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Chapter 3. Language Extensions |
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Absolute Variable Location
Variables may be located at absolute memory locations in your C program source modules using the _at_ keyword. The usage for this feature is:
type memory_space variable_name _at_ constant;
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where: |
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memory_space |
is the memory space for the variable. If missing from the |
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declaration, the default memory space is used. Refer to |
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“Memory Models” on page 94 for more information about |
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the default memory space. |
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type |
is the variable type. |
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variable_name |
is the variable name. |
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constant |
is the address at which to locate the variable. |
The absolute address following _at_ must conform to the physical boundaries of the memory space for the variable. The Cx51 compiler checks for invalid address specifications.
NOTE
If you use the _at_ keyword to declare a variable that accesses an XDATA peripheral, you may require the volatile keyword to ensure that the C compiler does not optimize out necessary memory accesses.
The following restrictions apply to absolute variable location:
1.Absolute variables cannot be initialized.
2.Functions and variables of type bit cannot be located at an absolute address.
Keil Software — Cx51 Compiler User’s Guide |
105 |
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The following example demonstrates how to locate several different variable types using the _at_ keyword.
struct link
{
struct link idata *next; char code *test; };
struct link list idata |
_at_ 0x40; |
char xdata text[256] |
_at_ 0xE000; |
int xdata i1 |
_at_ 0x8000; |
void main ( void ) { link.next = (void *) 0; i1 = 0x1234; text [0] = 'a';
}
/* list at idata 0x40 */ /* array at xdata 0xE000 */ /* int at xdata 0x8000 */
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You may wish to declare your variables in one source module and access them in another. Use the following external declarations to access the _at_ variables defined above in another source file.
struct link |
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{ |
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struct link idata *next; |
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char |
code *test; |
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}; |
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extern struct link idata list; |
/* list at idata 0x40 */ |
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extern char xdata text[256]; |
/* array at xdata 0xE000 */ |
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extern int xdata i1; |
/* int at xdata 0x8000 */ |
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