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Cx51 User's Guide
Cx51 Introduction Compiling Programs 
Language Extensions 
Keywords Memory Areas Program Memory Internal Data Memory External Data Memory Far Memory SFR Memory Memory Models Small Model Compact Model Large Model Memory Types bdata code data far idata pdata 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 Generic Pointers Memory-Specific Pointers Pointer Conversions Abstract Pointers Function Declarations Parameters and the Stack Parameters and Registers Return Values Memory Models Register Banks Register Bank Access Interrupt Functions Reentrant Functions Alien (PL/M-51) Function Real-Time Function Tasks Preprocessor Advanced Programming 8051 Derivatives Error Messages Library Reference AppendixMemory Models
The memory model determines the default memory type to use for function arguments, automatic variables, and declarations that include no explicit memory type. The C51 Compiler provides three memory models:
The following table lists the default memory areas used for each memory model.
| Memory Model | Parameters & Automatic Variables | Default Global Variables | Default Constant Variables | Default Pointer Definitions | Default Pointer Size |
|---|---|---|---|---|---|
| SMALL | data | data | data | * | 3 bytes |
| COMPACT | pdata | pdata | pdata | * | 3 bytes |
| LARGE | xdata | xdata | xdata | * | 3 bytes |
You may specify the memory model on the compiler command line using the SMALL, COMPACT, or LARGE directives or using #pragma in the source file. You may also specify the memory model on a function-by-function basis by adding the memory model to the function declaration.
Note
- Except in very special select applications, the SMALL memory model generates the fastest, most efficient code.
You may override the default memory type imposed by the memory model by explicitly declaring a variable with a memory type specifier.
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