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 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
end1. 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
-I
-E ：仅执行预处理
-o
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。