phdr =fill

...

}

explain:

 secname : section name

 start: starting address: runtime address (runtime addr); relocation address (relocate addr)

 AT (ldadr): optional (load addr: load address) If not written, LoadAddr = runtime addr

 Contents of { contents }: 

 start.o //The content is start.o file

 * (.text) All code snippet files

 start.o * (.text) files

elf file format

1 Link to get the elf file, which contains address information (load addr)

2 Using the loader

:: 2.1 For bare boards, it is the JTAG debugging tool

:: 2.2 For APP, the loader is also APP to parse the elf file and read it into the loading address of the memory

3 Run the program

4 If loadaddr != runtimeaddr the program itself needs to be relocated

The core program should be located at runtimeaddr (relocation addr) or link address when it is running

bin file

1 elf generates bin file 

2 Hardware mechanism startup

3 If the location of the bin file is not equal to runtimeaddr, the program itself implements relocation

The bin file/elf file does not save the bss segment. These are global variables with initial values ​​of 0 or not initialized.

When the program is running, the space corresponding to the bss segment is cleared

As an experiment, print out the global variable g_A in hexadecimal

/* 0xABCDEF12 */

void printHex(unsigned int val)

{

    int i;

    unsigned char arr[8];

    /* First take out the value of each bit*/

    for (i = 0; i < 8; i++)

    {

        arr[i] = val & 0xf;

        val >>= 4;   /* arr[0] = 2, arr[1] = 1, arr[2] = 0xF */

    }

    /* Print*/

    puts("0x");

    for (i = 7; i >=0; i--)

    {

        if (arr[i] >= 0 && arr[i] <= 9)

            putchar(arr[i] + '0');

        else if(arr[i] >= 0xA && arr[i] <= 0xF)

            putchar(arr[i] - 0xA + 'A');

    }

}

//Print the variable with initial value 0

int g_A = 0;

int g_B;

int main(void)

{

    uart0_init();

    puts("nrg_A = ");

    printHex(g_A);

    puts("nr");

In the above code, the bss segment g_A is equal to an inexplicable value and is not equal to 0, so the bss segment needs to be cleaned up.

Modify the lds link file

SECTIONS {

   .text   0  : { *(.text) }

   .rodata : { *(.rodata) }

   .data 0x30000000 : AT(0x700) 

   { 

      data_load_addr = LOADADDR(.data);

      data_start = . ;

      *(.data) 

      data_end = . ;

   }

   bss_start = .; //bss start address is the current position

   .bss  : { *(.bss) *(.COMMON) }

   bss_end = .; //bss end address is also the current position

}

Modify start.s and clear the bss segment

/* Clear BSS segment */

ldr r1, =bss_start

ldr r2, =bss_end

move r3, #0

clean:

    strb r3, [r1]

    add r1, r1, #1

    cmp r1, r2

    bne clean

    bl main

halt:

The current code global variable is 0. With just a few lines of code, you can save dozens or even thousands of global variables storage space.