基于Cortex-A8裸机，点亮LED灯程序分析

main.c

#include "s5pc100.h"

void delay(int ms)
{
int i;
while (ms--) {
i = 4000;
while (i--);
}
}

int main()
{
int num = 0;
char c;

GPG3.GPG3CON = 0x1111;//将GPG3端口与LED相连的引脚设置为输出模式
GPG3.GPG3DAT = 0;//数据寄存器的值设置为零

while (1) {
GPG3.GPG3DAT |= (1 << num);
delay(500);
GPG3.GPG3DAT &= ~(1 << num);
delay(500);
num++;
num %= 4;
}

return 0;
}

start.s

.text
.global _start
_start:
b	reset
ldr	pc, _undefined_instruction
ldr	pc, _software_interrupt
ldr	pc, _prefetch_abort
ldr	pc, _data_abort
ldr	pc, _not_used
ldr	pc, _irq
ldr	pc, _fiq

_undefined_instruction:
.word  _undefined_instruction
_software_interrupt:
.word  _software_interrupt
_prefetch_abort:
.word  _prefetch_abort
_data_abort:
.word  _data_abort
_not_used:
.word  _not_used
_irq:
.word  irq_handler
_fiq:
.word  _fiq

reset:
ldr	r0, =0x20008000
mcr	p15, 0, r0, c12, c0, 0  /*将中断向量表的地址设置为从0x20008000开始*/
init_stack:
ldr	r0, stackpointer	/* get stack pointer */
mov	sp, r0
sub	r0, #128 * 4		/* 512 byte  for irq mode of stack */
msr	cpsr, #0xD2
mov	sp, r0
sub	r0, #128 * 4		/* 512 byte  for fiq mode of stack */
msr 	cpsr, #0xD1
mov	sp, r0
sub	r0, #0
msr	cpsr, #0xD7
mov	sp, r0
sub	r0, #0
msr	cpsr, #0xDB
mov	sp, r0
sub	r0, #0
msr	cpsr, #0x10
mov	sp,r0			/* 1024 byte  for user mode of stack */

b	main

.align	5

irq_handler:
sub  lr,lr,#4
stmfd sp!,{r0-r12,lr}
@bl	do_irq
ldmfd sp!,{r0-r12,pc}^

stackpointer:
.word	stack + 4 * 512

.data
stack:		.space	4 * 512

.end

连接文件：

map.lds，其中代码段让start.o位于text的最前面

OUTPUT_FORMAT("elf32-littlearm", "elf32-littlearm", "elf32-littlearm")
/*OUTPUT_FORMAT("elf32-arm", "elf32-arm", "elf32-arm")*/
OUTPUT_ARCH(arm)
ENTRY(_start)
SECTIONS
{
. = 0x20008000;
. = ALIGN(4);
.text      :
{
start.o(.text)
*(.text)
}
. = ALIGN(4);
.rodata :
{ *(.rodata) }
. = ALIGN(4);
.data :
{ *(.data) }
. = ALIGN(4);
.bss :
{ *(.bss) }
}

Makefile

all:
arm-cortex_a8-linux-gnueabi-gcc -fno-builtin -nostdinc -c -o start.o start.S
arm-cortex_a8-linux-gnueabi-gcc -fno-builtin -nostdinc -c -o main.o main.c
arm-cortex_a8-linux-gnueabi-ld start.o main.o -Tmap.lds -o test.elf
arm-cortex_a8-linux-gnueabi-objcopy -O binary  test.elf test.bin
arm-cortex_a8-linux-gnueabi-objdump -D test.elf > test.dis
sudo cp test.bin /var/lib/tftpboot
clean:
rm -rf start.o main.o test.elf test.bin test.dis

其中-fno-builtin表示不接受不是两个下划线开头的内建函数(built-in function)，-nostdinc表示编译时不使用c库。

arm-cortex_a8-linux-gnueabi-objcopy -O binary  test.elf test.bin

这句话作用是将test.elf的格式头去掉并拷贝位test.bin

arm-cortex_a8-linux-gnueabi-objdump -D test.elf > test.dis

这句反汇编只是为了查看一下编译出来的汇编代码。

工程源码：点击打开链接