fixed registers in mips

[Eric Fisher]

Hello,

I'd like to ask a question about target macros of registers. In
mips.h, the fixed registers are defined as follows:

/* By default, fix the kernel registers ($26 and $27), the global
   pointer ($28) and the stack pointer ($29).  This can change
   depending on the command-line options.

   Regarding coprocessor registers: without evidence to the contrary,
   it's best to assume that each coprocessor register has a unique
   use.  This can be overridden, in, e.g., override_options() or
   CONDITIONAL_REGISTER_USAGE should the assumption be inappropriate
   for a particular target.  */

#define FIXED_REGISTERS							\
{									\
  1, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,			\
  0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 1, 1, 1, 0, 0,			\
  0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,			\
  0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,			\
  0, 0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 1, 1, 1, 1, 1,			\
  /* COP0 registers */							\
  1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,			\
  1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,			\
  /* COP2 registers */							\
  1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,			\
  1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,			\
  /* COP3 registers */							\
  1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,			\
  1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1			\
}

I'd like to know why mips doesn't define $30 and $31 as fix registers?
And when should they be defined true?

Thanks very much.

Eric.