This instructions computes a 8×8-bit multiply with the lower bits of the first operand (si4 which can be a register or an immediate value), and the higher byte of the second operand (snd), giving a 16-bit result that can be combined (by ADD) with the result of a previous MUL8L.
This is an unsigned operation, signed multiplies are not yet supported. Operands larger than 8 bits require a series of MUL8 and ADD.