Referring to Variables in Memory

For instructions that take a memory operand (such as the lwz instruction), follow these rules when using a C-language variable as an operand:

• Global variables and function parameters must:

  Be declared/assigned with the register.

  • Adhere to the syntax below when used as operands, so they are treated as an offset from zero.

    instrName  regName , 0(globalVarName ) 

    or

    instrName  regName , 0(parameterName ) 

Using C Variables with Instructions that Take a Memory Operand shows inline assembly language statements that correctly use C-language variables as operands in instructions that take a memory operand.

int my_global asm("r14")= 25; /* global variable */
int my_loc = 1;   /* my_loc is on memory, not in a register */
asm void red(register int *my_param)
{
lwz r4, 0(my_global) /* line 9 */
lwz r4, 0(my_param) /* line 10 */
lwz r4, my_loc /* line 11 */
lwz r4, my_loc(SP) 
blr
}

In Using C Variables with Instructions that Take a Memory Operand:

• The statement on line 9 is correct.
  • The operand is fully expressed (because it is an offset from zero).
  • The argument my_global is in a register.
• The statement on line 10 is correct for the same reasons as stated above.
• The statement on line 11 is correct.

  The CodeWarrior inline assembler automatically adds the contents of the SP register to local variable my_loc .

• The statement on line 12 is correct.

  Note that statements 11 and 12 are equivalent.

  As mentioned above, the inline assembler automatically adds the SP register to local variable my_loc , so explicitly including (SP) is redundant.