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

左手的ㄟ右手 2022-06-17 23:55 284阅读 0赞

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：

![0_1312946814ORwc.gif][]

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     0x00010000
    NVIC_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  ProcessorInit
    ProcessorInit
        ;
        ; 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_Handler
    Reset_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=2
    
    Reset_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 StartApplication
    StartApplication
        ;
        ; 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, r0
    VectorCopyLoop
            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_HANDLER
    NmiSR_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, r0
    VectorCopyLoop2
            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  Delay
    Delay
        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？

![0_13129552330N79.gif][]

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。

[0_1312946814ORwc.gif]: /images/20220618/0433b61d935c40b8a2f7e8a989ec717a.png
[0_13129552330N79.gif]: /images/20220618/36b8564714624c27b2f471eac59afd55.png