27.3 Freeing floating point registers (all processors)
You have to free all used floating point registers before exiting a subroutine, except for any register used for the result.
The fastest way of freeing one register is FSTP ST. The fastest way of freeing two registers is FCOMPP on PPlain and PMMX; on PPro, PII and PIII you may use either FCOMPP or two times FSTP ST, whichever fits best into the decoding sequence.
It is not recommended to use FFREE.