keil C51绝对地址访问

      在利用keil进行8051单片机编程的时,常常需要进行绝对地址进行访问.特别是对硬件操作,如DA AD 采样 ,LCD 液晶操作,打印操作.等等.
C51提供了三种访问绝对地址的方法:
1. 绝对宏:
在程序中,用“#include<absacc.h>”即可使用其中定义的宏来访问绝对地址,包括:
CBYTE、XBYTE、PWORD、DBYTE、CWORD、XWORD、PBYTE、DWORD
具体使用可看一看absacc.h便知

例如:
rval=CBYTE[0x0002];指向程序存贮器的0002h地址
rval=XWORD [0x0002];指向外RAM的0004h地址
2. _at_关键字
直接在数据定义后加上_at_ const即可,但是注意:
(1)绝对变量不能被初使化;
(2)bit型函数及变量不能用_at_指定。

例如:

idata struct link list _at_ 0x40;指定list结构从40h开始。
xdata char text[25b] _at_0xE000;指定text数组从0E000H开始
提示:如果外部绝对变量是I/O端口等可自行变化数据,需要使用volatile关键字进行描述,请参考absacc.h。

3. 连接定位控制
此法是利用连接控制指令code xdata pdata \data bdata对“段”地址进行,如要指定某具体变量地址,则很有局限性,不作详细讨论。

附:(c51)
/*-------------------------------------------------------------------------- 
ABSACC.H 

Direct access to 8051, extended 8051 and Philips 80C51MX memory areas. 
Copyright (c) 1988-2002 Keil Elektronik GmbH and Keil Software, Inc. 
All rights reserved. 
--------------------------------------------------------------------------*/ 

#ifndef __ABSACC_H__ 
#define __ABSACC_H__ 

#define CBYTE ((unsigned char volatile code *) 0) 
#define DBYTE ((unsigned char volatile data *) 0) 
#define PBYTE ((unsigned char volatile pdata *) 0) 
#define XBYTE ((unsigned char volatile xdata *) 0) 

#define CWORD ((unsigned int volatile code *) 0) 
#define DWORD ((unsigned int volatile data *) 0) 
#define PWORD ((unsigned int volatile pdata *) 0) 
#define XWORD ((unsigned int volatile xdata *) 0) 


#ifdef __CX51__ 
#define FVAR(object, addr) (*((object volatile far *) (addr))) 
#define FARRAY(object, base) ((object volatile far *) (base)) 
#define FCVAR(object, addr) (*((object const far *) (addr))) 
#define FCARRAY(object, base) ((object const far *) (base)) 
#else 
#define FVAR(object, addr) (*((object volatile far *) ((addr)+0x10000L))) 
#define FCVAR(object, addr) (*((object const far *) ((addr)+0x810000L))) 
#define FARRAY(object, base) ((object volatile far *) ((base)+0x10000L)) 
#define FCARRAY(object, base) ((object const far *) ((base)+0x810000L)) 
#endif 

#endif 

附:(c166) 

/*-------------------------------------------------------------------------- 
ABSACC.H 

Direct access to 166 memory areas for C166/EC++ Version 5. 
Copyright (c) 1992-2004 Keil Elektronik GmbH and Keil Software, Inc. 
All rights reserved. 
--------------------------------------------------------------------------*/ 

#ifndef __ABSACC_H__ 
#define __ABSACC_H__ 

#if (__MODEL__ == 0) 
#define MVAR(object, addr) (*((object volatile *) (addr))) 
#define MARRAY(object, base) ((object volatile *) (base)) 
#else 
#define MVAR(object, addr) (*((object volatile far *) (addr))) 
#define MARRAY(object, base) ((object volatile far *) (base)) 

#define HVAR(object, addr) (*((object volatile huge *) (addr))) 
#define HARRAY(object, base) ((object volatile huge *) (base)) 
#define XVAR(object, addr) (*((object volatile xhuge *) (addr))) 
#define XARRAY(object, base) ((object volatile xhuge *) (base)) 
#endif 

#endif 

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