The ADD and SUB instructions, respectively, add or subtract a byte to or from the accumulator. ADC and SBC do the same but with a carry or borrow if the condition code register carry bit (C) is set.
The MUL instruction performs an unsigned multiply of the index register (X) and the accumulator (A). The result is stored with the upper byte in X and the lower byte in A.
NEG, NEGA, and NEGX take the two's complement of a memory location, the accumulator, or the index register. The two's complement of a number is zero minus that number.