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

LJMP ?C_START

END

从上面一段代码，我们已经发现了一些与ATmega8初始化代码相同的操作：初始化堆栈和bss区。由于AT89C51的存储空间和寻址模式比ATmega8复杂，所以初始代码中增加了许多根据不同配置而进行初始化的代码