2 * mpvecsub(mpdigit *a, int alen, mpdigit *b, int blen, mpdigit *diff)
4 * diff[0:alen-1] = a[0:alen-1] - b[0:blen-1]
6 * prereq: alen >= blen, diff has room for alen digits
16 XORL BP,BP /* this also sets carry to 0 */
18 /* skip subraction if b is zero */
22 /* diff[0:blen-1],borrow = a[0:blen-1] - b[0:blen-1] */
36 /* diff[blen:alen-1] = a[blen:alen-1] - 0 */