crt0.s 分析

ENTRY(_main)

/*
 * Set up initial C runtime environment and call board_init_f(0).
 */

#if defined(CONFIG_SPL_BUILD) && defined(CONFIG_SPL_STACK)
ldr sp, =(CONFIG_SPL_STACK)
#else
ldr sp, =(CONFIG_SYS_INIT_SP_ADDR)
#endif
bic sp, sp, #7 /* 8-byte alignment for ABI compliance */
sub sp, sp, #GD_SIZE /* allocate one GD above SP */
bic sp, sp, #7 /* 8-byte alignment for ABI compliance */
mov r9, sp /* GD is above SP */
mov r0, #0
    /* 以上代码都是为board_init_f函数准备堆栈空间和gd空间，都在iram中 */
bl board_init_f /*  */
    /* 此时gd空间被初始化了 */
#if ! defined(CONFIG_SPL_BUILD)

/*
 * Set up intermediate environment (new sp and gd) and call
 * relocate_code(addr_moni). Trick here is that we'll return
 * 'here' but relocated.
 */
    /* 初始化sp堆栈 *//* 为relcate， 等函数装备运行环境。 */
ldr sp, [r9, #GD_START_ADDR_SP] /* sp = gd->start_addr_sp */
bic sp, sp, #7 /* 8-byte alignment for ABI compliance */
    /* [r9, #GD_BD]就是gd->bd， r9保存的是gd */
ldr r9, [r9, #GD_BD] /* r9 = gd->bd */
sub r9, r9, #GD_SIZE /* new GD is below bd */


adr lr, here    
ldr r0, [r9, #GD_RELOC_OFF] /* r0 = gd->reloc_off */
add lr, lr, r0  /* here加 gd->reloc_off，就是要返回的地址（代码重定位后执行）*/
ldr r0, [r9, #GD_RELOCADDR] /* r0 = gd->relocaddr */
b relocate_code /* 把自己从*（nand）拷贝到dram，返回后here是拷贝到dram中的here。 */
    /* 在此之前的代码都是在iram中执行的。 */
here:
    /* 此后代码在dram中执行了 */
/* Set up final (full) environment */
bl c_runtime_cpu_setup /* we still call old routine here */
ldr r0, =__bss_start /* this is auto-relocated! */
ldr r1, =__bss_end /* this is auto-relocated! */

mov r2, #0x00000000 /* prepare zero to clear BSS */

clbss_l:cmp r0, r1 /* while not at end of BSS */
strlo r2, [r0] /* clear 32-bit BSS word */
addlo r0, r0, #4 /* move to next */
blo clbss_l

bl coloured_LED_init
bl red_led_on

/* call board_init_r(gd_t *id, ulong dest_addr) */
mov     r0, r9                  /* gd_t */
ldr r1, [r9, #GD_RELOCADDR] /* dest_addr */
/* call board_init_r */
ldr pc, =board_init_r /* this is auto-relocated! */

/* we should not return here. */
#endif


ENDPROC(_main)