Intel IA-32 - Manual (Page 602)

17-10 Vol. 3A
IA-32 ARCHITECTURE COMPATIBILITY
is reserved on these processors. The addition of the SF flag on a 32-bit x87 FPU has no impact
on software. Existing exception handlers need not change, but may be upgraded to take advan-
tage of the additional information.
17.17.3 x87 FPU Control Word
Only affine closure is supported for infinity control on a 32-bit x87 FPU. The infinity control
flag (bit 12 of the x87 FPU control word) remains programmable on these processors, but has
no effect. This change was made to conform to the IEEE Standard 754 for Binary Floating-Point
Arithmetic. On a 16-bit IA-32 math coprocessor, both affine and projective closures are
supported, as determined by the setting of bit 12. After a hardware reset, the default value of bit
12 is projective. Software that requires projective infinity arithmetic may give different results.
17.17.4 x87 FPU Tag Word
When loading the tag word of a 32-bit x87 FPU, using an FLDENV, FRSTOR, or FXRSTOR
(Pentium III processor only) instruction, the processor examines the incoming tag and classifies
the location only as empty or non-empty. Thus, tag values of 00, 01, and 10 are interpreted by
the processor to indicate a non-empty location. The tag value of 11 is interpreted by the
processor to indicate an empty location. Subsequent operations on a non-empty register always
examine the value in the register, not the value in its tag. The FSTENV, FSAVE, and FXSAVE
(Pentium III processor only) instructions examine the non-empty registers and put the correct
values in the tags before storing the tag word.
The corresponding tag for a 16-bit IA-32 math coprocessor is checked before each register
access to determine the class of operand in the register; the tag is updated after every change to
a register so that the tag always reflects the most recent status of the register. Software can load
a tag with a value that disagrees with the contents of a register (for example, the register contains
a valid value, but the tag says special). Here, the 16-bit IA-32 math coprocessors honor the tag
and do not examine the register.
Software written to run on a 16-bit IA-32 math coprocessor may not operate correctly on a 16-bit
x87 FPU, if it uses the FLDENV, FRSTOR, or FXRSTOR instructions to change tags to values
(other than to empty) that are different from actual register contents.
The encoding in the tag word for the 32-bit x87 FPUs for unsupported data formats (including
pseudo-zero and unnormal) is special (10B), to comply with IEEE Standard 754. The encoding
in the 16-bit IA-32 math coprocessors for pseudo-zero and unnormal is valid (00B) and the
encoding for other unsupported data formats is special (10B). Code that recognizes the pseudo-
zero or unnormal format as valid must therefore be changed if it is ported to a 32-bit x87 FPU.
17.17.5 Data Types
This section discusses the differences of data types for the various x87 FPUs and math copro-
cessors.