Question

I have a simple code that I am trying to compile with lm32-rtems4.11-gcc.

I have the code, the compile command and the lst below. When I compile I see a bunch of code added on the top instead of the startup code that I want in there. The code I want the processor to start with after reset is at location 3f4 instead of 0. What I wanted help on is to figure out how the rest of the code got in and find a way to remove it or move all that code to addresses after my code. I appreciate the help.
Thanks

The code:

 //FILE: crt.S

.globl _start

.text
_start:
 xor r0, r0, r0
 mvhi sp, hi(_fstack)
 ori sp, sp, lo(_fstack)
 mv fp,r0
 mvhi r1, hi(_fbss)
 ori r1, r1, lo(_fbss)
 mvhi r2, hi(_ebss)
 ori r2, r2, lo(_ebss)
 1:
 bge r1, r2, 2f
 sw (r1+0), r0
 addi r1, r1, 4
 bi 1b
 2:
 calli main
 mvhi r1, 0xdead
 ori r2, r0, 0xbeef
 sw (r1+0), r2

 //FILE: hello_world.c

 void putc(char c)
 {
    char *tx = (char*)0xff000000;
    *tx = c;
 }

 void puts(char *s)
 {
    while (*s) putc(*s++);
 }

 void main(void)
 {
    puts("Hello World\n");
 }

 //FILE: linker.ld

OUTPUT_FORMAT("elf32-lm32")
ENTRY(_start)

__DYNAMIC = 0;

MEMORY {
    pmem : ORIGIN = 0x00000000, LENGTH = 0x8000
    dmem : ORIGIN = 0x00008000, LENGTH = 0x8000
}

SECTIONS
{
    .text :
    {
            _ftext = .;
            *(.text .stub .text.* .gnu.linkonce.t.*)
            _etext = .;
    } > pmem

    .rodata :
    {
            . = ALIGN(4);
            _frodata = .;
             *(.rodata .rodata.* .gnu.linkonce.r.*)
            *(.rodata1)
            _erodata = .;
    } > dmem

    .data :
    {
            . = ALIGN(4);
            _fdata = .;
            *(.data .data.* .gnu.linkonce.d.*)
            *(.data1)
            _gp = ALIGN(16);
            *(.sdata .sdata.* .gnu.linkonce.s.*)
            _edata = .;
    } > dmem

    .bss :
    {
            . = ALIGN(4);
            _fbss = .;
            *(.dynsbss)
            *(.sbss .sbss.* .gnu.linkonce.sb.*)
            *(.scommon)
            *(.dynbss)
            *(.bss .bss.* .gnu.linkonce.b.*)
            *(COMMON)
            . = ALIGN(4);
            _ebss = .;
            _end = .;
    } > dmem
}

The compile command

lm32-rtems4.11-gcc -Tlinker.ld -fno-builtin -o hello_world.elf crt.S hello_world.c
lm32-rtems4.11-objdump -DS hello_world.lst hello_world.elf

The lst file

00000000 <rtems_provides_crt0>:
#include <signal.h> /* sigset_t */
#include <time.h> /* struct timespec */
#include <unistd.h> /* isatty */

void rtems_provides_crt0( void ) {}  /* dummy symbol so file always has one */
0:   c3 a0 00 00     ret

00000004 <rtems_stub_malloc>:
#define RTEMS_STUB(ret, func, body) \
ret rtems_stub_##func body; \
ret func body

/* RTEMS provides some of its own routines including a Malloc family */
RTEMS_STUB(void *,malloc(size_t s), { return 0; })
4:   34 01 00 00     mvi r1,0
8:   c3 a0 00 00     ret

0000000c <malloc>:
c:   34 01 00 00     mvi r1,0
10:   c3 a0 00 00     ret
.
.
.
//omitting other such unrelated code that was inserted into the code and going to the 
//code at 3f4 that is the code I wanted at 0

000003f0 <__assert_func>:
3f0:   c3 a0 00 00     ret

000003f4 <_start>:
3f4:   98 00 00 00     xor r0,r0,r0
3f8:   78 1c 00 00     mvhi sp,0x0
3fc:   3b 9c ff fc     ori sp,sp,0xfffc
400:   b8 00 d8 00     mv fp,r0
404:   78 01 00 00     mvhi r1,0x0
408:   38 21 84 48     ori r1,r1,0x8448
40c:   78 02 00 00     mvhi r2,0x0
410:   38 42 84 48     ori r2,r2,0x8448
414:   4c 22 00 04     bge r1,r2,424 <_start+0x30>
418:   58 20 00 00     sw (r1+0),r0
41c:   34 21 00 04     addi r1,r1,4
420:   e3 ff ff fd     bi 414 <_start+0x20>
424:   f8 00 00 28     calli 4c4 <main>
428:   78 01 de ad     mvhi r1,0xdead
42c:   38 02 be ef     mvu r2,0xbeef
430:   58 22 00 00     sw (r1+0),r2
.
.
.

Answer 1

So I could not figure out why the compiler does not move the .start label to 0 when the linker.ld clearly tells it to do so. But I did figure a work around.

I created a section name for the startup code as shown in BOLD below. I then created a section in memory starting at 0 which I reserved only for this start up code. That seemed to do the trick. I ran the code and got a hello world 🙂 . All the changes I made are in BOLD and also commented //Change 1 //Change 2 and //Change 3.

//FILE: crt.S

.section .init// Change 1

.globl _start

.text
_start:
xor r0, r0, r0
mvhi sp, hi(_fstack)
ori sp, sp, lo(_fstack)
mv fp,r0
mvhi r1, hi(_fbss)
ori r1, r1, lo(_fbss)
.
.

//linker.ld
OUTPUT_FORMAT("elf32-lm32")
ENTRY(_start)

__DYNAMIC = 0;

MEMORY {

init : ORIGIN = 0x00000000, LENGTH = 0x40 //Change 2

    pmem : ORIGIN = 0x00000040, LENGTH = 0x8000
    dmem : ORIGIN = 0x00008000, LENGTH = 0x8000
}

SECTIONS
{

.init : {*(.init)}>init //Change 3

    .text :
    {
            _ftext = .;
            *(.text .stub .text.* .gnu.linkonce.t.*)
            _etext = .;
    } > pmem