GPIO控制（点亮LED）

LED灯GPIO连接示意图：

    ---------
    |GPM0   |--LED1
    ---------
    |GPM1   |--LED2
    ---------
    |GPM2   |--LED3
    ---------
    |GPM3   |--LED4
    ---------
    active low

相关寄存器：
  Register  Address     R/W     Description                     Reset Value
  GPMCON    0x7F008820  R/W     Port M Configuration Register   0x00222222
  GPMDAT    0x7F008824  R/W     Port M Data Register            Undefined

   GPMCON  Bit    Description                                  Initial State
   GPM0    [3:0]  0000 = Input           0001 = Output                 0010
                  0010 = Host I/F CSn    0011 = Ext. Interrupt[23]
                  0100 = Reserved        0101 = Reserved
                  0110 = CE_CF[0]        0111 = Reserved

点亮4盏灯的代码如下：

#define GPMCON    0x7F008820
#define GPMDAT    0x7F008824

    .text   
    .align  2
    .global _start
_start:
    mov ip, sp
    stmfd   sp!, {fp, ip, lr, pc}
    sub fp, ip, #4 
    @设为输入
    ldr r0, =GPMCON
    ldr r1, [r0]
    ldr r2, =0xffff
    bic r1, r1, r2
    ldr r2, =0x1111
    orr r1, r1, r2
    str r1, [r0]
    @LED全亮
    ldr r0, =GPMDAT
    ldr r1, [r0]
    ldr r2, =0xf
    bic r1, r1, r2
    str r1, [r0]
    
    sub sp, fp, #12
    ldmfd   sp, {fp, sp, lr}
    mov pc, lr

Makefile文件：

all:
    arm-linux-gcc -c start.S
    arm-linux-ld start.o -o start
    arm-linux-objcopy -O binary start start.bin
    mv start.bin /tftpboot