Complex sentence: Integral return values up to 64 bits in size are stored in RAX while values up to 128 bit are stored in RAX and RDX, while floating-point return values are similarly stored in XMM0 and XMM1.
Split into simpler sentences: Integral return values up to 64 bits in size are stored in RAX while values up to 128 bit are stored in RAX and RDX. Floating-point return values are similarly stored in XMM0 and XMM1.