Injecting tag into mantissa of IEEE floating-point number

We all know how it is done for fixnums, but how does one hacks a 64-bit FPU number to snatch a few mantissa bits for a tag? I want it unboxed!

>>4

Alternatively, one can break memory into 2^N sized chunks, putting the corresponding tag into the first byte of every chunk. That way memory address itself would tell the type.