Add Two 16-bit Numbers

Add two 16-bit numbers on an 8-bit AVR core, where the addition has to be done a byte at a time with an explicit carry.

Constraints

• Operands are in r24:r25 and r22:r23.
• Result goes in r24:r25.
• Use ADD for the low byte and ADC for the high byte.
• Do not use any 16-bit instruction such as ADIW.
• Preserve all other registers.
• Return with RET.