The NUL instruction outputs the value "0", and stores the result to a register, counter, or location in memory. For the latter, the address pointer must be setup before the ALU writes to memory. While technically a logical operation (the "0" evaluates as "false"), this is also useful for producing a zero value.

438A - Store the result to the "A" register.
438B - Store the result to the "B" register.
438C - Store the result to the "C" general purpose register.
438F - Store the result to the "F" output high register.

4380 - Store the result to memory, at the current active address.
4381 - Store the result to address pointer 1.
4382 - Store the result to address pointer 2.