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Memory Types

The Cx51 Compiler provides access to all 8051 memory areas. Variables may be explicitly assigned to a specific memory space (by including a memory type specifier in the declaration) or implicitly assigned (based on the memory model).

The following table summarizes the memory type specifiers you may use.

Memory Type Description
code Program memory (64 KBytes); accessed by opcode MOVC @A+DPTR.
data Directly addressable internal data memory; fastest access to variables (128 bytes).
idata Indirectly addressable internal data memory; accessed across the full internal address space (256 bytes).
bdata Bit-addressable internal data memory; supports mixed bit and byte access (16 bytes).
xdata External data memory (64 KBytes); accessed by opcode MOVX @DPTR.
far Extended RAM and ROM memory spaces (up to 16MB); accessed by user defined routines or specific chip extensions (Philips 80C51MX, Dallas 390).
pdata Paged (256 bytes) external data memory; accessed by opcode MOVX @Rn.

As with the signed and unsigned attributes, you may include memory type specifiers in the variable declaration. For example:

char data var1;
char code text[] = "ENTER PARAMETER:";
unsigned long xdata array[100];
float idata x,y,z;
unsigned int pdata dimension;
unsigned char xdata vector[10][4][4];
char bdata flags;

Note

  • For compatibility with previous versions of the C51 Compiler, you may specify the memory area before the data type. For example, the following two declarations are equivalent:

    data char x;   // Old-Style Memory Type Declaration
    char data x;   // New-Style Memory Type Declaration
    

    Nonetheless, this feature should not be used in new programs because it may not be supported in future versions of the Cx51 Compiler. Be careful when using the old C51 syntax with memory-specific pointers. For example, the following two declarations are equivalent:

    data char *x;   // Old-Style Memory Type Declaration
    char *data x;   // New-Style Memory Type Declaration
    
  • Accessing the internal data memory is considerably faster than accessing the external data memory. For this reason, place frequently used variables in internal data memory. Place larger, less frequently used variables in external data memory.

If no memory type is specified for a variable, the compiler implicitly locates the variable in the default memory space determined by the memory model: SMALL, COMPACT, or LARGE. Function arguments and automatic variables that cannot be located in registers are also stored in the default memory area. Refer to Memory Models for more information.

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