Description

   The aligned and unaligned qualifiers to this directive allow
   you to override the compiler's alignment assumptions. Using the
   directive for this purpose in certain cases can produce more
   efficient code.

   The read and written qualifiers to this directive allow implicit
   reads and writes of registers to be declared. They are generally
   used to declare the register usage of called routines and are
   useful for documenting your program.

   With one exception, the .SET_REGISTERS directive remains in
   effect (ensuring proper alignment processing) until the routine
   ends, unless you change the value in the register. The exception
   can occur under certain conditions when a flow path joins the
   code following a .SET_REGISTERS directive.

   The following example illustrates such an exception. R2 is
   declared aligned, and at a subsequent label, 10$, which is
   before the next write access to the register, a flow path joins
   the code. R2 will be treated as unaligned following the label,
   because it is unaligned from the other path.

           INCL R2          ; R2 is now unaligned
            .
            .
            .
           BLBC R0, 10$
            .
            .
            .
           MOVL R5, R2
           .SET_REGISTERS ALIGNED=R2
           MOVL R0, 4(R2)
     10$:  MOVL 4(R2), R3   ; R2 considered unaligned
                            ; due to BLBC branch