multiply-high
An instruction that gives the top half of a product, the bits that do not fit in one register.
The full product of two n-bit values needs 2n bits; multiply-high returns bits n..2n-1 (x86-64 one-operand mul/imul into rdx; ARM64 umulh/smulh; RISC-V mulhu/mulh), signed or unsigned.
Examples
ARM64
umulh x0, x1, x2withx1=x2= 2³² gives 1: the top 64 bits of 2⁶⁴.
See also