C51 中 XBYTE、CBYTE、DBYTE的定义在 ABSACC.H 中。
源文件位置：C:Keil952C51INC

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ABSACC.H Direct access to 8051, extended 8051 and NXP 8051MX memory areas.
Copyright © 1988-2010 Keil Elektronik GmbH and ARM Germany GmbH
All rights reserved.
--------------------------------------------------------------------------*/ #ifndef ABSACC_H
#define ABSACC_H #define CBYTE ((unsigned char volatile code *) 0)
#define DBYTE ((unsigned char volatile data *) 0)
#if !defined (CX2)
#define PBYTE ((unsigned char volatile pdata *) 0)
#endif
#define XBYTE ((unsigned char volatile xdata *) 0) #define CWORD ((unsigned int volatile code *) 0)
#define DWORD ((unsigned int volatile data *) 0)
#if !defined (CX2)
#define PWORD ((unsigned int volatile pdata *) 0)
#endif
#define XWORD ((unsigned int volatile xdata *) 0)