51单片机keil提供的启动代码解析（STARTUP.A51）

1.由Keil C生成的启动代码
程序清单1.1 AT89C51启动代码

$NOMOD51    //不使用51默认的特殊功能寄存器

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

; 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.

; 说的很清楚，该代码会在处理器上电复位后首先执行

;

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

;

;     A51 STARTUP.A51

;

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

; BL51 invocation:

;

;     BL51 , STARTUP.OBJ

; BL51是Keil使用的链接器(Linker),这是命令行的使用格式

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

;

; User-defined Power-On Initialization of Memory

; 下面将定义一些与存储器初始化相关的符号常量，与ICCAVR相比，Keil把符号常量显式声明出来更容易理解。

; With the following EQU statements the initialization of memory

; at processor reset can be defined:

;

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

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

;IDDATALEN指片内128字节RAM，I指internal

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

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

; XDATA指片外SRAM空间，X指external

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

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

; PDATA指程序存储器，p指program

; 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.

; 注意：8051 CPU的寄存器空间和存储器空间并不是统一编址的，而是通过指令来区分，所以不同的存储区的地址可能重叠(overlap)

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

;

; Reentrant Stack Initilization

;

; The following EQU statements define the stack pointer for reentrant

; functions and initialized it:

; Keil C默认情况不是用堆栈来传递参数的，所以造成函数不可重入(可重入概念参看线程安全和可重入函数一节)。Keil要求用户显示声明函数是否具有可重入属性，以便为C函数调用初始化栈。

; Stack Space for reentrant functions in the SMALL model.

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

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

;

; Stack Space for reentrant functions in the LARGE model.    

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

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

;

; Stack Space for reentrant functions in the COMPACT model.  

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

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

;不同内存模式下的堆栈。Keil 编译器中有三种模式设置：

Small：所有的变量都放在内部RAM区

Compact：所有变量在默认情况下都会放在外部RAM的低256字节中(可由R0寻址)

Large：所有变量都放在外部RAM中(DPTR寻址)

这是由51处理器繁多的寻址模式导致的，不同的寻址模式有不同的效率

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

;

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

;

; 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.

;

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

;

PPAGE           EQU     0       ; define PPAGE number.

;

PPAGE_SFR       DATA    0A0H    ; SFR that supplies uppermost address byte

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

;

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

; Standard SFR Symbols

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)

                PUBLIC ?C_STARTUP

                CSEG    AT      0   //代码段的起始点

?C_STARTUP:     LJMP    STARTUP1    //转跳到STARTUP1程序区中去。

                RSEG    ?C_C51STARTUP

STARTUP1:

//IF ENDIF是条件编译命令，只有当条件为真时，才编译代码

IF IDATALEN <> 0   //如果有IDATA，则把IDATA区的数据清零，类似于清BSS区

                MOV     R0,#IDATALEN - 1

                CLR     A

IDATALOOP:      MOV     @R0,A

                DJNZ    R0,IDATALOOP

ENDIF

//如果有外部数据区，则把外部数据区中从XDATASTART到XDATASTART+ XDATALEN的区域清零

//由于51的储存区类型和编译模型比较多，下面的代码就是根据不同的配置把相应的区域清零

IF XDATALEN <> 0

                MOV     DPTR,#XDATASTART

                MOV     R7,#LOW (XDATALEN)

IF (LOW (XDATALEN)) <> 0

                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

                MOV     R0,#LOW (PDATASTART)

                MOV     R7,#LOW (PDATALEN)

                CLR     A

PDATALOOP:      MOVX    @R0,A

                INC     R0

                DJNZ    R7,PDATALOOP

ENDIF

IF IBPSTACK <> 0

EXTRN DATA (?C_IBP)

                MOV     ?C_IBP,#LOW IBPSTACKTOP

ENDIF

IF XBPSTACK <> 0

EXTRN DATA (?C_XBP)

                MOV     ?C_XBP,#HIGH XBPSTACKTOP

                MOV     ?C_XBP+1,#LOW XBPSTACKTOP

ENDIF

IF PBPSTACK <> 0

EXTRN DATA (?C_PBP)

                MOV     ?C_PBP,#LOW PBPSTACKTOP

ENDIF

//该处是初始化堆栈指针

                MOV     SP,#?STACK-1

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

; EXTRN CODE (?B_SWITCH0)

;               CALL    ?B_SWITCH0      ; init bank mechanism to code bank 0