C, divide and carry macro

[mcweaver1]

Hi,

I am trying to write a macro which performs a divide and carry. I have been given a macro which performs a multiply and carry:



# define LIMB_MAC(r1 ,r0 ,e ,f ,g , h) \ (1)
{ \ (2)
DWORD t0 = ( DWORD )( e ); \ (3)
t0 = t0 * ( DWORD )( f ); \ (4)
t0 = t0 + ( DWORD )( g ); \ (5)
t0 = t0 + ( DWORD )( h ); \ (6)
\ (7)
r0 = ( WORD )( t0 ); \ (8)
r1 = ( WORD )( t0 >> BITSOF ( WORD ) ); \ (9)
}


where, for x/y:
r1 = the carry out
r0 = the result
e = each bit of x (dealt with in turn in a for loop)
f = each bit of y ( " " )
g = each bit of the temporary array (currently filled with 0s) used to hold the result
h = the carry in

To write my own macro I need to be able to understand what is happening here. Could someone please explain it to me what goes on in lines 5, 6, and 9?

Thanks in advance.