linux启动分析(4)---汇编部分(4)

函数__create_page_tables介绍：

假设内核起始物理地址是0xA0008000，虚拟地址是0xC0008000，下面的代码是建立内核起始处4MB空间的映射，
采用了一级映射方式，即段式(section)映射方式，每段映射范围为1MB空间。于是需要建立4个表项，实现：
虚拟地址0xC0000000~0xC0300000，映射到物理地址0xA0000000~0xA0300000。

     .macro  pgtbl, reg, rambase
     adr \reg, stext
     sub \reg, \reg, #0x4000    
     .endm
    
     .macro  krnladr, rd, pgtable, rambase
     bic \rd, \pgtable, #0x000ff000
     .endm
    
/*
 * Setup the initial page tables.  We only setup the barest
 * amount which are required to get the kernel running, which
 * generally means mapping in the kernel code.
 *     
 * We only map in 4MB of RAM, which should be sufficient in
 * all cases.
 *     
 * r5 = physical address of start of RAM
 * r6 = physical IO address
 * r7 = byte offset into page tables for IO
 * r8 = page table flags           
*/    
1 __create_page_tables:
/* r5中存放着内核启动的地址0xa0008000 */
/* pgtbl将启动地址减去0x4000，存放到r4=0xa0004000 */
2         pgtbl   r4, r5              @ page table address
        
/*
 * Clear the 16K level 1 swapper page table
 */
/* r0 = 0xa0004000 */
3         mov r0, r4
4         mov r3, #0         
/* r2 = 0xa0008000 */
5         add r2, r0, #0x4000
/* 清除16k空间，addr 0xa0004000: 0xa0008000 is page table, total 16K*/
6 1:      str r3, [r0], #4
7         str r3, [r0], #4
8         str r3, [r0], #4
9         str r3, [r0], #4
10        teq r0, r2
11        bne 1b 
      
/*
 * Create identity mapping for first MB of kernel to
 * cater for the MMU enable.  This identity mapping
 * will be removed by paging_init()
 */
/* r2 = 0xa0040000 & 0x000ff000 = 0xa00000000 */
12        krnladr r2, r4, r5          @ start of kernel
/* r3 = 0xa0000000 + 0x00000c0e = 0xa00000c0e */
/* r8 = 0x00000c0e在__lookup_processor_type函数中初始化 */
13       add r3, r8, r2          @ flags + kernel base
/* value r3=0xa0000c0e store to addr 0xa0006800*/
/* r4 = 0xa0006800 */
14        str r3, [r4, r2, lsr #18]       @ identity mapping   
/*
 * Now setup the pagetables for our kernel direct
 * mapped region.  We round TEXTADDR down to the
 * nearest megabyte boundary.
 */
/* TEXTADDR= 0xC0008000 有关TEXTADDR参考<> */
/* start of kernel, r0=0xa0007000 */
15        add r0, r4, #(TEXTADDR & 0xff000000) >> 18 @ start of kernel
/* r2=0xa0000c0e */
16        bic r2, r3, #0x00f00000
/* 0xa0000c0e的数据写入到0xa00070000 */
17        str r2, [r0]            @ PAGE_OFFSET + 0MB
/* r0=0xa0007000, no change */
18        add r0, r0, #(TEXTADDR & 0x00f00000) >> 18
       
19        str r3, [r0], #4            @ KERNEL + 0MB
20        add r3, r3, #1 << 20       
21        str r3, [r0], #4            @ KERNEL + 1MB
22        add r3, r3, #1 << 20       
23        str r3, [r0], #4            @ KERNEL + 2MB
24        add r3, r3, #1 << 20       
25        str r3, [r0], #4            @ KERNEL + 3MB
/*
 * Ensure that the first section of RAM is present.
 * we assume that:
 *  1. the RAM is aligned to a 32MB boundary
 *  2. the kernel is executing in the same 32MB chunk
 *     as the start of RAM.
 */   
26        bic r0, r0, #0x01f00000 >> 18   @ round down
27        and r2, r5, #0xfe000000     @ round down
28        add r3, r8, r2          @ flags + rambase
29        str r3, [r0]
   
30        bic r8, r8, #0x0c           @ turn off cacheable
   
31        mov pc, lr
       
 我已经把每一步涉及的地址详细列出了，读者可以自行对照阅读。第11～16行，清空页表项从0xA0004000到0xA00,8000，共16KB。
 第28行，取得__cpu_mmu_flags。第35～45行，填写页表项，共4项。读者可以对照XScale的地址映射手册，
 因为采用的是段式映射方式，所以每1MB虚拟空间映射到相同的页表表项，根据手册说明，段式映射只有一级表索引，
 是虚拟地址的前12位；而页式映射的页目录表是前12位，页表是接着的8位，最后12位才是页内偏移，
 读者一定不要和386的10位页目录表，10位页表的机制相混淆。我们举个例子说明，对于虚拟地址0xC00x,xxxxx，
 其前12位为C00，页表基址为0xA000,4000，所以表项地址为0xA000,4000+0xC00<<2=0xA000,7000，
 而这个地址内容为0xA0000C0E，其前12位0xA00为段基地址，后20位为一些flags，这是从刚才__bva0_proc_info中取得的。