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