在利用keil進(jìn)行8051單片機(jī)編程的時(shí),常常需要進(jìn)行絕對(duì)地址進(jìn)行訪問.特別是對(duì)硬件操作,如DA AD采樣,LCD液晶操作,打印操作.等等.
C51提供了三種訪問絕對(duì)地址的方法：
1.絕對(duì)宏：
在程序中，用“＃include”即可使用其中定義的宏來訪問絕對(duì)地址，包括：
CBYTE、XBYTE、PWORD、DBYTE、CWORD、XWORD、PBYTE、DWORD
具體使用可看一看absacc.h便知

例如：

＃include


#defineADstartXBYTE[0x8000]//總線方式訪問AD

#defineADLXBYTE[0x8000]

#defineADHXBYTE[0x8001]



rval=CBYTE[0x0002];指向程序存貯器的0002h地址
rval=XWORD [0x0002];指向外RAM的0004h地址



2. _at_關(guān)鍵字
直接在數(shù)據(jù)定義后加上_at_ const即可，但是注意：
(1)絕對(duì)變量不能被初使化；
(2)bit型函數(shù)及變量不能用_at_指定。

例如：


idata struct link list _at_ 0x40;指定list結(jié)構(gòu)從40h開始。
xdata char text[25b] _at_0xE000；指定text數(shù)組從0E000H開始
提示：如果外部絕對(duì)變量是I/O端口等可自行變化數(shù)據(jù)，需要使用volatile關(guān)鍵字進(jìn)行描述，請(qǐng)參考absacc.h。

3.連接定位控制
此法是利用連接控制指令code xdata pdata data bdata對(duì)“段”地址進(jìn)行，如要指定某具體變量地址，則很有局限性，不作詳細(xì)討論。

附:(c51)


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

Direct access to 8051, extended 8051 and Philips80C51MX 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