Volume 3: General-Purpose and System Instructions - Stanford ...

24594 Rev. 3.10 February 2005 AMD64 TechnologyThere are two significant ranges for the ModRM byte for x87opcodes: 00–BFh and C0–FFh. When the value of the ModRMbyte falls within the first range, 00–BFh, the opcode uses onlythe reg field to further define the opcode. When the value of theModRM byte falls within the second range, C0–FFh, the opcodeuses the entire ModRM byte to further define the opcode.Byte values shown as reserved or invalid in Table A-10 haveimplementation-specific functions, which can include aninvalid-opcode exception.The basic instructions FNSTENV, FNSTCW, FNCLEX, FNINIT,FNSAVE, FNSTSW, and FNSTSW do not check for possiblefloating point exceptions before operating. Utility versions ofthese mnemonics are provided that insert an FWAIT (opcode9B) before the corresponding non-waiting instruction. These areFSTENV, FSTCW, FCLEX, FINIT, FSAVE, and FSTSW. Forfurther information on wait and non-waiting versions of theseinstructions, see their corresponding pages in Volume 5.Appendix A: Opcode and Operand Encodings 393