请教，关于OK6410 NAND Flash 裸机调试

#define MEM_SYS_CFG     (*((volatile unsigned long *)0x7E00F120))

#define NFCONF          (*((volatile unsigned long *)0x70200000))

#define NFCONT          (*((volatile unsigned long *)0x70200004))

#define NFCMMD          (*((volatile unsigned long *)0x70200008))

#define NFADDR          (*((volatile unsigned long *)0x7020000C))

#define NFDATA          (*((volatile unsigned char *)0x70200010))

#define NFSTAT          (*((volatile unsigned long *)0x70200028))



void nand_select(void)

{

    NFCONT &= ~(1<<1);

}



void nand_deselect(void)

{

    NFCONT |= (1<<1);

}





void nand_cmd(unsigned char cmd)

{

    NFCMMD = cmd;

}



void nand_addr(unsigned char addr)

{

    NFADDR = addr;

}



unsigned char nand_get_data(void)

{

    return NFDATA;

}



void nand_send_data(unsigned char data)

{

    NFDATA = data;

}



void wait_ready(void)

{

    while ((NFSTAT & 0x1) == 0);

}



void nand_reset(void)

{

    /* 选中 */

    nand_select();

   

    /* 发出0xff命令 */

    nand_cmd(0xff);



    /* 等待就绪 */

    wait_ready();

   

    /* 取消选中 */

    nand_deselect();

}



void nand_init(void)

{

    /* 让xm0csn2用作nand flash cs0 片选引脚 */

    MEM_SYS_CFG &= ~(1<<1);



    /* 设置时间参数 */

#define TACLS     0

#define TWRPH0    1

#define TWRPH1    0

    NFCONF &= ~((1<<30) | (7<<12) | (7<<8) | (7<<4));

    NFCONF |= ((TACLS<<12) | (TWRPH0<<8) | (TWRPH1<<4));



    /* 使能nand flash controller */

    NFCONT |= 1;

    NFCONT &= ~(1<<16); /* 森止soft lock */



    nand_reset();

}



void nand_send_addr(unsigned int addr)

{

#if 0   

    unsigned int page = addr / 2048;



    /* 这两个地址表示从页内哪里开始 */

    nand_addr(addr & 0xff);

    nand_addr((addr >> 8) & 0xff);



    /* 下面三个地址表示哪一页 */

    nand_addr(page & 0xff);

    nand_addr((page >> 8) & 0xff);

    nand_addr((page >> 16) & 0xff);

#else

    nand_addr(addr & 0xff);         /* a0~a7 */

    nand_addr((addr >> 8) & 0x1f);   /* 程序的角度: a8~a12 */



    nand_addr((addr >> 13) & 0xff); /* 程序的角度: a13~a20 */

    nand_addr((addr >> 21) & 0xff); /* 程序的角度: a21~a28 */

    nand_addr((addr >> 29) & 0x7); /* 程序的角度: a29~    */

   

#endif

}



int nand_read(unsigned int nand_start, unsigned int ddr_start, unsigned int len)

{

    unsigned int addr = nand_start;

    int i = nand_start % 4096;//nand_start % 2048;

    int count = 0;

    unsigned char *dest = (unsigned char *)ddr_start;

   

    /* 选中芯片 */

    nand_select();



    while (count < len)

    {

        /* 发出命令0x00 */

        nand_cmd(0x00);



        /* 发出地址 */

        nand_send_addr(addr);



        /* 发出命令0x30 */

        nand_cmd(0x30);



        /* 等待就绪 */

        wait_ready();



        /* 读数据 */

        for (; i < 4096 && count < len; i++)

        {

            dest[count++] = nand_get_data();

            addr++;            

        }



        i = 0;        

    }



    /* 取消片选 */

    nand_deselect();

    return 0;

}



int copy2ddr(unsigned int nand_start, unsigned int ddr_start, unsigned int len)

{

    int ret;

   

    /* 初始化nand flash controller */

    nand_init();

   

    /* 读nand flash */

    ret = nand_read(nand_start, ddr_start, len);

   

    return ret;

}

.globl _start

_start:



/* 硬件相关的设置 */

    /* Peri port setup */

    ldr r0, =0x70000000

    orr r0, r0, #0x13

    mcr p15,0,r0,c15,c2,4       @ 256M(0x70000000-0x7fffffff)

   

/* 关看门狗 */

/* 往WTCON(0x7E004000)写0 */

   

    ldr r0, =0x7E004000

    mov r1, #0

    str r1, [r0]

   

    /* 设置栈 */

    ldr sp, =8*1024



    /* 设置时钟 */

    /*bl clock_init*/



    /* 设置DDR */

    bl ddr_init

   

    /* 重定位 */

/* 把程序的代码段、数据段复制到它的链接地址去 */   

    adr r0, _start   /* 获得_start指令当前所在的地址 : 0*/

    ldr r1, =_start  /* _start的链接地址 0x51000000 */

   

    ldr r2, =bss_start      /* bss段的起始链接地址 */

   

    sub r2, r2, r1

   

    cmp r0,r1

    beq clean_bss

   

    bl copy2ddr

    cmp r0, #0

    bne halt

        

    /* 清BSS */

/* 把BSS段对应的内存清零 */

clean_bss:

    ldr r0, =bss_start

    ldr r1, =bss_end

    mov r3, #0

    cmp r0, r1

    beq on_ddr

clean_loop:

    str r3, [r0], #4

    cmp r0, r1   

    bne clean_loop        



on_ddr:

    ldr pc, =main

halt:

    b halt    

SECTIONS {

    . = 0x51000000;

    .text : {

            start.o

            sdram.o

            nand.o

            * (.text)

    }



    .rodata : {

            * (.rodata)

    }



.data : {

            * (.data)

    }



    bss_start = .;

    .bss ALIGN(4)  : { *(.bss)  *(COMMON) }



   bss_end = .;

}