x86 Instruction Set Reference

FFREE - Free Floating-Point Register

Opcode Mnemonic Description
DD C0+i FFREE ST(i) Sets tag for ST(i) to empty.
Description

Sets the tag in the FPU tag register associated with register ST(i) to empty (11B). The contents of ST(i) and the FPU stack-top pointer (TOP) are not affected.
Operation 
Tag(i) = 3; //11B
FPU flags affected

C0, C1, C2, C3 undefined.

Floating-Point Exceptions
None.
Protected Mode Exceptions
#NMEM or TS in CR0 is set.
Real-Address Mode Exceptions
#NMEM or TS in CR0 is set.
Virtual-8086 Mode Exceptions
#NMEM or TS in CR0 is set.