help with the conception of floating point

[Eric Fisher]

I guess that the macro NGARDS is relevant to the guard digit. But I
still failed to have a clear conception about what it means. Others
are easy to know by IEEE 754 and "What Every Computer Scientist Should
Know about Floating-Point Arithmetic". Except,

define NGARDS 8L
define GARDROUND 0x7f
define GARDMASK  0xff
define GARDMSB   0x80

what do these mean? :-)

Especially when unpack a signal floating point, the fraction takes
such an operation at last, that make me much confused.
dst->fraction.ll = (fraction << NGARDS) | IMPLICIT_1;

I just want to know what does this mean.:-)

Thanks a lot.
Eirc.