ref: e81c54ba2ecc673a4d5f8aed0e9b52841fe07b0d
dir: /os/pc64/fpsave.s/
TEXT FPsave(SB), 1, $0 /* save FPU environment without waiting */ MOVQ RARG, AX FSTENV 0(AX) RET TEXT FPrestore(SB), 1, $0 /* restore FPU environment without waiting */ MOVQ RARG, AX FLDENV 0(AX) RET