1.7: C Startup

The C compiler uses a memory model where the RAM is divided into four contiguous sections. The linker provides the symbols needed to make sure the initial state meet the requirement of this memory model. So I need to write a piece of code to use those symbols to initialize or clear the RAM accordingly.

Section	Description
data	static initialized, initial value in flash
bss	static unassigned, cleared
heap	dynamically allocated, user managed
stack	automatically allocated, stack frame managed

My bootstrap since the first boot.c already initializes the stack. I need now to copy the initial values from flash to the data section and clear the bss section.

You can check your understanding of the C memory model by looking at the C test code below and figuring where the linker will allocate the variables.

/** Test code: main.c *********************************************************/

const char hexa[] = "0123456789abcdef" ;
long first = 1 ;
long i ;

int main( void) {
    static char c = 'a' ;
    char *cp = &c ;

    *cp += i ;
    i += hexa[ 13] - c + first++ ;
    return 0 ;
}

• data section holds first and c, for a total of 8 bytes as sections are word aligned.

• bss section holds i for a total of 4 bytes.

• text section holds hexa with all the const data located after the code. As it is a zero terminated string, it occupies 17 bytes and is padded with 3 zero for word alignment.

• text section holds the initial value of first and c for a total of 8 bytes located after the const data.

• stack section holds cp, it is dynamically managed by the code generated by the C compiler.

• after executing main(), hexa and c are unchanged, first has the value 2, i has the value 4 and cp has been deallocated.

Evolving the bootstrap

First I make a copy of boot.c into cstartup.c.

I add the symbols defined by the linker:

• __etext, start of initial value copy in FLASH.

• __data_start, start of initialized data in RAM.

• __bss_start, start of unitialized data in RAM, it is the same location as __data_end.

• __bss_end, first location after the bss section.

I rework Reset_handler() to:

• Initialize the data section with the initial values stored in flash.

• Clear the bss section

• Call the main() C function.

• Fallback to idle loop after main() has been executed.

Finally I append the test code for validation.

/* Memory locations defined by linker script */
extern long __StackTop ;        /* &__StackTop points after end of stack */
void Reset_Handler( void) ;     /* Entry point for execution */
extern const long __etext[] ;   /* start of initialized data copy in flash */
extern long __data_start__[] ;
extern long __bss_start__[] ;
extern long __bss_end__ ;       /* &__bss_end__ points after end of bss */

/* Interrupt vector table:
 * 1  Stack Pointer reset value
 * 15 System Exceptions
 * NN Device specific Interrupts
 */
typedef void (*isr_p)( void) ;
isr_p const isr_vector[ 2] __attribute__((section(".isr_vector"))) = {
    (isr_p) &__StackTop,
/* System Exceptions */
    Reset_Handler
} ;


extern int main( void) ;

void Reset_Handler( void) {
    const long  *f ;    /* from, source constant data from FLASH */
    long    *t ;        /* to, destination in RAM */

/* Assume:
**  __bss_start__ == __data_end__
**  All sections are 4 bytes aligned
*/
    f = __etext ;
    for( t = __data_start__ ; t < __bss_start__ ; t += 1)
        *t = *f++ ;

    while( t < &__bss_end__)
        *t++ = 0 ;

    main() ;
    for( ;;) ;
}

/** Test code: main.c *********************************************************/

const char hexa[] = "0123456789abcdef" ;
long first = 1 ;
long i ;

int main( void) {
    static char c = 'a' ;
    char *cp = &c ;

    *cp += i ;
    i += hexa[ 13] - c + first++ ;
    return 0 ;
}

Build

Building a binary, I can see that the data and bss section are not empty anymore and their size match my estimations.

$ make cstartup.bin
cstartup.elf
   text    data     bss     dec     hex filename
    121       8       4     133      85 cstartup.elf
cstartup.hex
rm cstartup.o cstartup.elf

If I look further in the cstartup.map generated by the linker.

.text           0x08000000       0x79
 *(.isr_vector)
 .isr_vector    0x08000000        0x8 cstartup.o
                0x08000000                isr_vector
 *(.text*)
 .text          0x08000008       0x34 cstartup.o
                0x08000008                Reset_Handler
 .text.startup  0x0800003c       0x2c cstartup.o
                0x0800003c                main
 *(.rodata*)
 .rodata        0x08000068       0x11 cstartup.o
                0x08000068                hexa
.data           0x20000000        0x8 load address 0x0800007c
                0x20000000                __data_start__ = .
 *(.data*)
 .data          0x20000000        0x8 cstartup.o
                0x20000004                first
                0x20000008                . = ALIGN (0x4)
                0x20000008                __data_end__ = .
.bss            0x20000008        0x4 load address 0x08000084
                0x20000008                . = ALIGN (0x4)
                0x20000008                __bss_start__ = .
 *(.bss*)
 .bss           0x20000008        0x0 cstartup.o
 *(COMMON)
 COMMON         0x20000008        0x4 cstartup.o
                0x20000008                i
                0x2000000c                . = ALIGN (0x4)
                0x2000000c                __bss_end__ = .
 *(.stack*)
                0x20001000                __StackTop = (ORIGIN (RAM) + LENGTH (RAM))

• hexa is located in .rodata at 0x08000068

• first is located in .data at 0x20000004

• i is located in .bss at 0x20000008

• c is not listed as it doesn’t have global scope, but we can guess it’s located at 0x20000000.

• Initial values for the .data section are located at 0x0800007c.

A hexadecimal dump of cstartup.bin confirms that the initial value of c is at offset 0x7c, which also means that c has been located at 0x20000000.

$ hexdump -C cstartup.bin
00000000  00 10 00 20 09 00 00 08  10 b5 08 4a 08 4b 09 49  |... .......J.K.I|
00000010  8b 42 06 d3 00 21 08 4a  93 42 05 d3 00 f0 0e f8  |.B...!.J.B......|
00000020  fe e7 01 ca 01 c3 f3 e7  02 c3 f5 e7 7c 00 00 08  |............|...|
00000030  00 00 00 20 08 00 00 20  0c 00 00 20 30 b5 08 49  |... ... ... 0..I|
00000040  08 4c 08 68 23 78 1b 18  da b2 63 68 22 70 5d 1c  |.L.h#x....ch"p].|
00000050  9b 1a 64 33 1b 18 65 60  00 20 0b 60 30 bd c0 46  |..d3..e`. .`0..F|
00000060  08 00 00 20 00 00 00 20  30 31 32 33 34 35 36 37  |... ... 01234567|
00000070  38 39 61 62 63 64 65 66  00 00 00 00 61 00 00 00  |89abcdef....a...|
00000080  01 00 00 00                                       |....|
00000084

Debug

I use the STM32 Cube Programmer to check that the code behaves as expected by checking the RAM state after main() has been executed.

Checkpoint

I have now a sample bootstrap that puts the RAM memory in a state required for a C startup.

Next I will merge the C startup initialization with the ledtick code.

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© 2020-2021 Renaud Fivet