Sorry. In the old spec(version 0.41), nanboxing is not totally disabled, 
but "NaN-boxing is limited to |XLEN| bits, not |FLEN| bits". Taking 
misa.mxl into acount, if misa.mxl is RV32, and maximum is RV64, this 
should be sign-extended. Is there any other new update for nanboxing  to 
the spec?

在 2021/12/26 上午6:00, Richard Henderson 写道:
> On 12/24/21 7:13 PM, liweiwei wrote:
>> In RV64 case, this should be nan-boxing value( upper bits are all 
>> ones).  However, zfinx will not check nan-boxing of source, the upper 
>> 32 bits have no effect on the final result. So I think both 
>> zero-extended or sign-extended are OK.
>
> There is no nanboxing in zfinx at all -- values are sign-extended.
>
>
> r~