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C251 User's Guide
C251 Introduction Compiling Programs 
Language Extensions 
Keywords Memory Areas Program Memory Internal Data Memory External Data Memory Near Memory Far Memory Huge Memory SFR Memory Memory Models Tiny Model XTiny Model Small Model XSmall Model Large Model Memory Types Implicit Memory Types bdata code data ebdata far huge idata near xdata Data Types Special Function Registers sfr sfr16 sbit Bit-Addressable Objects bit Standard C Types void char int enum float double Type Modifiers signed unsigned short long Type Qualifiers const volatile Storage Classes auto register extern static Absolute Variable Location Pointers Memory Type in Pointer Declarations Pointer Conversions Function Declarations Memory Models Register Banks Interrupt Functions Reentrant Functions Alien (PL/M-51) Function Real-Time Function Tasks Preprocessor Advanced Programming Error Messages Library Reference AppendixLarge Model
In the Large model, all variables, by default, reside in external data memory (which may be up to 64K Bytes). This is the same as if they were explicitly declared using the xdata memory type specifier.
The data pointer (DPTR) is used to address external memory. It is important to note that memory access through the data pointer is inefficient and slow, especially on variables that are two or more bytes long. This type of data access mechanism generates more code than the XTiny Model or the XSmall Model.
Therefore try to use the XTiny or XSmall memory model instead of the Large. Using the HOLD directive, you may direct variables above a specific object size to alternative memory areas.
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