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;
where:
memory_space is the memory space for the variable. If missing from the
declaration, the default memory space is used. Refer to
“Memory Models” on page 94 for more information about
the default memory space.
type is the variable type.
variable_name is the variable name.
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
3
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; /* list at idata 0x40 */
char xdata text[256] _at_ 0xE000; /* array at xdata 0xE000 */
int xdata i1 _at_ 0x8000; /* int at xdata 0x8000 */
void main ( void ) {
link.next = (void *) 0;
i1 = 0x1234;
text [0] = 'a';
}
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
{
struct link idata *next;
char code *test;
};
extern struct link idata list; /* list at idata 0x40 */
extern char xdata text[256]; /* array at xdata 0xE000 */
extern int xdata i1; /* int at xdata 0x8000 */
The _at_ Keyword
The third method of accessing absolute memory locations is to use the _at_
keyword when you declare variables in your C source files. 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; /* list at idata 0x40 */
char xdata text[256] _at_ 0xE000; /* array at xdata 0xE000 */
int xdata i1 _at_ 0x8000; /* int at xdata 0x8000 */
void main ( void ) {
link.next = (void *) 0;
i1 = 0x1234;
text [0] = 'a';
}
Refer to “Absolute Variable Location” on page 104 for more information about
the _at_ keyword.
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.