Keil Software — C51 Compiler User’s Guide |
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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:
!memory_space" type 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 61 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. |
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The absolute address following _at_ must conform to the physical boundaries of the memory space for the variable. C51 checks for invalid address specifications.
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.
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Chapter 3. Language Extensions |
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The following example demonstrates how to locate several different variable types using the _at_ keyword.
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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idata struct link list _at_ 0x40; |
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xdata char text[256] _at_ 0xE000; |
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xdata int i1 |
_at_ 0x8000; |
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void main ( void ) { |
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link.next = (void *) 0; |
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i1 |
= 0x1234; |
text [0] |
= 'a'; |
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} |
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/* list at idata 0x40 */ /* array at xdata 0xE000 */ /* int at xdata 0x8000 */
Often, 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 idata struct link list; |
/* list at idata 0x40 */ |
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extern xdata char text[256]; |
/* array at xdata 0xE000 */ |
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extern xdata int i1; |
/* int at xdata 0x8000 */ |
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