Floating point operations; Exception, Flags, and Trap Handlers I am reading over the article found here specifically §D.4.4 Exceptions, Flags and Trap Handlers and am confused by the table D-4 in that section. 

Specifically the arguments sent to the trap handler for the overflow and and underflow exceptions. 

It shows round(x2-α) as the argument for both the over and underflow conditions (EDIT: It is not). I am not sure what x2 is at all, I see no reference to it in the section. 

As for α footnote says:


  *xis the exact result of the operation,  α= 192 for single precision, 1536 for double, and xmax= 1.11 …11 ×2^e_max.


So it says  "α = 192 for single precision, 1536 for double" which is also confusing. Where do these values come from?

I am not looking for anything super in-depth, just something succinct.
Your problem is typographical. Look at a well-formatted copy and it should all make sense. So for example, the argument to the trap handler on overflow is $\hbox{round}(x 2^{-\alpha})$, where $x$ is the exact value.

The value of $\alpha$ comes from the fact that the exponent is 8 bits in binary32 (previously called "single") and 11 bits in binary64 (previously called "double"). You can think of $\alpha$ as an unsigned integer the size of the exponent field, with the two most significant bits set. Why it would be that number in particular is left as an exercise; think the possible range of an overflowing operation and what would be required to rescale it back into a representable range.