.text
.globl _start
_start:ldr r0, =0x70000000orr r0, r0, #0x13mcr p15, 0, r0, c15, c2, 4ldr r0, =0x7e004000mov r1, #0str r1, [r0]bl clock_initldr sp, =8*1024bl xxxxb .

start.S 文件，调用完时钟初始化函数后调用跑马灯程序。

.globl clock_init
clock_init:ldr r0, =0x7E00F000ldr r1, =0x0000FFFFstr r1, [r0]str r1, [r0, #4]str r1, [r0, #8]#define OTHERS       0x7e00f900ldr r0, =OTHERSldr r1, [r0]bic r1, r1, #0xc0str r1, [r0]loop1:ldr r0, =OTHERSldr r1, [r0]and r1, r1, #0xf00                 cmp r1, #0bne loop1     #define ARM_RATIO    0   /* ARMCLK = DOUTAPLL / (ARM_RATIO + 1)    */
#define HCLKX2_RATIO 1   /* HCLKX2 = HCLKX2IN / (HCLKX2_RATIO + 1) */
#define HCLK_RATIO   1   /* HCLK = HCLKX2 / (HCLK_RATIO + 1)       */
#define PCLK_RATIO   3   /* PCLK   = HCLKX2 / (PCLK_RATIO + 1)     */
#define MPLL_RATIO   0   /* DOUTMPLL = MOUTMPLL / (MPLL_RATIO + 1) */ldr r0, =0x7E00F020  /* CLK_DIV0 */ldr r1, =(ARM_RATIO) | (MPLL_RATIO << 4) | (HCLK_RATIO << 8) | (HCLKX2_RATIO << 9) | (PCLK_RATIO << 12)str r1, [r0]#define APLL_CON_VAL  ((1<<31) | (266 << 16) | (3 << 8) | (1))ldr r0, =0x7E00F00Cldr r1, =APLL_CON_VALstr r1, [r0]/** APLL_CON, FOUTAPL = MDIV * Fin / (PDIV*2^SDIV) = 266*12/(3*2^1) = 532MHz*/#define MPLL_CON_VAL  ((1<<31) | (266 << 16) | (3 << 8) | (1))ldr r0, =0x7E00F010ldr r1, =MPLL_CON_VALstr r1, [r0]/** MPLL_CON, FOUTMPL = MDIV * Fin / (PDIV*2^SDIV) = 266*12/(3*2^1) = 532MHz */ldr r0, =0x7E00F01Cldr r1, =0x03str r1, [r0]mov pc, lr 

clock.S：时钟初始化。

all:start.o clock.o led.oarm-linux-ld -Ttext 0 -o led.elf start.o clock.o led.oarm-linux-objcopy -O binary led.elf led.binarm-linux-objdump -D led.elf > led.discp led.bin /tftpboot/led.bin
%.o:%.Sarm-linux-gcc -c $< -o $@
%.o:%.carm-linux-gcc -c -o $@ $<
clean:rm -rf *.o *.elf *.dis *.bin /tftpboot/*.bin

Makefile:在跑马灯的基础上添加clock.o

void delay ()
{int i;for (i = 0; i < 100000; i++);
}int xxxx (void)
{volatile unsigned long *gpmcon = (volatile unsigned long *)0x7f008820;        volatile unsigned long *gpmdat = (volatile unsigned long *)0x7f008824;int i;for (i = 1; i <= 4; i++){if (1 == i){*gpmcon &= ~0xffff;*gpmcon |= 0x1;}else if (2 == i){*gpmcon &= ~0xffff;*gpmcon |= 0x10;}else if (3 == i){*gpmcon &= ~0xffff;*gpmcon |= 0x100;}else if (4 == i){*gpmcon &= ~0xffff;*gpmcon |= 0x1000;i = 0;}*gpmdat = 0;delay();}return 0;
}

led.c：同上一章一样。

结果：初始化时钟后，跑马灯速度明显快于不初始化时钟的跑马灯程序。