以arm为例，分析一下kernel的启动过程;
内核版本：linux-3.2.tar.gz
一、arch/arm/kernel/head.s
/** Kernel startup entry point.
* ---------------------------
*
* This is normally called from the decompressor code. The requirements
* are: MMU = off, D-cache = off, I-cache = dont care, r0 = 0,
* r1 = machine nr, r2 = atags or dtb pointer.
*
* This code is mostly position independent, so if you link the kernel at
* 0xc0008000, you call this at __pa(0xc0008000).
*
* See linux/arch/arm/tools/mach-types for the complete list of machine
* numbers for r1.
*
* We're trying to keep crap to a minimum; DO NOT add any machine specific
* crap here - that's what the boot loader (or in extreme, well justified
* circumstances, zImage) is for.
*/
.arm
__HEAD @#define __HEAD .section ".head.text","ax"
ENTRY(stext)
THUMB( adr r9, BSYM(1f) ) @ Kernel is always entered in ARM.
THUMB( bx r9 ) @ If this is a Thumb-2 kernel,
THUMB( .thumb ) @ switch to Thumb now.
THUMB(1: )
setmode PSR_F_BIT | PSR_I_BIT | SVC_MODE, r9 @ ensure svc mode 关闭普通中断，快速中断，使能svc模式
@ and irqs disabled
mrc p15, 0, r9, c0, c0 @ get processor id 获得芯片ID
bl __lookup_processor_type @ r5=procinfo r9=cpuid 获得处理器型号，r5 == id，#1
movs r10, r5 @ invalid processor (r5=0)? 校验正确性，0错误
THUMB( it eq ) @ force fixup-able long branch encoding
beq __error_p @ yes, error 'p'
#ifndef CONFIG_XIP_KERNEL
adr r3, 2f
ldmia r3, {r4, r8}
sub r4, r3, r4 @ (PHYS_OFFSET - PAGE_OFFSET)
add r8, r8, r4 @ PHYS_OFFSET
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#1 :arch/arm/kernel/head-common.h
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/*
* Read processor ID register (CP#15, CR0), and look up in the linker-built
* supported processor list. Note that we can't use the absolute addresses
* for the __proc_info lists since we aren't running with the MMU on
* (and therefore, we are not in the correct address space). We have to
* calculate the offset.
*
* r9 = cpuid
* Returns:
* r3, r4, r6 corrupted
* r5 = proc_info pointer in physical address space
* r9 = cpuid (preserved)
*/
__CPUINIT
__lookup_processor_type:
adr r3, __lookup_processor_type_data @adr 相对偏移读取，读取下面type_data地址
ldmia r3, {r4 - r6} @将该地址存放的值 放入r4(.)，r5(begin)，r6(end)
sub r3, r3, r4 @ get offset between virt&phys 链接地址-实际地址=偏移量
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继续arch/arm/kernel/head.s
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/*
* r1 = machine no, r2 = atags or dtb,
* r8 = phys_offset, r9 = cpuid, r10 = procinfo
*/
bl __vet_atags @#1,head-common.s
#ifdef CONFIG_SMP_ON_UP
bl __fixup_smp @略
#endif
#ifdef CONFIG_ARM_PATCH_PHYS_VIRT
bl __fixup_pv_table @略
#endif
bl __create_page_tables @#2
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#1 :arch/arm/kernel/head-common.h
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