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C COMPILER->8051 Specific Features->Absolute Object Location

C variables can be located at absolute locatations using the _at keyword.  This feature can be used to simply reserve space used by other programs such as a monitor, or to simplify access to memory mapped I/O.

For example:

long _xdata variable _at 0X8000;

will locate at address 0X8000 and access to it will write or read the four bytes at 0X8000, 0X8001, 0X8002 and 0X8003.

Another example:

struct tag8255 {
    unsigned char PortA;
    unsigned char PortB;
    unsigned char PortC;
    unsigned char I_O;

#define BASE8255        0xFF00
volatile struct tag8255 _xdata S8255 _at BASE8255;

    S8255.PortA = 0X88;    // write to address 0XFF00
    S8255.PortC = 0X98;     // write to address 0XFF01