A.5.79 FFREE: Flag Floating-Point Register as Unused
FFREE fpureg ; DD C0+r [8086,FPU]
FFREEP fpureg ; DF C0+r [286,FPU,UNDOC]
FFREE marks the given register as being empty.
FFREEP marks the given register as being empty, and then pops the
register stack.