打开Freedom KL25的例程包，任意找一个code example，以lower_power_dma_uart_demo为例，分析一下启动汇编文件的代码。

代码主要分三部分：堆栈等的配置、向量表、初始启动跳转。

1. 堆栈配置

; <h> Stack Configuration
;   <o> Stack Size (in Bytes) <0x0-0xFFFFFFFF:8>
; </h>

Stack_Size      EQU     0x00000800

                AREA    STACK, NOINIT, READWRITE, ALIGN=3
Stack_Mem       SPACE   Stack_Size
__initial_sp    EQU     0x20002ff8

; <h> Heap Configuration
;   <o>  Heap Size (in Bytes) <0x0-0xFFFFFFFF:8>
; </h>

Heap_Size       EQU     0x00000000

                AREA    HEAP, NOINIT, READWRITE, ALIGN=3
__heap_base
Heap_Mem        SPACE   Heap_Size
__heap_limit

注：AREA关键字的作用是告诉汇编器，该section是独立的一段，应该重新开始解析。AREA后面的一系列表示该段的属性。

SPACE关键字告诉其后的一段memory存储空间，不被初始化为某些特定值，与FILL相对。

EQU可以理解为等于。

2. 向量表

; Vector Table Mapped to Address 0 at Reset

                AREA    RESET, DATA, READONLY
                EXPORT  __Vectors
                EXPORT  __Vectors_End
                EXPORT  __Vectors_Size

__Vectors       DCD     __initial_sp  ; Top of Stack
                DCD     Reset_Handler  ; Reset Handler
                DCD     NMI_Handler  ; NMI Handler
                DCD     HardFault_Handler  ; Hard Fault Handler
                DCD     0  ; Reserved
                DCD     0  ; Reserved
                DCD     0  ; Reserved
                DCD     0  ; Reserved
                DCD     0  ; Reserved
                DCD     0  ; Reserved
                DCD     0  ; Reserved
                DCD     SVC_Handler  ; SVCall Handler
                DCD     0  ; Reserved
                DCD     0  ; Reserved
                DCD     PendSV_Handler  ; PendSV Handler
                DCD     SysTick_Handler  ; SysTick Handler

                ; External Interrupts
                DCD     DMA0_IRQHandler  ; DMA channel 0 transfer complete interrupt
                DCD     DMA1_IRQHandler  ; DMA channel 1 transfer complete interrupt
                DCD     DMA2_IRQHandler  ; DMA channel 2 transfer complete interrupt
                DCD     DMA3_IRQHandler  ; DMA channel 3 transfer complete interrupt
                DCD     Reserved20_IRQHandler  ; Reserved interrupt 20
                DCD     FTFA_IRQHandler  ; FTFA interrupt
                DCD     LVD_LVW_IRQHandler  ; Low Voltage Detect, Low Voltage Warning
                DCD     LLW_IRQHandler  ; Low Leakage Wakeup
                DCD     I2C0_IRQHandler  ; I2C0 interrupt
                DCD     I2C1_IRQHandler  ; I2C0 interrupt 25
                DCD     SPI0_IRQHandler  ; SPI0 interrupt
                DCD     SPI1_IRQHandler  ; SPI1 interrupt
                DCD     UART0_IRQHandler  ; UART0 status/error interrupt
                DCD     UART1_IRQHandler  ; UART1 status/error interrupt
                DCD     UART2_IRQHandler  ; UART2 status/error interrupt
                DCD     ADC0_IRQHandler  ; ADC0 interrupt
                DCD     CMP0_IRQHandler  ; CMP0 interrupt
                DCD     TPM0_IRQHandler  ; TPM0 fault, overflow and channels interrupt
                DCD     TPM1_IRQHandler  ; TPM1 fault, overflow and channels interrupt
                DCD     TPM2_IRQHandler  ; TPM2 fault, overflow and channels interrupt
                DCD     RTC_IRQHandler  ; RTC interrupt
                DCD     RTC_Seconds_IRQHandler  ; RTC seconds interrupt
                DCD     PIT_IRQHandler  ; PIT timer interrupt
                DCD     Reserved39_IRQHandler  ; Reserved interrupt 39
                DCD     USB0_IRQHandler  ; USB0 interrupt
                DCD     DAC0_IRQHandler  ; DAC interrupt
                DCD     TSI0_IRQHandler  ; TSI0 interrupt
                DCD     MCG_IRQHandler  ; MCG interrupt
                DCD     LPTimer_IRQHandler  ; LPTimer interrupt
                DCD     Reserved45_IRQHandler  ; Reserved interrupt 45
                DCD     PORTA_IRQHandler  ; Port A interrupt
                DCD     PORTD_IRQHandler  ; Port D interrupt

向量表很长，此处只截取一部分，这些向量表中的向量的默认名字就在这里指定了。开发人员只需将中断向量函数的具体功能实现即可。当然，有一些默认的dummy 异常handler已经有了，都是死循环：

; Dummy Exception Handlers (infinite loops which can be modified)

NMI_Handler     PROC
                EXPORT  NMI_Handler               [WEAK]
                B       .
                ENDP
HardFault_Handler\
                PROC
                EXPORT  HardFault_Handler         [WEAK]
                B       .
                ENDP
SVC_Handler     PROC
                EXPORT  SVC_Handler               [WEAK]
                B       .
                ENDP
PendSV_Handler  PROC
                EXPORT  PendSV_Handler            [WEAK]
                B       .
                ENDP
SysTick_Handler PROC
                EXPORT  SysTick_Handler           [WEAK]
                B       .
                ENDP

3. 启动代码

启动代码就放在Reset复位向量里面，因为Reset向量是MCU启动时候第一个进入到PC寄存器并开始执行的地方。

; Reset Handler

Reset_Handler   PROC
                EXPORT  Reset_Handler             [WEAK]
                IMPORT  SystemInit
                IMPORT  __main
                LDR     R0, =SystemInit
                BLX     R0
                LDR     R0, =__main
                BX      R0
                ENDP

启动代码首先将SystemInit的地址load到R0寄存器中，然后使用BLX跳转指令跳到SystemInit函数开始处去执行，执行完毕以后再执行main函数的执行。

接下去需要找SystemInit的代码查看。