pub fn wmul256(a: &[u64; 4], b: &[u64; 4]) -> [u64; 4]Expand description
The low 256 bits of a * b — schoolbook over LIMBS limbs, carries past the top limb dropped
(the wraparound a fixed 256-bit datapath produces). This is bigint_mul (64×64) composed with
the proven bigint_alu carry-adds; the reference for the soft-core’s BigIntWide Mul.