内存管理单元MMU 地址映射一

这个实例将开启MMU，并将虚拟地址0xA0000000-0xA0100000映射到物理地址0x56000000-0x56100000（GPBCON物理地址为0x56000010，GPBDAT物理地址为0x56000014），来驱动LED。

将虚拟地址0xB0000000-0xB3FFFFFF映射到物理地址0x30000000-0x33FFFFFF，在连接程序时，将一部分代码的运行地址指定为0xB0004000.

这个程序只使用一级页表，以段的方式进行地址映射，32位CPU虚拟地址空间达到4G，一级页表使用4096个描述符来表示4G空间（每个描述符对应1MB），每个描述符占4字节，所以一级页表占16KB。这个程序使用SDRAM的开始16KB存放一级页表，所以剩下的内存开始地址就为0x30004000，这个地址最终会对应虚拟地址0xB0004000（所以代码运行地址为0xB0004000）

程序分为两部分：第一部分的运行地址为0，它用来初始化SDRAM，复制第二部分的代码到SDRAM中（存放在0x30004000）、设置页表、启动MMU，最后跳到SDRAM中（地址0xB0004000），第二部分运行地址设为0xB0004000，用来驱动LED

先看连接文件mmu.lds

SECTIONS { 

  firtst    0x00000000 : { head.o init.o }

  second    0xB0004000 : AT(2048) { leds.o }

} 

程序分两个段：first和second。first由head.o和init.o组成，加载和运行地址都是0，second由leds.o组成，加载地址为2048，重定位地址为0xB0004000。

@*************************************************************************

@ File：head.S

@ 功能：设置SDRAM，将第二部分代码复制到SDRAM，设置页表，启动MMU，

@       然后跳到SDRAM继续执行

@*************************************************************************       

.text

.global _start

_start:

    ldr sp, =4096                       @ 设置栈指针，以下都是C函数，调用前需要设好栈

    bl  disable_watch_dog               @ 关闭WATCHDOG，否则CPU会不断重启

    bl  memsetup                        @ 设置存储控制器以使用SDRAM

    bl  copy_2th_to_sdram               @ 将第二部分代码复制到SDRAM

    bl  create_page_table               @ 设置页表

    bl  mmu_init                        @ 启动MMU，启动以后下面代码都用虚拟地址

    ldr sp, =0xB4000000                 @ 重设栈指针，指向SDRAM顶端(使用虚拟地址)

    ldr pc, =0xB0004000                 @ 跳到SDRAM中继续执行第二部分代码

halt_loop:

    b   halt_loop

#define WTCON           (*(volatile unsigned long *)0x53000000)

#define MEM_CTL_BASE    0x48000000

void disable_watch_dog(void)

{

    WTCON = 0;  // 关闭WATCHDOG很简单，往这个寄存器写0即可

}

void memsetup(void)

{

    unsigned long  const    mem_cfg_val[]={ 0x22011110,     //BWSCON

                                            0x00000700,     //BANKCON0

                                            0x00000700,     //BANKCON1

                                            0x00000700,     //BANKCON2

                                            0x00000700,     //BANKCON3  

                                            0x00000700,     //BANKCON4

                                            0x00000700,     //BANKCON5

                                            0x00018005,     //BANKCON6

                                            0x00018005,     //BANKCON7

                                            0x008C07A3,     //REFRESH

                                            0x000000B1,     //BANKSIZE

                                            0x00000030,     //MRSRB6

                                            0x00000030,     //MRSRB7

                                    };

    int     i = 0;

    volatile unsigned long *p = (volatile unsigned long *)MEM_CTL_BASE;

    for(; i < 13; i++)

        p[i] = mem_cfg_val[i];        //循环复制13个寄存器到内存控制器基址

}

void copy_2th_to_sdram(void)

{

    unsigned int *pdwSrc  = (unsigned int *)2048;        //第二段代码加载地址2048

    unsigned int *pdwDest = (unsigned int *)0x30004000;        //0x30004000前放页表

    while (pdwSrc < (unsigned int *)4096) //4kb最大4096

    {

        *pdwDest = *pdwSrc;

        pdwDest++;

        pdwSrc++;

    }

}

void create_page_table(void)

{

#define MMU_FULL_ACCESS     (3 << 10)   

#define MMU_DOMAIN          (0 << 5)    

#define MMU_SPECIAL         (1 << 4)    

#define MMU_CACHEABLE       (1 << 3)    

#define MMU_BUFFERABLE      (1 << 2)    

#define MMU_SECTION         (2)         

#define MMU_SECDESC         (MMU_FULL_ACCESS | MMU_DOMAIN | MMU_SPECIAL |

                             MMU_SECTION)

#define MMU_SECDESC_WB      (MMU_FULL_ACCESS | MMU_DOMAIN | MMU_SPECIAL |

                             MMU_CACHEABLE | MMU_BUFFERABLE | MMU_SECTION)

#define MMU_SECTION_SIZE    0x00100000        

    unsigned long virtuladdr, physicaladdr;

    unsigned long *mmu_tlb_base = (unsigned long *)0x30000000;        

    virtuladdr = 0;

    physicaladdr = 0;

     //虚拟地址[31:20]用于索引一级页表，找到它对应的描述符，对应于(virtualaddr>>20)

     //段描述符中[31:20]保存段的物理地址，对应(physicaladdr & 0xFFF00000)

    *(mmu_tlb_base + (virtuladdr >> 20)) = (physicaladdr & 0xFFF00000) |

                                            MMU_SECDESC_WB;

    [page]

    virtuladdr = 0xA0000000;

    physicaladdr = 0x56000000;

    *(mmu_tlb_base + (virtuladdr >> 20)) = (physicaladdr & 0xFFF00000) |

                                            MMU_SECDESC;

    virtuladdr = 0xB0000000;

    physicaladdr = 0x30000000;

    while (virtuladdr < 0xB4000000)

    {

        *(mmu_tlb_base + (virtuladdr >> 20)) = (physicaladdr & 0xFFF00000) |

                                                MMU_SECDESC_WB;

        virtuladdr += 0x100000;        //右移20位就是1

        physicaladdr += 0x100000;        //右移20位就是1

    }

}

void mmu_init(void)

{

    unsigned long ttb = 0x30000000;

__asm__(

    "mov    r0, #0 "

    "mcr    p15, 0, r0, c7, c7, 0 "    

    "mcr    p15, 0, r0, c7, c10, 4 "   

    "mcr    p15, 0, r0, c8, c7, 0 "    

    "mov    r4, %0 "                   

    "mcr    p15, 0, r4, c2, c0, 0 "    

    "mvn    r0, #0 "                   

    "mcr    p15, 0, r0, c3, c0, 0 "        

    "mrc    p15, 0, r0, c1, c0, 0 "    

    "bic    r0, r0, #0x3000 "          

    "bic    r0, r0, #0x0300 "          

    "bic    r0, r0, #0x0087 "          

    "orr    r0, r0, #0x0002 "          

    "orr    r0, r0, #0x0004 "          

    "orr    r0, r0, #0x1000 "          

    "orr    r0, r0, #0x0001 "          

    "mcr    p15, 0, r0, c1, c0, 0 "    

    :

    : "r" (ttb) );

}

#define GPBCON      (*(volatile unsigned long *)0xA0000010)     // 物理地址0x56000010

#define GPBDAT      (*(volatile unsigned long *)0xA0000014)     // 物理地址0x56000014

#define GPB5_out    (1<<(5*2))

#define GPB6_out    (1<<(6*2))

#define GPB7_out    (1<<(7*2))

#define GPB8_out    (1<<(8*2))

static inline void wait(unsigned long dly)

{

    for(; dly > 0; dly--);

}

int main(void)

{

    unsigned long i = 0;

    // 将LED1-4对应的GPB5/6/7/8四个引脚设为输出

    GPBCON = GPB5_out|GPB6_out|GPB7_out|GPB8_out;       

    while(1){

        wait(30000);

        GPBDAT = (~(i<<5));     // 根据i的值，点亮LED1-4

        if(++i == 16)

            i = 0;

    }

    return 0;

}

最后是Makefile

objs := head.o init.o leds.o

mmu.bin : $(objs)

arm-linux-ld -Tmmu.lds -o mmu_elf $^

arm-linux-objcopy -O binary -S mmu_elf $@

arm-linux-objdump -D -m arm mmu_elf > mmu.dis

%.o:%.c

arm-linux-gcc -Wall -O2 -c -o $@ $<

%.o:%.S

arm-linux-gcc -Wall -O2 -c -o $@ $<

clean:

rm -f mmu.bin mmu_elf mmu.dis *.o