AVR Assembly - Operations with two 8-bit registers in 16-bit
The \$x\$ and \$y\$ and \$z\$ registers are mostly for indirect references: &quot;These registers are 16-bit address pointers for indirect addressing of the data space.&quot; There aren't any ALU operations that support 16-bit because the ALU is an 8-bit ALU, not a 16-bit ALU. There's no escaping that requirement here.