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        让我们先来看看STARTUP.A51文件（默认设置）吧！根据源文件中的说明文字，我们很容易分析出STARTUP.A51文件的工作方式和作用！让我们把它先汉化一下看看！
         $NOMOD51;Ax51宏汇编器控制命令：禁止预定义的8051

        ;------------------------------------------------------------------------------

        ; This file is part of the C51 Compiler package

        ; Copyright (c) 1988-2002 Keil Elektronik GmbH and Keil Software, Inc.

        ;------------------------------------------------------------------------------

         ; STARTUP.A51: This code is executed after processor reset.

; STARTUP.A51: STARTUP.A51文件所生成的代码将在单片机复位后被执行！

        ; To translate this file use A51 with the following invocation:

; 将按照下面的命令行语句调用A51编译器进行编译产生目标文件

        ; A51 STARTUP.A51

        ;

        ; To link the modified STARTUP.OBJ file to your application use the following

        ; BL51 invocation:

; 将按照下面的命令行语句调用BL51连接器把STARTUP.OBJ定位连接到您的程序代码中

; BL51 <your object file list>, STARTUP.OBJ <controls>

         ;

        ;------------------------------------------------------------------------------

        ;

        ; User-defined Power-On Initialization of Memory

 ; 自定义上电后需要初始化的储存区域

        ; With the following EQU statements the initialization of memory

         ; at processor reset can be defined:

; 使用下列EQU伪指令定义初始化的存储区域在单片机复位后定义生效

        ;

        ; the absolute start-address of IDATA memory is always 0

        IDATALEN EQU 80H                     ; the length of IDATA memory in bytes.

; IDATA（间接寻址区）其起始地址固定为0

;IDATALEN用于指定需要初始化的 IDATA区长度（以字节为单位）*

        XDATASTART EQU 0H                    ; the absolute start-address of XDATA memory 

;XDATA （外部直接寻址区） XDATASTART用于指定需要初始化的XDATA区起始地址

        XDATALEN EQU 0H                         ; the length of XDATA memory in bytes.

;XDATALEN 用于指定需要初始化的XDATA区长度（以字节为单位）*

        PDATASTART EQU 0H                     ; the absolute start-address of PDATA memory

;PDATA（页寻址区）PDATASTART用于指定需要初始化的PDATA区起始地址

        PDATALEN EQU 0H                          ; the length of PDATA memory in bytes.

;PDATALEN用于指定需要初始化的;PDATA区长度（以字节为单位）*

        ; Notes: The IDATA space overlaps physically the DATA and BIT areas of the

        ; 8051 CPU. At minimum the memory space occupied from the C51

        ; run-time routines must be set to zero.
;注释：8051中 IDATA 区物理上已经包括了DATA区（直接寻址区）以及 BIT区（位寻址区）。C51（库）占用了最小化内存空间，运行时程序需要把它设为0
        ;------------------------------------------------------------------------------

        ;

        ; Reentrant Stack Initilization

        ; 重入堆栈初始化

        ; The following EQU statements define the stack pointer for reentrant

        ; functions and initialized it:

 ; 下面的EQU语句定义重入函数的堆栈指针并初始化它

        ; Stack Space for reentrant functions in the SMALL model.

 ; SMALL模式下的重入函数的堆栈空间

        IBPSTACK EQU 0                     ; set to 1 if small reentrant is used.

;如果再SMALL模式下使用重入则设为1

        IBPSTACKTOP EQU 0FFH+1                     ; set top of stack to highest location+1.

;设置堆栈顶最高位置+1

        ;

        ; Stack Space for reentrant functions in the LARGE model.

; LARGE模式下的重入函数的堆栈空间        

        XBPSTACK EQU 0                     ; set to 1 if large reentrant is used.

;如果再LARGE模式下使用重入则设为1

        XBPSTACKTOP EQU 0FFFFH+1             ; set top of stack to highest location+1.

;设置堆栈顶最高位置+1

        ;

        ; Stack Space for reentrant functions in the COMPACT model.

; COMPACT模式下的重入函数的堆栈空间

        PBPSTACK EQU 0                     ; set to 1 if compact reentrant is used.

;如果再COMPACT模式下使用重入则设为1

        PBPSTACKTOP EQU 0FFFFH+1                    ; set top of stack to highest location+1.

;设置堆栈顶最高位置+1

        ;

        ;------------------------------------------------------------------------------

        ;

        ; Page Definition for Using the Compact Model with 64 KByte xdata RAM

; 使用COMPACT模式时为64KB的XDATA RAM定义页

        ; The following EQU statements define the xdata page used for pdata

        ; variables. The EQU PPAGE must conform with the PPAGE control used

        ; in the linker invocation.

 ; 下面的EQU语句定义PDATA变量的使用了XDATA页

        PPAGEENABLE EQU 0                     ; set to 1 if pdata object are used.

        ;

        PPAGE EQU 0                     ; define PPAGE number.

;定义页号;如果使用PDATA页则设为1

        ;

        PPAGE_SFR DATA 0A0H                     ; SFR that supplies uppermost address byte

;SFR的最高地址字节

        ; (most 8051 variants use P2 as uppermost address byte)

; (大多数8051变量要用P2的最高地址字节)

        ;------------------------------------------------------------------------------
; Standard SFR Symbols ; 标准SFR符号

        ACC DATA 0E0H

        B DATA 0F0H

        SP DATA 81H

        DPL DATA 82H

        DPH DATA 83H

        NAME ?C_STARTUP

        ?C_C51STARTUP SEGMENT CODE

        ?STACK SEGMENT IDATA

                RSEG ?STACK

                DS 1

                EXTRN CODE (?C_START)

;外部代码(这个标号将代表用户程序的启始地址)

                PUBLIC ?C_STARTUP

;给外部使用的符号

                CSEG AT 0

;在code段的0地址处放以下代码（使用AT指令进行绝对地址的定位）

       ?C_STARTUP: LJMP STARTUP1

        RSEG ?C_C51STARTUP

        STARTUP1:

        IF IDATALEN <> 0

;如果长度大于1则初始化IDATA

                MOV R0,#IDATALEN - 1

                CLR A

        IDATALOOP: MOV @R0,A

                DJNZ R0,IDATALOOP

        ENDIF

        IF XDATALEN <> 0

;如果长度大于1则初始化XDATA

                MOV DPTR,#XDATASTART

                MOV R7,#LOW (XDATALEN)

        IF (LOW (XDATALEN)) <> 0

;预置初始化时的外循环次数到R6

                MOV R6,#(HIGH (XDATALEN)) +1

        ELSE

                MOV R6,#HIGH (XDATALEN)

        ENDIF

                CLR A

        XDATALOOP: MOVX @DPTR,A

                INC DPTR

                DJNZ R7,XDATALOOP

                DJNZ R6,XDATALOOP

        ENDIF

        IF PPAGEENABLE <> 0

                MOV PPAGE_SFR,#PPAGE

        ENDIF

        IF PDATALEN <> 0

;如果长度大于1则初始化PDATA

                MOV R0,#LOW (PDATASTART)

                MOV R7,#LOW (PDATALEN)

                CLR A

        PDATALOOP:

                MOVX @R0,A

                INC R0

                DJNZ R7,PDATALOOP

        ENDIF

        IF IBPSTACK <> 0

;SMALL模式下使用重入函数时要设置的堆栈

        EXTRN DATA (?C_IBP)
                MOV ?C_IBP,#LOW IBPSTACKTOP

        ENDIF

        IF XBPSTACK <> 0

;COMPACT模式下使用重入函数时要设置的堆栈

        EXTRN DATA (?C_XBP)
                MOV ?C_XBP,#HIGH XBPSTACKTOP

                MOV ?C_XBP+1,#LOW XBPSTACKTOP

        ENDIF

        IF PBPSTACK <> 0

;LARGE模式下使用重入函数时要设置的堆栈

        EXTRN DATA (?C_PBP)

                MOV ?C_PBP,#LOW PBPSTACKTOP

        ENDIF

                MOV SP,#?STACK-1

        ; This code is required if you use L51_BANK.A51 with Banking Mode 4

; 如果你的程序使用了Mode 4 程序分组技术（BANKING）请启用下面的程序代码; EXTRN CODE (?B_SWITCH0)

        ; CALL ?B_SWITCH0

        ; init bank mechanism to code bank 0

;程序从第一个块（bank0）开始执行

        LJMP ?C_START ;从这里跳到你的程序入口
        END