The IEEE 754 floating-point standard defines two types of not-a-number (NaN) data: “signalling” NaNs and “quiet” NaNs. The original version of the standard did not specify how these two types should be distinguished. Most implementations followed the i387 model, in which the first bit of the significand is set for quiet NaNs and clear for signalling NaNs. However, the original MIPS implementation assigned the opposite meaning to the bit, so that it was set for signalling NaNs and clear for quiet NaNs.
The 2008 revision of the standard formally suggested the i387 choice
and as from Sep 2012 the current release of the MIPS architecture
therefore optionally supports that form. Code that uses one NaN encoding
would usually be incompatible with code that uses the other NaN encoding,
so MIPS ELF objects have a flag (EF_MIPS_NAN2008) to record which
encoding is being used.
Assembly files can use the .nan directive to select between the
two encodings. ‘.nan 2008’ says that the assembly file uses the
IEEE 754-2008 encoding while ‘.nan legacy’ says that the file uses
the original MIPS encoding. If several .nan directives are given,
the final setting is the one that is used.
The command-line options -mnan=legacy and -mnan=2008
can be used instead of ‘.nan legacy’ and ‘.nan 2008’
respectively. However, any .nan directive overrides the
command-line setting.
‘.nan legacy’ is the default if no .nan directive or
-mnan option is given.
Note that gnu as does not produce NaNs itself and
therefore these directives do not affect code generation. They simply
control the setting of the EF_MIPS_NAN2008 flag.
Traditional MIPS assemblers do not support these directives.