MMU

Posted 2020-08-12 LeoSanford

1.MMU功能

将虚拟地址转化为物理地址；
访问权限管理。

2.地址转化

2.1 总体分析

2.2 一级转换格式

 

位解释：（段设置）

B：表示是否使能write buffer； 

C:  表示是否开启cache；

XN（The Execute-Never ）：determines if the region is Executable (0) or Not-executable(1)。

Domain：the ARM1176JZF-S supports 16 Domains in the Secure world and 16 Domains in the Non-secure world. Domains provide support for multi-user operating systems.  
P：If the P bit is supported and set for the memory region, it indicates to the system memory controller that this memory region has ECC enabled. ARM1176JZF-S processors do not support the P bit.  （设为0）
AP：访问权限

APX：provides an extra access permission bit. 
TEX：remap bit

S:determines if the translation is for Non-Shared (0), or Shared (1)memory. 

nG：determines if the translation is marked as global (0), or process-specific (1) in the TLB. 
NS:The Non-secure (NS) bit determines if the program execution is in the Secure or Non-secure world. 

3.MMU配置与使用

3.1 建立一级页表

 1 void create_page_table(void)
 2 {
 3     unsigned long *ttb = (unsigned long *)0x50000000;
 4     unsigned long vaddr, paddr;
 5 
 6     vaddr = 0xA0000000;
 7     paddr = 0x7f000000;
 8     *(ttb + (vaddr >> 20)) = (paddr & 0xFFF00000) | MMU_SECDESC;
 9 
10     vaddr = 0x50000000;
11     paddr = 0x50000000;
12     while (vaddr < 0x54000000)
13     {
14         *(ttb + (vaddr >> 20)) = (paddr & 0xFFF00000) | MMU_SECDESC_WB;
15         vaddr += 0x100000;
16         paddr += 0x100000;
17     }
18     
19 }

create_page_table

3.2 初始化MMU

 1 void mmu_init()
 2 {
 3    __asm__(
 4     
 5     /*设置TTB*/
 6     "ldr    r0, =0x50000000\\n"                  
 7     "mcr    p15, 0, r0, c2, c0, 0\\n"    
 8     
 9     /*不进行权限检查*/
10     "mvn    r0, #0\\n"                   
11     "mcr    p15, 0, r0, c3, c0, 0\\n"    
12     
13     
14    /*使能MMU*/
15     "mrc    p15, 0, r0, c1, c0, 0\\n"    
16     "orr    r0, r0, #0x0001\\n"          
17     "mcr    p15, 0, r0, c1, c0, 0\\n"    
18     : 
19     : 
20   );
21 }

void mmu_init

3.3 主函数

#define GPKCON (volatile unsigned long*)0xA0008800
#define GPKDAT (volatile unsigned long*)0xA0008808


/* 
 * 用于段描述符的一些宏定义
 */ 
#define MMU_FULL_ACCESS     (3 << 10)   /* 访问权限 */
#define MMU_DOMAIN          (0 << 5)    /* 属于哪个域 */
#define MMU_SPECIAL         (1 << 4)    /* 必须是1 */
#define MMU_CACHEABLE       (1 << 3)    /* cacheable */
#define MMU_BUFFERABLE      (1 << 2)    /* bufferable */
#define MMU_SECTION         (2)         /* 表示这是段描述符 */
#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)


int main()
{
    create_page_table();
    mmu_init();
    
    *(GPKCON) = 0x11110000;
    *(GPKDAT) = 0xa0;
    
    return 0;    
}

mian