Yes, due to the poor quality of the IEEE 754 specifications. In 1984,
the distinction was left completely unspecified (even in intent). In
2008, there is a recommendation (no more) that the top bit of the payload
is used, with no specification of what to do if that is zero (which is
the most obvious default). That appears to be the Intel specification,
though it may not have always been so even for x86. I have certainly
seen more than one convention for different architectures, but have no
idea how many are extant nor how many are documented in the relevant
architecture manuals.
In GCC, the qnan_msb_set bit of struct real_format is what specifies the
convention in use for a particular floating-point mode.
A single bit necessarily conveys at most one bit of information.
That can indicate whether the MSB being zero is the criterion;
it cannot indicate what the criterion is if that is not the case.
Regards,
Nick Maclaren.
