基于ARM 的Linux 的启动分析报告——ARM+Linux的启动分析（4）

.long _end @ r7
.long processor_id @ r4
.long __machine_arch_type @ r5
.long __atags_pointer @ r6
.long cr_alignment @ r7
.long init_thread_union + THREAD_START_SP @ sp
/*
* The following fragment of code is executed with the MMU on in MMU
mode,
* and uses absolute addresses; this is not position independent.
* r0 = cp#15 control register
* r1 = machine ID
* r2 = atags pointer
* r9 = processor ID
*/
.type __mmap_switched, %function
__mmap_switched:
//把sp指针指向init_task_union+8192（include/linux/sched.h）处，即第
//一个进程的task_struct和系统堆栈的地址；清空BSS段；保存processor
ID
//和machine type到全局变量processor_id和__machine_arch_type，这些值
//以后要用到；r0为"A"置位的control register 值，r2为"A"清空的
//control register 值，即对齐检查（Alignment fault checking）位，并保
//存到cr_alignment，和cr_no_alignment（在文件entry-armv.S中）。最
//后跳转到start_kernel（init/main.c）
adr r3, __switch_data + 4
ldmia r3!, {r4, r5, r6, r7} @ r2 = compat//r2＝0xc0000000
cmp r4, r5 @ Copy data segment if needed //r4＝0xc00c04e0；
__bss_start
1: cmpne r5, r6 //r5＝0xc00e02a8；_end //r6＝0xc00c0934；
processor_id
ldrne fp, [r4], #4 //r7＝0xc00c0930；__machine_arch_type
strne fp, [r5], #4 //r8＝0xc00bcb88；cr_alignment
bne 1b //sp＝0xc00bc000;(init_task_union)+8192
mov fp, #0 @ Clear BSS (and zero fp)
1: cmp r6, r7
strcc fp, [r6],#4
bcc 1b
ldmia r3, {r4, r5, r6, r7, sp}
str r9, [r4] @ Save processor ID
str r1, [r5] @ Save machine type
str r2, [r6] @ Save atags pointer
bic r4, r0, #CR_A @ Clear 'A' bit
stmia r7, {r0, r4} @ Save control register values
b start_kernel //下面就开始真正的内核了：）
/*
* Enable the MMU. This completely changes the structure of the
visible
* memory space. You will not be able to trace execution through
this.
* If you have an enquiry about this, *please* check the linux-armkernel
* mailing list archives BEFORE sending another post to the list.
*
* r0 = cp#15 control register
* r13 = *virtual* address to jump to upon completion
*
* other registers depend on the function called upon completion
*/
.align 5
.type __turn_mmu_on, %function
__turn_mmu_on:
mov r0, r0
mcr p15, 0, r0, c1, c0, 0 @ write control reg
mrc p15, 0, r3, c0, c0, 0 @ read id reg
mov r3, r3
mov r3, r3
mov pc, r13
/*
* 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.
*
* r8 = machinfo
* r9 = cpuid
* r10 = procinfo
*
* Returns:
* r0, r3, r6, r7 corrupted
* r4 = physical page table address
*/
.type __create_page_tables, %function
__create_page_tables:
pgtbl r4 @ page table address