arch\arm\kernel\head.S
该文件末尾包含了语句：#include "head-common.S"，__lookup_processor_type函数在arch\arm\kernel\head-common.S中定义。
关键数据结构：struct proc_info_list，在arch\arm\include\asm\procinfo.h文件中定义，Cortex对此数据结构的填充在arch\arm\mm\proc-v7.s文件中定义。
head.s中，kernel的关键入口如下：
ENTRY(stext)
ARM( msr cpsr_c, #PSR_F_BIT | PSR_I_BIT | SVC_MODE ) @ ensure svc mode
THUMB( mov r9, #PSR_F_BIT | PSR_I_BIT | SVC_MODE )
THUMB( msr cpsr_c, r9 ) @ ensure svc mode
@ and irqs disabled
mrc p15, 0, r9, c0, c0 @ get processor id
bl __lookup_processor_type @ r5=procinfo r9=cpuid
movs r10, r5 @ invalid processor (r5=0)?
beq __error_p @ yes, error 'p'
bl __lookup_machine_type @ r5=machinfo
movs r8, r5 @ invalid machine (r5=0)?
beq __error_a @ yes, error 'a'
bl __vet_atags
bl __create_page_tables
/*
* The following calls CPU specific code in a position independent
* manner. See arch/arm/mm/proc-*.S for details. r10 = base of
* xxx_proc_info structure selected by __lookup_machine_type
* above. On return, the CPU will be ready for the MMU to be
* turned on, and r0 will hold the CPU control register value.
*/
ldr r13, __switch_data @ address to jump to after
@ mmu has been enabled
badr lr, __enable_mmu @ return (PIC) address
ARM( add pc, r10, #PROCINFO_INITFUNC )
THUMB( add r12, r10, #PROCINFO_INITFUNC )
THUMB( mov pc, r12 )
ENDPROC(stext)
语句“add pc, r10, #PROCINFO_INITFUNC”通过查表调用proc-v7.s中__v7_setup函数，该函数末尾通过将lr寄存器赋给pc，导致对__enable_mmu的调用，完成使能mmu的操作，之后将r13寄存器值赋给pc，调用__switch_data数据结构中的第一个函数__mmap_switched，该函数指向最后的初始化函数，定义在head-common.s文件中，该函数最终调用init\main.c文件中的start_kernel函数。
语句“mrc p15, 0, r9, c0, c0”将协处理器寄存器CP15读入寄存器r9，此寄存器保存CPUID；之后调用函数__lookup_processor_type查找
