If you need to place a local variable in a specific register, you can use this construct:

register type var asm("reg") [= init];

Typically, these variables are used in inline assembler statements.

Alternatively, you can use the @ operator and the #pragma location directive like this: #pragma location=r9 or @r9, see Controlling data and function placement in memory. Note that the compiler requires that the register is free to be used throughout the lifetime of the variable.

For example:

int test(void)
{
  register int myVar asm("r10");
  asm volatile("// do something with %0" : "=r"(myVar));
  return myVar;
}