Short version : Both operands (registers snd and si4, which can also contain a 4-bit sign-extended immediate value) are added together and the result is written to the destination register (snd).
Long version : The source register (snd) and the immediate field (Imm16, sign-extended with YASEP32) are added and the result is written to the destination register (si4).
The result is truncated to the 16 (YASEP16) or 32 (YASEP32) lower bits. The carry bit is later accessed with the CARRY condition.