Logic control word ← 037Fh status word ← 0000h tag word ← 0FFFFh
FINIT/FNINIT put the FPU in a known state, unaffected by any previous activity. In fact, FINIT/FNINIT affect the FPU in the same way as a hardware RESET signal with Built-In Self-Test. They do not affect the synchronization between the 80x86 and the 80x87.
Before initializing the FPU, FINIT checks for unmasked FP error conditions (using an FWAIT instruction); FNINIT does not make this check.
FINIT/FNINIT do the following: - set the FPU control word to 037Fh, i.e. round to nearest, 64-bit precision, all exceptions masked - clear the status word, i.e. no exception flags set and stack register R0 = ST(0) - tag all stack registers as empty - clear instruction and error data pointers (80486 FPU+ only)