The DEC instruction subtracts 1 from the "A" register, 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.

4F0A - Store the result to the "A" register.
4F0B - Store the result to the "B" register.
4F0C - Store the result to the "C" general purpose register.
4F0F - Store the result to the "F" output high register.

4F00 - Store the result to memory, at the current active address.
4F01 - Store the result to address pointer 1.
4F02 - Store the result to address pointer 2.