/*

  *  linux/arch/arm/kernel/head.S

  *

  *  Copyright (C) 1994-2002 Russell King

  *  Copyright (c) 2003 ARM Limited

  *  All Rights Reserved

  *

  * This program is free software; you can redistribute it and/or modify

  * it under the terms of the GNU General Public License version 2 as

  * published by the Free Software Foundation.

  *

  *  Kernel startup code for all 32-bit CPUs

  */

 #include <linux/linkage.h>

 #include <linux/init.h>

 #include <asm/assembler.h>

 #include <asm/domain.h>

 #include <asm/ptrace.h>

 #include <asm/asm-offsets.h>

 #include <asm/memory.h>

 #include <asm/thread_info.h>

 #include <asm/system.h>

 #if (PHYS_OFFSET & 0x001fffff)

 #error "PHYS_OFFSET must be at an even 2MiB boundary!"

 #endif

 /*

 PAGE_OFFSET            0xC0000000     页表偏移地址

 PHYS_OFFSET            0x30000000     物理内存偏移地址

 TEXT_OFFSET            0x00008000

 KERNEL_RAM_VADDR    0xC0008000     内核虚拟映射地址

 KERNEL_RAM_PADDR      0x30008000     内核物理（RAM）地址

 */

 #define KERNEL_RAM_VADDR    (PAGE_OFFSET + TEXT_OFFSET)

 #define KERNEL_RAM_PADDR    (PHYS_OFFSET + TEXT_OFFSET)

 /*

  * swapper_pg_dir is the virtual address of the initial page table.

  * We place the page tables 16K below KERNEL_RAM_VADDR.  Therefore, we must

  * make sure that KERNEL_RAM_VADDR is correctly set.  Currently, we expect

  * the least significant 16 bits to be 0x8000, but we could probably

  * relax this restriction to KERNEL_RAM_VADDR >= PAGE_OFFSET + 0x4000.

  */

  /*

  swapper_pg_dir是初始化页表的虚拟地址

  我们把页表放在KERNEL_RAM_VADDR下面,因此我们必须保证KERNEL_RAM_VADDR设置得正确。

  当前我们要求最低有效的16位数值位0x8000,

  但我们可能放宽此限制为KERNEL_RAM_VADDR >= PAGE_OFFSET + 0x4000

  */

 #if (KERNEL_RAM_VADDR & 0xffff) != 0x8000

 #error KERNEL_RAM_VADDR must start at 0xXXXX8000

 #endif

 /*

     .equ 虽然数据段主要用于定义变量数据，但是也可以在这里声明静态数据符号。

     .equ 命令用于把常量值设置为可以在文本段中使用的符号

 */

     .globl    swapper_pg_dir

     .equ    swapper_pg_dir, KERNEL_RAM_VADDR - 0x4000

 //定义rgtbl 宏 功能是将KERNEL_RAM_PADDR - 0x4000复制给rd寄存器

     .macro    pgtbl, rd

     ldr    \rd, =(KERNEL_RAM_PADDR - 0x4000)

     .endm

 /*CONFIG_XIP_KERNEL未设置

 *#define KERNEL_START    KERNEL_RAM_VADDR

 *#define KERNEL_END        _end

 */

 //XIP片上执行

 #ifdef CONFIG_XIP_KERNEL

 #define KERNEL_START    XIP_VIRT_ADDR(CONFIG_XIP_PHYS_ADDR)

 #define KERNEL_END    _edata_loc

 #else

 #define KERNEL_START    KERNEL_RAM_VADDR

 #define KERNEL_END    _end

 #endif

 /*

  * 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,

  * 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.

  */

  /**

  内核运行入口

  MCR指令将ARM处理器的寄存器中的数据传送到协处理器的寄存器中。如果协处理器不能成功地执行该操作，将产生未定义的指令异常中断。

  MRC指令将协处理器的寄存器中数值传送到ARM处理器的寄存器中。如果协处理器不能成功地执行该操作，将产生未定义的指令异常中断。

  **/

     .section ".text.head", "ax"

 ENTRY(stext)

     setmode    PSR_F_BIT | PSR_I_BIT | SVC_MODE, r9 @ ensure svc mode

                         @ and irqs disabled

     mrc    p15, , r9, c0, c0        @ get processor id            //获取处理器ID            r9 = cpuid

     bl    __lookup_processor_type        @ r5=procinfo r9=cpuid     //处理器类型是否支持

     beq    __error_p            @ yes, error 'p'

     bl    __lookup_machine_type        @ r5=machinfo            //机器（板子）类型是否支持

     movs    r8, r5                @ invalid machine (r5=)?                            r8 = machinfo

     beq    __error_a            @ yes, error 'a'

     bl    __vet_atags                //确定r2 atags指针的有效性   无参数？？？

     //r8 = machinfo

     //r9 = cpuid

     //r10 = procinfo

     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      r13 = __mmap_switched(虚拟地址)

                         @ mmu has been enabled

     adr    lr, BSYM(__enable_mmu)        @ return (PIC) address

  ARM(    add    pc, r10, #PROCINFO_INITFUNC    )

  THUMB(    add    r12, r10, #PROCINFO_INITFUNC    )

  THUMB(    mov    pc, r12                )

 ENDPROC(stext)

 #if defined(CONFIG_SMP)

 ENTRY(secondary_startup)

     /*

      * Common entry point for secondary CPUs.

      *

      * Ensure that we're in SVC mode, and IRQs are disabled.  Lookup

      * the processor type - there is no need to check the machine type

      * as it has already been validated by the primary processor.

      */

     setmode    PSR_F_BIT | PSR_I_BIT | SVC_MODE, r9

     mrc    p15, , r9, c0, c0        @ get processor id

     bl    __lookup_processor_type

     movs    r10, r5                @ invalid processor?

     moveq    r0, #'p'            @ yes, error 'p'

     beq    __error

     /*

      * Use the page tables supplied from  __cpu_up.

      */

     adr    r4, __secondary_data

     ldmia    r4, {r5, r7, r12}        @ address to jump to after

     sub    r4, r4, r5            @ mmu has been enabled

     ldr    r4, [r7, r4]            @ get secondary_data.pgdir

     adr    lr, BSYM(__enable_mmu)        @ return address

     mov    r13, r12            @ __secondary_switched address

  ARM(    add    pc, r10, #PROCINFO_INITFUNC    ) @ initialise processor

                           @ (return control reg)

  THUMB(    add    r12, r10, #PROCINFO_INITFUNC    )

  THUMB(    mov    pc, r12                )

 ENDPROC(secondary_startup)

     /*

      * r6  = &secondary_data

      */

 ENTRY(__secondary_switched)

     ldr    sp, [r7, #]            @ get secondary_data.stack

     mov    fp, #

     b    secondary_start_kernel

 ENDPROC(__secondary_switched)

     .type    __secondary_data, %object

 __secondary_data:

     .long    .

     .long    secondary_data

     .long    __secondary_switched

 #endif /* defined(CONFIG_SMP) */

 /*

  * Setup common bits before finally enabling the MMU.  Essentially

  * this is just loading the page table pointer and domain access

  * registers.

  */

 __enable_mmu:

 #ifdef CONFIG_ALIGNMENT_TRAP

     orr    r0, r0, #CR_A            //执行

 #else

     bic    r0, r0, #CR_A

 #endif

 #ifdef CONFIG_CPU_DCACHE_DISABLE

     bic    r0, r0, #CR_C            //未执行

 #endif

 #ifdef CONFIG_CPU_BPREDICT_DISABLE

     bic    r0, r0, #CR_Z            //未执行

 #endif

 #ifdef CONFIG_CPU_ICACHE_DISABLE

     bic    r0, r0, #CR_I            //未执行

 #endif

     mov    r5, #(domain_val(DOMAIN_USER, DOMAIN_MANAGER) | \

               domain_val(DOMAIN_KERNEL, DOMAIN_MANAGER) | \

               domain_val(DOMAIN_TABLE, DOMAIN_MANAGER) | \

               domain_val(DOMAIN_IO, DOMAIN_CLIENT))    

 //MRC：协处理器寄存器到ARM处理器寄存器的数据传送指令(读出协处理器寄存器)。

 //MCR：ARM处理器寄存器到协处理器寄存器的数据传送指令(写入协处理器寄存器)。

     //c3 DOMAIN ACCESS CONTROL REGISTER

     mcr    p15, , r5, c3, c0,         @ load domain access register

     //r4 = 30004000 放进协处理器c2

     mcr    p15, , r4, c2, c0,         @ load page table pointer

     b    __turn_mmu_on

 ENDPROC(__enable_mmu)

 /*

  * 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-arm-kernel

  * 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

 __turn_mmu_on:

     mov    r0, r0

     mcr    p15, , r0, c1, c0,         @ write control reg

     mrc    p15, , r3, c0, c0,         @ read id reg

     mov    r3, r3

     mov    r3, r13

     mov    pc, r3

 ENDPROC(__turn_mmu_on)

 /*

  * 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

  */

  //创建页表

 __create_page_tables:

 /*

 *宏定义 *.macro pgtbl, rd *ldr \rd, =(KERNEL_RAM_PADDR - 0x4000) *.endm *

 *分析：内存为 4G = 4*1024 MB；需要4096个表单表示，每一页表单为4bytes；因此需要16K内存，即0x4000；

 *根据 =(KERNEL_RAM_PADDR - 0x4000)得，页表存放于-内核在内存中的物理地址之前。

 */

 //定义rgtbl 宏 功能是将KERNEL_RAM_PADDR - 0x4000复制给rd寄存器

     pgtbl    r4                @ page table address       //r4 = 30004000

     /*

      * Clear the 16K level 1 swapper page table

      */

      /* * 按16个bytes一次，将页表清空 */

     mov    r0, r4        //r0 = 30004000

     mov    r3, #        //r3 = 0

     add    r6, r0, #0x4000 //r6 = 30008000

     //将30004000-30008000 的空间置0

 :    str    r3, [r0], #

     str    r3, [r0], #

     str    r3, [r0], #

     str    r3, [r0], #

     teq    r0, r6

     bne    1b

 /* *r10 = proc_info_list类型结构体的基地址 *PROCINFO_MM_MMUFLAGS 8 /* offsetof(struct proc_info_list, __cpu_mm_mmu_flags) @ */

     ldr    r7, [r10, #PROCINFO_MM_MMUFLAGS] @ mm_mmuflags   r7 = 00000C1E __cpu_mm_mmu_flags

     /*

      * Create identity mapping for first MB of kernel to

      * cater for the MMU enable.  This identity mapping

      * will be removed by paging_init().  We use our current program

      * counter to determine corresponding section base address.

      */

      /*

      为第一个MB内核创建标识映射，以满足MMU启用。 此标识映射将通过paging_init（）删除。 我们使用当前的程序计数器来确定相应的部分基址。

      */

 /*

 *下面代码建立kernel对应的section页表项。 *

 *1. 通过PC值的高12位(右移20位)，得到kernel的section，并存储在r6中。

 *2. 获取32bit的页表表单值

 *3. 将页表表单值存放在页表内存区中。 *

 *注意点： *a. lsr 20 因为虚拟地址分区中，后20位为相对地址，前12位为段地址

 *b. lsl 2 因为每一个页表项为4字节，所以需要左移2位

 */

 /*页表将4GB的地址空间分成若干个1MB的段(section)，因此页表包含4096个页表项(section entry)。

 每个页表项是32bits(4 bytes)，因而页表占用4096*4=16k的内存空间。下面的代码是将这16k的页表清0。

 */

     mov    r6, pc   //r6= 3000xxxx 通过pc值的高12位(右移20位),得到kernel的section,并存储到r6中.因为当前是通过运行时地址得到的kernel的section,因而是物理地址.

     mov    r6, r6, lsr #            @ start of kernel section  r6 =

     orr    r3, r7, r6, lsl #        @ flags + kernel base   r3 = 30000C1E = r7 | (r6 << ); flags + kernel base,得到页表中需要设置的值.

     str    r3, [r4, r6, lsl #]        @ identity mapping  //设置页表: mem[r4 + r6 * 4] = r3   r4 = 30004000

                                                         //         mem(30004000 + 300 << 2) = 30000C1E

                                                         //         mem(30004000 + 300 * 4)  = 30000C1E

                                                         //            mem(30004C00) = 30000C1E

                                                         //这里,因为页表的每一项是32 bits(4 bytes),所以要乘以4(<<2). 

     /*

      * Now setup the pagetables for our kernel direct

      * mapped region.

      */

 /*

 * 下面的 add r0, r4, #(KERNEL_START & 0xff000000) >> 18 涉及到一个立即数的概念：

 * 关于arm汇编立即数，可以参考下面网站：http://blog.csdn.net/a99778800/article/details/6759825 *

 * 下面这段代码就是存储kernel物理地址。建立页表，虚拟地址和物理地址之间建立连接。

 * 即：将内核中所有的物理地址（1M为单位）都存放到了页表中，与虚拟地址一一对应。 */     

 //KERNEL_START = 0xC0008000

 //这样分开写是由于arm的立即数只能是8位表示。

     add    r0, r4,  #(KERNEL_START & 0xff000000) >>      //r0 = 30007000 R4 =30004000

     str    r3, [r0, #(KERNEL_START & 0x00f00000) >> ]!

     ldr    r6, =(KERNEL_END - )

     add    r0, r0, #

     add    r6, r4, r6, lsr #

 :    cmp    r0, r6

     add    r3, r3, # <<

     strls    r3, [r0], #

     bls    1b

 /*

 运行到此 段表在内存中如下：

 J-Link>mem32 30007000 10

 30007000 = 30000C1E 30100C1E 30200C1E 30300C1E

 30007010 = 30400C1E 00000000 00000000 00000000

 30007020 = 00000000 00000000 00000000 00000000

 30007030 = 00000000 00000000 00000000 00000000

 J-Link>h

 PC: (R15) = 300080D8, CPSR = 200000D3 (SVC mode, ARM FIQ dis. IRQ dis.)

 Current:

      R0 =30007014, R1 =000007CF, R2 =00000000, R3 =30500C1E

      R4 =30004000, R5 =00000000, R6 =30007013, R7 =00000C1E

      R8 =3001EF70, R9 =41129200, R10=3001EF3C, R11=00000020, R12=306EFD84

      R13=3049D990, R14=30008028, SPSR=00000010

 USR: R8 =3001EF70, R9 =41129200, R10=3001EF3C, R11=00000020, R12=306EFD84

      R13=00000000, R14=00000000

 FIQ: R8 =00000000, R9 =00000000, R10=00000000, R11=00000000, R12=00000000

      R13=00000000, R14=00000000, SPSR=00000010

 IRQ: R13=00000000, R14=00000000, SPSR=00000010

 SVC: R13=3049D990, R14=30008028, SPSR=00000010

 ABT: R13=00000000, R14=00000000, SPSR=00000010

 UND: R13=00000000, R14=00000000, SPSR=00000010

 */

 /*

 * XIP介绍:

 * XIP是指 (EXECUTE IN PLACE) 是指直接从存放代码的位置上启动运行。

 * 非XIP方式是指在运行之前需对代码进行重定位。该类型的内核以非压缩方式存放在Flash中，启动时由Bootloader加载到内存后运行。 *

 * 如果是XIP技术的内核，上面的映射只能映射内核代码和只读数据部分

 * 这里我们再映射一些RAM来作为.data and .bss空间。

 */

 #ifdef CONFIG_XIP_KERNEL XIP 未定义

     /*

      * Map some ram to cover our .data and .bss areas.

      */

     orr    r3, r7, #(KERNEL_RAM_PADDR & 0xff000000)

     .if    (KERNEL_RAM_PADDR & 0x00f00000)

     orr    r3, r3, #(KERNEL_RAM_PADDR & 0x00f00000)

     .endif

     //这样分开写是由于arm的立即数只能是8位表示。

     add    r0, r4,  #(KERNEL_RAM_VADDR & 0xff000000) >>

     str    r3, [r0, #(KERNEL_RAM_VADDR & 0x00f00000) >> ]!

     ldr    r6, =(_end - )

     add    r0, r0, #

     add    r6, r4, r6, lsr #

 :    cmp    r0, r6

     add    r3, r3, # <<

     strls    r3, [r0], #

     bls    1b

 #endif

     /*

      * Then map first 1MB of ram in case it contains our boot params.

      * 然后映射第一个1MB的RAM，以防它包含我们的引导参数。

      */

 /*

 * 下面的代码用来设置RAM中大小为1M虚拟地址的页表。之所以要设置这个页表项的原因是该区域存储着boot params。

 * 因此需要为它建立map，这样开启MMU后就可以访问

 */     

 //这样分开写是由于arm的立即数只能是8位表示。

 /*

 *下面的代码用来设置RAM中起始地址为0x30000000、大小为1M虚拟地址的页表，之所以要设置这个页表项的原因是该区域起始地址为0x30000100存

 *储着boot params。因此需要为它建立map，这样开启MMU后就可以访问这些参数了。

 */

     add    r0, r4, #PAGE_OFFSET >>

     orr    r6, r7, #(PHYS_OFFSET & 0xff000000)

     .if    (PHYS_OFFSET & 0x00f00000)

     orr    r6, r6, #(PHYS_OFFSET & 0x00f00000)

     .endif

     str    r6, [r0]

 #ifdef CONFIG_DEBUG_LL

     ldr    r7, [r10, #PROCINFO_IO_MMUFLAGS] @ io_mmuflags

     /*

      * Map in IO space for serial debugging.

      * This allows debug messages to be output

      * via a serial console before paging_init.

      */

     ldr    r3, [r8, #MACHINFO_PGOFFIO]

     add    r0, r4, r3

     rsb    r3, r3, #0x4000            @ PTRS_PER_PGD*sizeof(long)

     cmp    r3, #0x0800            @ limit to 512MB

     movhi    r3, #0x0800

     add    r6, r0, r3

     ldr    r3, [r8, #MACHINFO_PHYSIO]

     orr    r3, r3, r7

 :    str    r3, [r0], #

     add    r3, r3, # <<

     teq    r0, r6

     bne    1b

 #if defined(CONFIG_ARCH_NETWINDER) || defined(CONFIG_ARCH_CATS)

     /*

      * If we're using the NetWinder or CATS, we also need to map

      * in the 16550-type serial port for the debug messages

      */

     add    r0, r4, #0xff000000 >>

     orr    r3, r7, #0x7c000000

     str    r3, [r0]

 #endif

 #ifdef CONFIG_ARCH_RPC

     /*

      * Map in screen at 0x02000000 & SCREEN2_BASE

      * Similar reasons here - for debug.  This is

      * only for Acorn RiscPC architectures.

      */

     add    r0, r4, #0x02000000 >>

     orr    r3, r7, #0x02000000

     str    r3, [r0]

     add    r0, r4, #0xd8000000 >>

     str    r3, [r0]

 #endif

 #endif

     mov    pc, lr

 ENDPROC(__create_page_tables)

     .ltorg

 #include "head-common.S"

 /*

  *  linux/arch/arm/kernel/head-common.S

  *

  *  Copyright (C) 1994-2002 Russell King

  *  Copyright (c) 2003 ARM Limited

  *  All Rights Reserved

  *

  * This program is free software; you can redistribute it and/or modify

  * it under the terms of the GNU General Public License version 2 as

  * published by the Free Software Foundation.

  *

  */

 #define ATAG_CORE 0x54410001

 #define ATAG_CORE_SIZE ((2*4 + 3*4) >> 2)

 #define ATAG_CORE_SIZE_EMPTY ((2*4) >> 2)

     .align

     .type    __switch_data, %object

 __switch_data:

     .long    __mmap_switched

     .long    __data_loc            @ r4

     .long    _data                @ r5

     .long    __bss_start            @ r6

     .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

  */

  /*

 R0~R15 和 r0~r15 (16 个通用寄存器);

 a1~a4(参数,结果或临时寄存器,同 R0~R3);

 v1~v8(变量寄存器,同 R4~R11);

 SB 和 sb(静态基址,同 R9);

 SL 和 sl(堆栈限制,同 R10);

 FP 和 fp(帧指针);

 IP 和 ip(过程调用中间临时寄存器,同 R12);

 SP 和 sp(堆栈指针,同 R13);

 LR 和 lr(链接寄存器,同 R14);

 PC 和 pc(程序计数器,同 R15).

  */

 __mmap_switched:

     adr    r3, __switch_data + 

 //R4 =C0460000, R5 =C0460000, R6 =C049D8E0, R7 =C04D1D38

     ldmia    r3!, {r4, r5, r6, r7}

     cmp    r4, r5                @ Copy data segment if needed

     //以下ne都不会执行

 :    cmpne    r5, r6

     ldrne    fp, [r4], #

     strne    fp, [r5], #

     bne    1b

     mov    fp, #                @ Clear BSS (and zero fp)

 :    cmp    r6, r7

     strcc    fp, [r6],#

     bcc    1b

  ARM(    ldmia    r3, {r4, r5, r6, r7, sp})

  THUMB(    ldmia    r3, {r4, r5, r6, r7}    )

  THUMB(    ldr    sp, [r3, #]        )

     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

 ENDPROC(__mmap_switched)

 /*

  * Exception handling.  Something went wrong and we can't proceed.  We

  * ought to tell the user, but since we don't have any guarantee that

  * we're even running on the right architecture, we do virtually nothing.

  *

  * If CONFIG_DEBUG_LL is set we try to print out something about the error

  * and hope for the best (useful if bootloader fails to pass a proper

  * machine ID for example).

  */

 __error_p:

 #ifdef CONFIG_DEBUG_LL

     adr    r0, str_p1

     bl    printascii

     mov    r0, r9

     bl    printhex8

     adr    r0, str_p2

     bl    printascii

     b    __error

 str_p1:    .asciz    "\nError: unrecognized/unsupported processor variant (0x"

 str_p2:    .asciz    ").\n"

     .align

 #endif

 ENDPROC(__error_p)

 __error_a:

 #ifdef CONFIG_DEBUG_LL

     mov    r4, r1                @ preserve machine ID

     adr    r0, str_a1

     bl    printascii

     mov    r0, r4

     bl    printhex8

     adr    r0, str_a2

     bl    printascii

     adr    r3, 4f

     ldmia    r3, {r4, r5, r6}        @ get machine desc list

     sub    r4, r3, r4            @ get offset between virt&phys

     add    r5, r5, r4            @ convert virt addresses to

     add    r6, r6, r4            @ physical address space

 :    ldr    r0, [r5, #MACHINFO_TYPE]    @ get machine type

     bl    printhex8

     mov    r0, #'\t'

     bl    printch

     ldr     r0, [r5, #MACHINFO_NAME]    @ get machine name

     add    r0, r0, r4

     bl    printascii

     mov    r0, #'\n'

     bl    printch

     add    r5, r5, #SIZEOF_MACHINE_DESC    @ next machine_desc

     cmp    r5, r6

     blo    1b

     adr    r0, str_a3

     bl    printascii

     b    __error

 ENDPROC(__error_a)

 str_a1:    .asciz    "\nError: unrecognized/unsupported machine ID (r1 = 0x"

 str_a2:    .asciz    ").\n\nAvailable machine support:\n\nID (hex)\tNAME\n"

 str_a3:    .asciz    "\nPlease check your kernel config and/or bootloader.\n"

     .align

 #endif

 __error:

 #ifdef CONFIG_ARCH_RPC

 /*

  * Turn the screen red on a error - RiscPC only.

  */

     mov    r0, #0x02000000

     mov    r3, #0x11

     orr    r3, r3, r3, lsl #

     orr    r3, r3, r3, lsl #

     str    r3, [r0], #

     str    r3, [r0], #

     str    r3, [r0], #

     str    r3, [r0], #

 #endif

 :    mov    r0, r0

     b    1b     //1b b指back 1为标号

 ENDPROC(__error)

 /*

  * 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 物理内存中的proc_info指针

  *    r9 = cpuid (preserved)

  */

  //ADR : 小范围的地址读取伪指令.ADR 指令将基于 PC 相对偏移的地址值读取到寄存器中.

  //LDMIA 和 STMIA 批量加载/存储指令可以实现在一组寄存器和一块连续的内存单元之间传输数据

 __lookup_processor_type:

     adr    r3, 3f    //3f f指forward 3为标号

     ldmia    r3, {r5 - r7}

 /*

     r5 __proc_info_begin     地址

     r6 __proc_info_end        地址

     r7 .                    当前地址

     R0 =00000000, R1 =000007CF, R2 =016F2818, R3 =300081B8

     R4 =30008000, R5 =C001EF3C, R6 =C001EF70, R7 =C00081C0

     R8 =016F2818, R9 =41129200, R10=00000004, R11=00000020, R12=306EFD84

     R13=3049D990, R14=3000800C, SPSR=00000010

 */

     add    r3, r3, #

     sub    r3, r3, r7            @ get offset between virt&phys  R3 =  = 300081C0 - C00081C0

     add    r5, r5, r3            @ convert virt addresses to

     add    r6, r6, r3            @ physical address space

 :    ldmia    r5, {r3, r4}            @ value, mask

     and    r4, r4, r9            @ mask wanted bits

     teq    r3, r4

     beq    2f                    //ID正确，跳转回去

     add    r5, r5, #PROC_INFO_SZ        @ sizeof(proc_info_list) ID不正确的话，寻找下一个list对比

     cmp    r5, r6                //比较是否小于__proc_info_end的地址，如果小于则说明还有list可以比较，否则就已经没有了

     blo    1b

     mov    r5, #                @ unknown processor  返回r5 = 0表示没有对应的处理器

 :    mov    pc, lr

 ENDPROC(__lookup_processor_type)

 /*

  * This provides a C-API version of the above function.

  */

 ENTRY(lookup_processor_type)

     stmfd    sp!, {r4 - r7, r9, lr}

     mov    r9, r0

     bl    __lookup_processor_type

     mov    r0, r5

     ldmfd    sp!, {r4 - r7, r9, pc}

 ENDPROC(lookup_processor_type)

 /*

  * Look in <asm/procinfo.h> and arch/arm/kernel/arch.[ch] for

  * more information about the __proc_info and __arch_info structures.

  */

 //proc_info和arch_info位置

     .align

 :    .long    __proc_info_begin

     .long    __proc_info_end

 :    .long    .

     .long    __arch_info_begin

     .long    __arch_info_end

 /*

  * Lookup machine architecture in the linker-build list of architectures.

  * Note that we can't use the absolute addresses for the __arch_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.

  *

  *  r1 = machine architecture number

  * Returns:

  *  r3, r4, r6 corrupted

  *  r5 = mach_info pointer in physical address space

  */

 __lookup_machine_type:

     adr    r3, 4b       //将 .long    . 的物理地址加载至r3寄存器 R3 =300081C0

     ldmia    r3, {r4, r5, r6} //R4 =C00081C0, R5 =C001EF70, R6 =C001EFA4,

     //将r5  r6的__arch_info_begin和__arch_info_end地址由虚拟地址转换成物理地址

     sub    r3, r3, r4            @ get offset between virt&phys

     add    r5, r5, r3            @ convert virt addresses to

     add    r6, r6, r3            @ physical address space        

 :    ldr    r3, [r5, #MACHINFO_TYPE]    @ get machine type

     teq    r3, r1                @ matches loader number?

     beq    2f                @ found

     add    r5, r5, #SIZEOF_MACHINE_DESC    @ next machine_desc

     cmp    r5, r6

     blo    1b

     mov    r5, #                @ unknown machine

 :    mov    pc, lr

 ENDPROC(__lookup_machine_type)

 /*

  * This provides a C-API version of the above function.

  */

 ENTRY(lookup_machine_type)

     stmfd    sp!, {r4 - r6, lr}

     mov    r1, r0

     bl    __lookup_machine_type

     mov    r0, r5

     ldmfd    sp!, {r4 - r6, pc}

 ENDPROC(lookup_machine_type)

 /* Determine validity of the r2 atags pointer.  The heuristic requires

  * that the pointer be aligned, in the first 16k of physical RAM and

  * that the ATAG_CORE marker is first and present.  Future revisions

  * of this function may be more lenient with the physical address and

  * may also be able to move the ATAGS block if necessary.

  *

  * r8  = machinfo

  *

  * Returns:

  *  r2 either valid atags pointer, or zero

  *  r5, r6 corrupted

  */

 __vet_atags:

     tst    r2, #0x3            @ aligned?

     bne    1f

     ldr    r5, [r2, #]            @ is first tag ATAG_CORE?

     cmp    r5, #ATAG_CORE_SIZE

     cmpne    r5, #ATAG_CORE_SIZE_EMPTY

     bne    1f

     ldr    r5, [r2, #]

     ldr    r6, =ATAG_CORE

     cmp    r5, r6

     bne    1f

     mov    pc, lr                @ atag pointer is ok

 :    mov    r2, #

     mov    pc, lr

 ENDPROC(__vet_atags)