register pair

Two registers used together as one double-width number, like rdx:rax on x86-64, high half first.

A register pair such as rdx:rax holds a 2n-bit value with the high half in the first register; x86-64 mul writes its product there and div reads its dividend from there.

Examples

x86-64 mul rdi writes the 128-bit product into rdx:rax, the high half in rdx.