amw-zero/6502-rs — pull request #14 by akeeton (2014-10-20 01:22:14+00:00)
Title: Implement the DEC instruction

From http://www.obelisk.demon.co.uk/6502/reference.html#DEC:

> DEC - Decrement Memory
>
> M,Z,N = M-1
>
> Subtracts one from the value held at a specified memory location setting the zero and negative flags as appropriate.