Global Variable Address Modifier (@address)

You can assign global variables to specific addresses with the global variable address modifier. These variables are called absolute variables. They are useful for accessing memory mapped I/O ports and have the following syntax:

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

where:

<TypeSpec> is the type specifier, e.g., int, char
<Declarator> is the identifier of the global object, e.g., i, glob
<Address> is the absolute address of the object, e.g., 0xff04, 0x00+8
<Initializer> is the value to which the global variable is initialized.

A segment is created 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. Otherwise, there would be a linker error (overlapping segments). 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. An alternate way to assign global variables to specific addresses is setting the PLACEMENT section in the Linker parameter file (see the following listing).

Listing: Assigning global variables to specific addresses

#pragma DATA_SEG [__SHORT_SEG] <segment_name>

An older method of accomplishing this is shown in the following listing.

Listing: Another means of assigning global variables to specific addresses

<segment_name> INTO  READ_ONLY <Address> ;

The following listing is an example using correctly and incorrectly the global variable address modifier and the following listing is a possible PRM file that corresponds with the example Listing.

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 Linker 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