================================

Author: taoyuetao
Email: tao_yuetao@yahoo.com.cn
Blog: http://www.eetop.cn/blog/?11145

07-04-18

================================

 函数__mmap_switched介绍：
     
/*
 * The following fragment of code is executed with the MMU on, and uses
 * absolute addresses; this is not position independent.
 *
 *  r0  = processor control register
 *  r1  = machine ID
 *  r9  = processor ID
*/

/* 下面按4字节对齐 */
1      .align  5
2 __mmap_switched:
/* r3 = __bss_start */
3     adr r3, __switch_data + 4
4       ldmia   r3, {r4, r5, r6, r7, r8, sp}@ r2 = compat
                             @ sp = stack pointer
5       mov fp, #0              @ Clear BSS (and zero fp)
6 1:    cmp r4, r5
7       strcc   fp, [r4],#4
8       bcc 1b

9       str r9, [r6]            @ Save processor ID
10      str r1, [r7]            @ Save machine type
11      orr r0, r0, #2          @ ...........A.
12      bic r2, r0, #2          @ Clear 'A' bit
13      stmia   r8, {r0, r2}            @ Save control register values
14      b   SYMBOL_NAME(start_kernel)

程序的4行执行完成之后的结果是r4=__bss_start，r5=_end，r6=processor_id，r7=__machine_arch_type，
r8=cr_alignment，sp=init_task_union+8192，第5-8行将__bss_start到_end清零，定义在vmlinux.lds文件中，如下：

  .bss : {                                             
        __bss_start = .;    /* BSS              */       
       *(.bss)
       *(COMMON)
  _end = . ;
  }  
 
第9、10行分别将处理器类型和机器类型存储到变量processor_id和__machine_arch_type中，这些变量以后会
在start_kernel->setup_arch中使用，来得到当前处理器的struct proc_info_list结构和当前系统的machine_desc结构的数据。
第10-13将processor control register保存到cr_alignment中，14行跳转到init/main.c中的start_kernel进入内核启动的第二阶段。