TI Cortex M3串口转以太网例程分析2-----bootloader

        bootloader是TI串口转以太网代码的一小部分，位于Flash开始的4KB空间内。它的一个重要作用是在应用远程升级，可以通过串口、USB、IIC、以太网等通道进行远程固件升级。bootloader是CPU启动后最先执行的程序，它会把自己拷贝到SRAM，并判断是否有固件升级，如果有升级请求，则执行升级程序；反之，执行用户程序。

  一.流程图       

           由于这里只考虑基于以太网的bootloader，其流程图如图2-1所示：

图2-1

 二.配置文件     

        由于bootlaoder可以使用串口、USB、IIC、以太网等通道进行远程固件升级，那么怎么样配置才可以使用以太网呢？这就牵扯到bl_config文件。此文件是专门配置bootloader的。代码就不贴了，看一下这里面几个必须配置的选项：

1. 以下至少且只能定义一个，用于指明使用何种方式升级。

        CAN_ENABLE_UPDATE,       

        ENET_ENABLE_UPDATE,

        I2C_ENABLE_UPDATE,

        SSI_ENABLE_UPDATE,

        UART_ENABLE_UPDATE,

        USB_ENABLE_UPDATE

2. 以下必须定义

        APP_START_ADDRESS                        用户程序启动地址

        VTABLE_START_ADDRESS                 用户程序向量表起始地址

        FLASH_PAGE_SIZE                               Flash页大小，TI的目前为止都为1K

        STACK_SIZE                                           堆栈大小

3. 当选择了以太网升级后，以下必须定义

        CRYSTAL_FREQ                                     目标板晶振频率

三.bootloader启动代码分析

          不少人不喜欢分析汇编文件，甚至总想绕过汇编。网络上也出现一些人教导初学者学习单片机的时候直接用C语言编程，避开汇编。我个人是极其不同意这种“速成”方法的。作为一名合格的嵌入式工程师或者说爱好者，汇编绝不可回避。汇编能帮助理解硬件，特别是CPU结构、存储和寻址等等；现在的嵌入式程序虽然绝大多数是用C编写的，但要想精通C语言，必须具有汇编基础，任何技术都是入门容易，精通难，因此要想深入理解C的指针、数组甚至是变量存储，还非少不了汇编不可；再者，有些地方必须使用汇编，比如一些实时性要求高的模块（不常见），还有就是接下来要说的启动代码。先附源代码。

;******************************************************************************;; bl_startup_rvmdk.S - Startup code for RV-MDK.;; Copyright (c) 2007-2010 Texas Instruments Incorporated.  All rights reserved.; Software License Agreement; ; Texas Instruments (TI) is supplying this software for use solely and; exclusively on TI's microcontroller products. The software is owned by; TI and/or its suppliers, and is protected under applicable copyright; laws. You may not combine this software with "viral" open-source; software in order to form a larger program.; ; THIS SOFTWARE IS PROVIDED "AS IS" AND WITH ALL FAULTS.; NO WARRANTIES, WHETHER EXPRESS, IMPLIED OR STATUTORY, INCLUDING, BUT; NOT LIMITED TO, IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR; A PARTICULAR PURPOSE APPLY TO THIS SOFTWARE. TI SHALL NOT, UNDER ANY; CIRCUMSTANCES, BE LIABLE FOR SPECIAL, INCIDENTAL, OR CONSEQUENTIAL; DAMAGES, FOR ANY REASON WHATSOEVER.; ; This is part of revision 6288 of the Stellaris Firmware Development Package.;;******************************************************************************    include bl_config.inc;******************************************************************************;; A couple of defines that would normally be obtained from the appropriate C; header file, but must be manually provided here since the Keil compiler does; not have a mechanism for passing assembly source through the C preprocessor.; 以下定义通常在C头文件中定义过,但仍要在这里定义,因为keil编译器没有从汇编器直接; 调用C预编译器的机制.;;******************************************************************************SYSCTL_RESC                     equ     0x400fe05c  ;复位原因SYSCTL_RESC_MOSCFAIL            equ     0x00010000NVIC_VTABLE                     equ     0xe000ed08  ;向量表偏移量寄存器;******************************************************************************;; Put the assembler into the correct configuration.;;******************************************************************************    thumb   ;thumb指令    require8    preserve8;******************************************************************************;; The stack gets placed into the zero-init section.; 将堆放到零初始化区;;******************************************************************************    area    ||.bss||, noinit, align=2 ;4字节对齐,2的2次幂;******************************************************************************;; Allocate storage for the stack.; 为堆分配空间,STACK_SIZE在bl_config.h中定义的宏,通过bl_config.inc加载armcc;;******************************************************************************g_pulStack    space   _STACK_SIZE * 4;******************************************************************************;; This portion of the file goes into the reset section.;;******************************************************************************    area    RESET, code, readonly, align=3  ;8字节对齐?;******************************************************************************;; The minimal vector table for a Cortex-M3 processor.;;******************************************************************************    export  __Vectors__Vectors    dcd     g_pulStack + (_STACK_SIZE * 4)  ; Offset 00: Initial stack pointer 初始化堆栈指针    if      :def:_FLASH_PATCH_COMPATIBLE    dcd     Reset_Handler + 0x1000          ; Offset 04: Reset handler 为某些Flash打了补丁的器件    dcd     NmiSR + 0x1000                  ; Offset 08: NMI handler    dcd     FaultISR + 0x1000               ; Offset 0C: Hard fault handler    else    dcd     Reset_Handler                   ; Offset 04: Reset handler    dcd     NmiSR                           ; Offset 08: NMI handler    dcd     FaultISR                        ; Offset 0C: Hard fault handler    endif    dcd     IntDefaultHandler               ; Offset 10: MPU fault handler    dcd     IntDefaultHandler               ; Offset 14: Bus fault handler    dcd     IntDefaultHandler               ; Offset 18: Usage fault handler    dcd     0                               ; Offset 1C: Reserved    dcd     0                               ; Offset 20: Reserved    dcd     0                               ; Offset 24: Reserved    dcd     0                               ; Offset 28: Reserved    if      :def:_FLASH_PATCH_COMPATIBLE    dcd     UpdateHandler + 0x1000          ; Offset 2C: SVCall handler  SVC异常    else    dcd     UpdateHandler                   ; Offset 2C: SVCall handler    endif    dcd     IntDefaultHandler               ; Offset 30: Debug monitor handler    dcd     0                               ; Offset 34: Reserved    dcd     IntDefaultHandler               ; Offset 38: PendSV handler    if      :def:_ENET_ENABLE_UPDATE    import  SysTickIntHandler    dcd     SysTickIntHandler               ; Offset 3C: SysTick handler    else    dcd     IntDefaultHandler               ; Offset 3C: SysTick handler    endif    if      :def:_UART_ENABLE_UPDATE :land: :def:_UART_AUTOBAUD    import  GPIOIntHandler    dcd     GPIOIntHandler                  ; Offset 40: GPIO port A handler    else    dcd     IntDefaultHandler               ; Offset 40: GPIO port A handler    endif    if      :def:_USB_ENABLE_UPDATE :lor:                                     \            (_APP_START_ADDRESS != _VTABLE_START_ADDRESS) :lor:               \            :def:_FLASH_PATCH_COMPATIBLE    dcd     IntDefaultHandler               ; Offset 44: GPIO Port B    dcd     IntDefaultHandler               ; Offset 48: GPIO Port C    dcd     IntDefaultHandler               ; Offset 4C: GPIO Port D    dcd     IntDefaultHandler               ; Offset 50: GPIO Port E    dcd     IntDefaultHandler               ; Offset 54: UART0 Rx and Tx    dcd     IntDefaultHandler               ; Offset 58: UART1 Rx and Tx    dcd     IntDefaultHandler               ; Offset 5C: SSI0 Rx and Tx    dcd     IntDefaultHandler               ; Offset 60: I2C0 Master and Slave    dcd     IntDefaultHandler               ; Offset 64: PWM Fault    dcd     IntDefaultHandler               ; Offset 68: PWM Generator 0    dcd     IntDefaultHandler               ; Offset 6C: PWM Generator 1    dcd     IntDefaultHandler               ; Offset 70: PWM Generator 2    dcd     IntDefaultHandler               ; Offset 74: Quadrature Encoder 0    dcd     IntDefaultHandler               ; Offset 78: ADC Sequence 0    dcd     IntDefaultHandler               ; Offset 7C: ADC Sequence 1    dcd     IntDefaultHandler               ; Offset 80: ADC Sequence 2    dcd     IntDefaultHandler               ; Offset 84: ADC Sequence 3    dcd     IntDefaultHandler               ; Offset 88: Watchdog timer    dcd     IntDefaultHandler               ; Offset 8C: Timer 0 subtimer A    dcd     IntDefaultHandler               ; Offset 90: Timer 0 subtimer B    dcd     IntDefaultHandler               ; Offset 94: Timer 1 subtimer A    dcd     IntDefaultHandler               ; Offset 98: Timer 1 subtimer B    dcd     IntDefaultHandler               ; Offset 9C: Timer 2 subtimer A    dcd     IntDefaultHandler               ; Offset A0: Timer 2 subtimer B    dcd     IntDefaultHandler               ; Offset A4: Analog Comparator 0    dcd     IntDefaultHandler               ; Offset A8: Analog Comparator 1    dcd     IntDefaultHandler               ; Offset AC: Analog Comparator 2    dcd     IntDefaultHandler               ; Offset B0: System Control    if      :def:_FLASH_PATCH_COMPATIBLE    dcd     0x00000881                      ; Offset B4: FLASH Control    else    dcd     IntDefaultHandler               ; Offset B4: FLASH Control    endif    endif    if      :def:_USB_ENABLE_UPDATE :lor:                                     \            (_APP_START_ADDRESS != _VTABLE_START_ADDRESS)    dcd     IntDefaultHandler               ; Offset B8: GPIO Port F    dcd     IntDefaultHandler               ; Offset BC: GPIO Port G    dcd     IntDefaultHandler               ; Offset C0: GPIO Port H    dcd     IntDefaultHandler               ; Offset C4: UART2 Rx and Tx    dcd     IntDefaultHandler               ; Offset C8: SSI1 Rx and Tx    dcd     IntDefaultHandler               ; Offset CC: Timer 3 subtimer A    dcd     IntDefaultHandler               ; Offset D0: Timer 3 subtimer B    dcd     IntDefaultHandler               ; Offset D4: I2C1 Master and Slave    dcd     IntDefaultHandler               ; Offset D8: Quadrature Encoder 1    dcd     IntDefaultHandler               ; Offset DC: CAN0    dcd     IntDefaultHandler               ; Offset E0: CAN1    dcd     IntDefaultHandler               ; Offset E4: CAN2    dcd     IntDefaultHandler               ; Offset E8: Ethernet    dcd     IntDefaultHandler               ; Offset EC: Hibernation module    if      :def: _USB_ENABLE_UPDATE    import  USB0DeviceIntHandler    dcd     USB0DeviceIntHandler            ; Offset F0: USB 0 Controller    else    dcd     IntDefaultHandler               ; Offset F0: USB 0 Controller    endif    endif;******************************************************************************;; Initialize the processor by copying the boot loader from flash to SRAM, zero; filling the .bss section, and moving the vector table to the beginning of; SRAM.  The return address is modified to point to the SRAM copy of the boot; loader instead of the flash copy, resulting in a branch to the copy now in; SRAM.; 初始化处理器,将boot loader从flash拷贝到SRAM,将.bss区用零填充并将向量表重映射到; SRAM的开始处.;;******************************************************************************    export  ProcessorInitProcessorInit    ;    ; Copy the code image from flash to SRAM.    ;    if      :def:_FLASH_PATCH_COMPATIBLE    movs    r0, #0x1000    else    movs    r0, #0x0000     endif    movs    r1, #0x0000    movt    r1, #0x2000 ;将16位的立即数放到寄存器的高16位,低位不受影响    import  ||Image$SRAM$ZI$Base||  ;为汇编器提供一个在当前汇编程序中未定义的符号    ldr     r2, =||Image$SRAM$ZI$Base||;SRAM区中的ZI输出节执行地址copy_loop        ldr     r3, [r0], #4        str     r3, [r1], #4        cmp     r1, r2        blt     copy_loop    ;    ; Zero fill the .bss section.将.bss区用零填充    ;    movs    r0, #0x0000    import  ||Image$SRAM$ZI$Limit||   ;SRAM区中ZI 输出节末尾地址后面的字节地址    ldr     r2, =||Image$SRAM$ZI$Limit||zero_loop        str     r0, [r1], #4        cmp     r1, r2        blt     zero_loop    ;    ; Set the vector table pointer to the beginning of SRAM.; 将向量表指针指向SRAM开始处    ;    movw    r0, #(NVIC_VTABLE & 0xffff);放入r0低16位,高位清零    movt    r0, #(NVIC_VTABLE >> 16);NVIC_VTABLE=0xe000ed08(向量表偏移量寄存器)    movs    r1, #0x0000    movt    r1, #0x2000    str     r1, [r0];向量表重定位到0x2000 0000处    ;    ; Return to the caller.返回    ;    bx      lr;******************************************************************************;; The reset handler, which gets called when the processor starts.;;******************************************************************************    export  Reset_HandlerReset_Handler    ;    ; Initialize the processor.    ;    bl      ProcessorInit    ;    ; Branch to the SRAM copy of the reset handler.    ;

    ldr     pc, =Reset_Handler_In_SRAM       ;进入SRAM执行程序

;******************************************************************************;; The NMI handler.;;******************************************************************************NmiSR    if      :def:_ENABLE_MOSCFAIL_HANDLER    ;    ; Grab the fault frame from the stack (the stack will be cleared by the    ; processor initialization that follows).    ;    ldm     sp, {r4-r11}    mov     r12, lr    ;    ; Initialize the processor.    ;    bl      ProcessorInit    ;    ; Branch to the SRAM copy of the NMI handler.    ;    ldr     pc, =NmiSR_In_SRAM    else    ;    ; Loop forever since there is nothing that we can do about a NMI.    ;    b       .    endif;******************************************************************************;; The hard fault handler.;;******************************************************************************FaultISR    ;    ; Loop forever since there is nothing that we can do about a hard fault.    ;    b       .;******************************************************************************;; The update handler, which gets called when the application would like to; start an update.; 升级服务函数,当应用程序想要开始升级时,调用这个函数.;;******************************************************************************UpdateHandler    ;    ; Initialize the processor.初始化处理器    ;    bl      ProcessorInit  ;调用子程序    ;    ; Branch to the SRAM copy of the update handler.    ;    ldr     pc, =UpdateHandler_In_SRAM;******************************************************************************;; This portion of the file goes into the text section.;;******************************************************************************    align   4    area    ||.text||, code, readonly, align=2Reset_Handler_In_SRAM    ;    ; Call the user-supplied low level hardware initialization function    ; if provided.; 如果用户提供了底层硬件初始化函数,则调用这个函数    ;    if      :def:_BL_HW_INIT_FN_HOOK    import  $_BL_HW_INIT_FN_HOOK    bl      $_BL_HW_INIT_FN_HOOK    endif    ;    ; See if an update should be performed.; 检查是否有升级请求    ;    import  CheckForceUpdate    bl      CheckForceUpdate    cbz     r0, CallApplication   ;结果为零则转移(只能跳到下一行)    ;    ; Configure the microcontroller.    ;EnterBootLoader    if      :def:_ENET_ENABLE_UPDATE    import  ConfigureEnet    bl      ConfigureEnet    elif    :def:_CAN_ENABLE_UPDATE    import  ConfigureCAN    bl      ConfigureCAN    elif    :def:_USB_ENABLE_UPDATE    import  ConfigureUSB    bl      ConfigureUSB    else    import  ConfigureDevice    bl      ConfigureDevice    endif    ;    ; Call the user-supplied initialization function if provided.; 如果用户提供了初始化函数,则调用.    ;    if      :def:_BL_INIT_FN_HOOK    import  $_BL_INIT_FN_HOOK    bl      $_BL_INIT_FN_HOOK    endif    ;    ; Branch to the update handler.    ; 进入升级处理程序;    if      :def:_ENET_ENABLE_UPDATE    import  UpdateBOOTP    b       UpdateBOOTP    elif    :def:_CAN_ENABLE_UPDATE    import  UpdaterCAN    b       UpdaterCAN    elif    :def:_USB_ENABLE_UPDATE    import  UpdaterUSB    b       UpdaterUSB    else    import  Updater    b       Updater    endif    ;    ; This is a second symbol to allow starting the application from the boot    ; loader the linker may not like the perceived jump.    ;    export StartApplicationStartApplication    ;    ; Call the application via the reset handler in its vector table.  Load the    ; address of the application vector table.    ;CallApplication    ;    ; Copy the application's vector table to the target address if necessary.    ; Note that incorrect boot loader configuration could cause this to    ; corrupt the code!  Setting VTABLE_START_ADDRESS to 0x20000000 (the start    ; of SRAM) is safe since this will use the same memory that the boot loader    ; already uses for its vector table.  Great care will have to be taken if    ; other addresses are to be used.; 如果必要的话,复制应用程序的向量表到目标地址.; 请注意,不正确的boot loader配置会破坏整个程序!设置VTABLE_START_ADDRESS为; 0x2000 0000(从SRAM启动)也是可以的,因为这将和boot loader使用同样的内存    ;    if (_APP_START_ADDRESS != _VTABLE_START_ADDRESS) ;看应用程序的起始地址是否和应用程序的向量表存储地址相同    movw    r0, #(_VTABLE_START_ADDRESS & 0xffff)    if (_VTABLE_START_ADDRESS > 0xffff)    movt    r0, #(_VTABLE_START_ADDRESS >> 16)    endif    movw    r1, #(_APP_START_ADDRESS & 0xffff)    if (_APP_START_ADDRESS > 0xffff)    movt    r1, #(_APP_START_ADDRESS >> 16)    endif    ;    ; Calculate the end address of the vector table assuming that it has the    ; maximum possible number of vectors.  We don't know how many the app has    ; populated so this is the safest approach though it may copy some non    ; vector data if the app table is smaller than the maximum.; 计算向量表的结束地址,假设向量表有最大数目. 我们不知道应用程序使用了多少; 向量表,但这样是最安全的    ;    movw    r2, #(70 * 4)    adds    r2, r2, r0VectorCopyLoop        ldr     r3, [r1], #4        str     r3, [r0], #4        cmp     r0, r2        blt     VectorCopyLoop    endif    ;    ; Set the vector table address to the beginning of the application.; 将向量表重定位到应用程序开始处    ;    movw    r0, #(_VTABLE_START_ADDRESS & 0xffff)    if (_VTABLE_START_ADDRESS > 0xffff)    movt    r0, #(_VTABLE_START_ADDRESS >> 16)    endif    movw    r1, #(NVIC_VTABLE & 0xffff) ;向量表偏移寄存器    movt    r1, #(NVIC_VTABLE >> 16)    str     r0, [r1]    ;    ; Load the stack pointer from the application's vector table.; 从应用程序向量表装载用户堆栈.    ;    if (_APP_START_ADDRESS != _VTABLE_START_ADDRESS)    movw    r0, #(_APP_START_ADDRESS & 0xffff)    if (_APP_START_ADDRESS > 0xffff)    movt    r0, #(_APP_START_ADDRESS >> 16)    endif    endif    ldr     sp, [r0]    ;    ; Load the initial PC from the application's vector table and branch to    ; the application's entry point.    ;    ldr     r0, [r0, #4]    bx      r0;******************************************************************************;; The update handler, which gets called when the application would like to; start an update.; 升级处理函数,当用户程序想要开始升级时,调用此函数;;******************************************************************************UpdateHandler_In_SRAM    ;    ; Load the stack pointer from the vector table.; 从boot loader向量表中装载堆栈指针    ;    if      :def:_FLASH_PATCH_COMPATIBLE    movs    r0, #0x1000    else    movs    r0, #0x0000    endif    ldr     sp, [r0]    ;    ; Call the user-supplied low level hardware initialization function    ; if provided.; 调用用户提供的底层硬件初始化函数    ;    if      :def:_BL_HW_INIT_FN_HOOK    bl      $_BL_HW_INIT_FN_HOOK    endif    ;    ; Call the user-supplied re-initialization function if provided.; 调用用户提供的初始化函数    ;    if      :def:_BL_REINIT_FN_HOOK    import  $_BL_REINIT_FN_HOOK    bl      $_BL_REINIT_FN_HOOK    endif    ;    ; Branch to the update handler.; 进入升级例程    ;    if      :def:_ENET_ENABLE_UPDATE    b       UpdateBOOTP   ;在bl_enet.c中    elif    :def:_CAN_ENABLE_UPDATE    import  AppUpdaterCAN    b       AppUpdaterCAN    elif    :def:_USB_ENABLE_UPDATE    import  AppUpdaterUSB    b       AppUpdaterUSB    else    b       Updater    endif;******************************************************************************;; The NMI handler.; NMI异常服务例程,处理主振荡器失败;;******************************************************************************    if      :def:_ENABLE_MOSCFAIL_HANDLERNmiSR_In_SRAM    ;    ; Restore the stack frame.    ;    mov     lr, r12    stm     sp, {r4-r11}    ;    ; Save the link register.    ;    mov     r9, lr    ;    ; Call the user-supplied low level hardware initialization function    ; if provided.    ;    if      :def:_BL_HW_INIT_FN_HOOK    bl      _BL_HW_INIT_FN_HOOK    endif    ;    ; See if an update should be performed.    ;    bl      CheckForceUpdate    cbz     r0, EnterApplication        ;        ; Clear the MOSCFAIL bit in RESC.        ;        movw    r0, #(SYSCTL_RESC & 0xffff)        movt    r0, #(SYSCTL_RESC >> 16)        ldr     r1, [r0]        bic     r1, r1, #SYSCTL_RESC_MOSCFAIL        str     r1, [r0]        ;        ; Fix up the PC on the stack so that the boot pin check is bypassed        ; (since it has already been performed).        ;        ldr     r0, =EnterBootLoader        bic     r0, #0x00000001        str     r0, [sp, #0x18]                ;        ; Return from the NMI handler.  This will then start execution of the        ; boot loader.        ;        bx      r9    ;    ; Restore the link register.    ;EnterApplication    mov     lr, r9    ;    ; Copy the application's vector table to the target address if necessary.    ; Note that incorrect boot loader configuration could cause this to    ; corrupt the code!  Setting VTABLE_START_ADDRESS to 0x20000000 (the start    ; of SRAM) is safe since this will use the same memory that the boot loader    ; already uses for its vector table.  Great care will have to be taken if    ; other addresses are to be used.    ;    if (_APP_START_ADDRESS != _VTABLE_START_ADDRESS)    movw    r0, #(_VTABLE_START_ADDRESS & 0xffff)    if (_VTABLE_START_ADDRESS > 0xffff)    movt    r0, #(_VTABLE_START_ADDRESS >> 16)    endif    movw    r1, #(_APP_START_ADDRESS & 0xffff)    if (_APP_START_ADDRESS > 0xffff)    movt    r1, #(_APP_START_ADDRESS >> 16)    endif    ;    ; Calculate the end address of the vector table assuming that it has the    ; maximum possible number of vectors.  We don't know how many the app has    ; populated so this is the safest approach though it may copy some non    ; vector data if the app table is smaller than the maximum.    ;    movw    r2, #(70 * 4)    adds    r2, r2, r0VectorCopyLoop2        ldr     r3, [r1], #4        str     r3, [r0], #4        cmp     r0, r2        blt     VectorCopyLoop2    endif    ;    ; Set the application's vector table start address.  Typically this is the    ; application start address but in some cases an application may relocate    ; this so we can't assume that these two addresses are equal.    ;    movw    r0, #(_VTABLE_START_ADDRESS & 0xffff)    if (_VTABLE_START_ADDRESS > 0xffff)    movt    r0, #(_VTABLE_START_ADDRESS >> 16)    endif    movw    r1, #(NVIC_VTABLE & 0xffff)    movt    r1, #(NVIC_VTABLE >> 16)    str     r0, [r1]    ;    ; Remove the NMI stack frame from the boot loader's stack.    ;    ldmia   sp, {r4-r11}    ;    ; Get the application's stack pointer.    ;    if (_APP_START_ADDRESS != _VTABLE_START_ADDRESS)    movw    r0, #(_APP_START_ADDRESS & 0xffff)    if (_APP_START_ADDRESS > 0xffff)    movt    r0, #(_APP_START_ADDRESS >> 16)    endif    endif    ldr     sp, [r0, #0x00]    ;    ; Fix up the NMI stack frame's return address to be the reset handler of    ; the application.    ;    ldr     r10, [r0, #0x04]    bic     r10, #0x00000001    ;    ; Store the NMI stack frame onto the application's stack.    ;    stmdb   sp!, {r4-r11}    ;    ; Branch to the application's NMI handler.    ;    ldr     r0, [r0, #0x08]    bx      r0    endif;******************************************************************************;; The default interrupt handler.;;******************************************************************************IntDefaultHandler    ;    ; Loop forever since there is nothing that we can do about an unexpected    ; interrupt.    ;    b       .;******************************************************************************;; Provides a small delay.  The loop below takes 3 cycles/loop.; 提供一个小的延时函数. 循环一次需要3个时钟周期.;;******************************************************************************    export  DelayDelay    subs    r0, #1    bne     Delay    bx      lr;******************************************************************************;; This is the end of the file.;;******************************************************************************    align   4    end

1. 汇编文件正文的第一句

      include bl_config.inc

包含bl_config.inc，这个文件是什么，从哪里来，有什么作用？再看bootloader工程Options---User---Run User Programs Before Build/Rebuild内的用户命令（见图2-2）又是什么？

图2-2

         所有的一切，要从keil MDK的汇编器说起，在启动代码中要用到配置文件bl_config.h中定义的一些配置选项，但因为MDK汇编器不能通过C预处理器运行汇编代码，所以bl_config.h中的相关内容需要 转化为汇编格式并包含到MDK的启动代码中。这需要手动运行C预编译器进行格式转化。图2-2中红色部分圈出的内容正是为了完成这个转换。在点击Build/Rebuild编译按钮之后，会先运行图2-2指定的命令，再进行编译。先来分析一下这条命令：

                                armcc --device DLM -o bl_config.inc -E bl_config.c

          这条命令的作用是将bl_config.c（包含bl_config.h文件）进行而且仅进行预编译处理，并生成bl_config.inc文件。

          armcc是Keil MDK提供的C编译工具，语法为：

                                 armcc [Options]  file1  file2  ...  file n

           介绍一下这里用到的Options选项：

                                   --device<dev>：设置目标的设备类型，DLM为Luminary的设备标识。

                                   -I<directory>   ：目录列表

                                   -E                      ：仅执行预处理

                                   -o<file>            ：指定输出文件的名字

2. 看一下目标板上电后启动代码的运行流程

          上电后程序先到Flash地址0x00处装载堆栈地址，这跟以前接触过的处理器不同，以前0x00处都是放置的复位处理代码，但Cortex M3内核却不是，0x00处是放置的堆栈地址，而不是跳转指令。

           堆栈设置完成后，跳转到Reset处理程序处，调用处理器初始化函数ProcessorInit，该函数将bootloader从Flash拷贝到SRAM，将.bss区用零填充并将向量表重映射到SRAM开始处。

           之后跳转到Reset_Handler_In_SRAM函数，在该函数中，如果用户提供了底层硬件初始化函数（在bl_config.h中使能），则调用这个函数。然后调用CheckForceUpdate函数，检查是否有升级请求。如果没有升级请求，跳转到CallApplication函数，在该函数中，将向量表重映射到应用程序开始处（这里为地址0x1000），装载用户程序堆栈地址，跳转到用户程序的Reset服务函数。

           如果调用CheckForceUpdate函数检测到有升级请求，则配置以太网，跳转到升级程序UpdateBOOTP处执行。

3. 如何在用户程序中调用升级程序

          用户程序存在于Flash地址0x1000处，bootloader存放于Flash地址0x00处，并且用户程序在执行的时候已经将向量表重映射到了Flash地址0x1000处了，那么应用程序是如何调用位于bootloader中的升级程序呢？

        再看bootloader启动代码的中断向量表，在Flash地址的0x2C中存放的是CPU SVC异常服务跳转地址：

                    dcd     UpdateHandler                   ; Offset 2C: SVCall handler

        而bootloader正是用这个异常来处理升级请求的。那么，应用程序只要执行该地址处的跳转指令，就能进行一次程序升级，在应用程序中的swupdate.c中，使用了如下C代码来执行位于Flash地址0x2C内的跳转程序：

                    (*((void (*)(void))(*(unsigned long *)0x2c)))();  

         对C语言还没有入门的同学可能会比较的头痛，这像谜一样的语句是如何执行位于bootloader的SVC异常服务例程呢？还是分解一下吧：

                      (*(unsigned long *)0x2c)：将0x2C强制转化为unsigned long类型指针，并指向该地址所在的数据；

                      void (*)(void)                      ：函数指针，指针名为空，该函数参数为空，返回值为空

                     (void (*)(void))(*(unsigned long *)0x2c)：将Flash地址0x2C中的内容强制转化为函数指针，该函数参数为空，返回值为空

                     (*((void (*)(void))(*(unsigned long *)0x2c)))()；：调用函数，即开始从启动代码中的UpdateHandler标号处开始执行。