Short version : Both operands (registers snd and si4 (which can be a register or 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 si4 (which can be a register or a 6-bits or 16-bits sign-extended immediate) are added and the result is written to the destination register (dst).
The result is truncated to the 16 (YASEP16) or 32 (YASEP32) lower bits. The carry bit is later accessed with the CARRY condition.