Global Variable Address Modifier (@address)

Use the global variable address modifier to assign global variables to specific addresses and to access memory-mapped I/O ports. These variables, called absolute variables, have the following syntax:

Declaration = <TypeSpec> <Declarator>[@<Address>|@"<Section>"]

              [= <Initializer>];

<TypeSpec> is the type specifier, for example, int, char

<Declarator> is the identifier of the global object, for example, i, glob

<Address> is the absolute address of the object, for example, 0xff04, 0x00+8

<Initializer> is the value to which the global variable is initialized.

The frontend creates a segment for each global object specified with an absolute address. This address must not be inside any address range in the SECTIONS entries of the link parameter file, or a linker error (overlapping segments) occurs. If the specified address has a size greater than that used for addressing the default data page, pointers pointing to this global variable must be __far. The following listing shows an alternate way to assign global variables to specific addresses.

Listing: Assigning Global Variables to Specific Addresses

#pragma DATA_SEG [__SHORT_SEG] <segment_name>

This sets the PLACEMENT section in the linker parameter file. An older method of accomplishing this is shown in the following listing.

Listing: Another Method of Assigning Global Variables to Specific Addresses

<segment_name> INTO  READ_ONLY <Address> ;

Use the LINEAR attribute to specify linear addresses in extended memory. Since extended memory is Flash memory, use const as the global variable.

Listing: Using LINEAR to Specify an Address

const char const_data @ LINEAR 0x01A6AB /*0x06A6AB*/ = 0x30;

Listing: Using the Global Variable Address Modifier shows a correct and incorrect example of using the global variable address modifier and Listing: Corresponding Link Parameter File Settings (PRM file) shows a possible PRM file that corresponds with Listing: Using the Global Variable Address Modifier.

Listing: Using the Global Variable Address Modifier

int glob @0x0500 = 10; // OK, global variable "glob" is
                       // at 0x0500, initialized with 10

void g() @0x40c0;      // error (the object is a function)

void f() {

  int i @0x40cc;       // error (the object is a local variable)

}

Listing: Corresponding Link Parameter File Settings (PRM file)

/* the address 0x0500 of "glob" must not be in any address
   range of the SECTIONS entries */

SECTIONS

    MY_RAM    = READ_WRITE 0x0800 TO 0x1BFF;

    MY_ROM    = READ_ONLY  0x2000 TO 0xFEFF;

    MY_STACK  = READ_WRITE 0x1C00 TO 0x1FFF;

    MY_IO_SEG = READ_WRITE 0x0400 TO 0x4ff;

END

PLACEMENT

    IO_SEG       INTO  MY_IO_SEG;

    DEFAULT_ROM  INTO  MY_ROM;

    DEFAULT_RAM  INTO  MY_RAM;

    SSTACK       INTO  MY_STACK;

END