8-bit CPU: ALU and Flags
Now that I have a pair of devices that can store a byte, I can proceed to do something more interesting. Build a device that add them together, for example. In other words - let's build an ALU. The design follows same basic idea as Ben's - two 4-bit adders, XOR gates and an output buffer. I'm using HC logic here, so I used 74HC283, 74HC86 and 74HC245 chips. Since I've built only one Register and did not want to build another, I connected my Program Counter board as register B. If I keep the counting function disabled, it serves the purpose just fine. It took a lot of wires, but I'm just getting started, let's build Flags register as well. There were no surprises with Zero flag - 4 NOR and 3 AND gates does the job - just following the Ben's design. Another latch (74HC173) chip to store it, done. The fun starts with the Carry flag: I wanted to support a carry-in signal for the ALU. It is very useful if one wants to perform the addition or subtraction for wid