C51: Correctly Declaring Variables with Memory Spaces

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Information in this article applies to:

• C51 All Versions

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QUESTION

When I declare a variable with a memory space, what format should I use?

[Data type] [Memory Space] Variable_name

or

[Memory space] [Data type] Variable_name

ANSWER

Either way is acceptable (see below for more information). For example:

int data var;
data int var;

both declare an integer in the data memory space. The place to be careful is in complex declarations involving pointers where multiple memory spaces may be required. For example:

data int *p;

is a generic pointer to an integer. The integer may be in any memory space, but the pointer is stored in the data memory space.

int data *p;

is a memory-specific pointer to an integer in the data memory space. The pointer is stored in the default memory space.

xdata int data *p;

is a pointer (stored in xdata) that points to an integer (stored in data).

Note that the older method of declaring variables:

[Memory space] [Data type] Variable_name

may not be supported in future versions of the compiler. For this reason,

[Data type] [Memory Space] Variable_name

is the preferred method of declaring variables and

[Data type] [Data type Memory Space] * [Variable Memory Space] Variable_name

is the preferred method of declaring pointers. For example:

int data * xdata p;

is a pointer (stored in xdata) that points to an integer (stored in data) using the new format. The following example code:

void main (void)
{
int data * xdata p;

p = 0x08;

*p = 0xAA;

while (1);
}

when compiled, yields the following:

             ; FUNCTION main (BEGIN)
                                           ; SOURCE LINE # 5
0000 900000      R     MOV     DPTR,#p
0003 7408              MOV     A,#08H
0005 F0                MOVX    @DPTR,A
                                           ; SOURCE LINE # 7
0006 E0                MOVX    A,@DPTR
0007 F8                MOV     R0,A
0008 7600              MOV     @R0,#00H
000A 08                INC     R0
000B 76AA              MOV     @R0,#0AAH
000D         ?C0001:
                                           ; SOURCE LINE # 9
000D 80FE              SJMP    ?C0001
             ; FUNCTION main (END)

As you can see, the compiler loads the address of p (the pointer) into the DPTR register and uses the MOVX instruction to access p.

SEE ALSO

• C51: Pointer Storage Size
• C51: Memory Classes for Strings and String Pointers
• C51: Locating Variables to Far Const Space
• C51: How to Use 256 Bytes of Data Space

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Last Reviewed: Thursday, February 25, 2021

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