1.2.2. Placing an Object at an Absolute Address: __at() and __atbit(). Altium TriCore

TASKING VX-toolset for TriCore User Guide

To declare and initialize a string in A0 memory: char __a0 string[] = "TriCore";

If you use the

__near

memory qualifier, the compiler generates faster access code for those (frequently used) variables. Pointers are always 32-bit.

Functions are by default allocated in ROM. In this case you can omit the a memory qualifier. You cannot use memory qualifiers for function return values.

Some examples of using pointers with memory qualifiers: int __near *p; /* pointer to int in __near memory

(pointer has 32-bit size) */ int __far *g; /* pointer to int in __far memory

(pointer has 32-bit size) */ g = p; /* the compiler issues a warning */

You cannot use memory qualifiers in structure declarations: struct S {

__near int i; /* put an integer in near

memory: Incorrect ! */

__far int *p; /* put an integer pointer in

far memory: Incorrect ! */

}

If a library function declares a variable in near memory and you try to redeclare the variable in far memory, the linker issues an error: extern int __near foo; /* extern int in near memory*/ int __far foo; /* int in far memory */

The usage of the variables is always without a storage specifier: char __near example; example = 2;

The generated assembly would be: mov16 d15,2 st.b example,d15

All allocations with the same storage specifiers are collected in units called 'sections'. The section with the

__near

attribute will be located within the first 16 kB of each 256 MB block.

1.2.2. Placing an Object at an Absolute Address: __at() and __atbit()

Just like you can declare a variable in a specific part of memory (using memory qualifiers), you can also place an object at an absolute address in memory.

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C Language

With the attribute

__at()

you can specify an absolute address. The address is a 32-bit linear address.

If you use this keyword on

__bit

objects, the address is a bit address.

Examples

unsigned char Display[80*24] __at( 0x2000 );

The array

Display

is placed at address 0x2000. In the generated assembly, an absolute section is created. On this position space is reserved for the variable

Display

.

int i __at(0x1000) = 1;

Restrictions

Take note of the following restrictions if you place a variable at an absolute address:

• The argument of the

__at()

attribute must be a constant address expression.

• You can place only global variables at absolute addresses. Parameters of functions, or automatic variables within functions cannot be placed at absolute addresses.

• You cannot place functions at absolute addresses.

• A variable that is declared extern

, is not allocated by the compiler in the current module. Hence it is not possible to use the keyword

__at()

on an external variable. Use

__at()

at the definition of the variable.

• You cannot place structure members at an absolute address.

• Absolute variables cannot overlap each other. If you declare two absolute variables at the same address, the assembler and/or linker issues an error. The compiler does not check this.

Declaring a bit variable with __atbit()

If you have defined a 32-bit base variable ( int

, long

) you can declare a single bit of that variable as a bit variable with the keyword

__atbit()

. The syntax is:

__atbit(name,offset) where, name is the name of an integer variable in which the bit is located. offset (range 0-31) is the bit-offset within the variable.

If you have defined an absolute integer variable with the keyword

__at()

, you can declare a single bit of that variable as an absolute bit variable with

__atbit()

.

The following restrictions apply:

• This keyword can only be applied to

__bit

type symbols.

• When a variable is

__atbit()

qualified it represents an alias of a bit in another variable. Therefore, it cannot be initialized.

• You can only use the

__atbit()

qualifier on variables which have either a global scope or file scope.

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