mul is a multiplication instruction. When using mul for multiplication: the two numbers to be multiplied: either both are 8 bits or both are 16 bits.
8-bit: AL and 8-bit register or memory byte unit;
16-bit: AX and 16-bit register or memory word unit.
result
8 bits: AX;
16 bits: DX (high bit) and AX (low bit).
Format: mul register or mul memory unit
Reference: <<Assembly Language>> Wang Shuang