So, a 6-bit ALU, or a 5-bit including the overflow as actual data? You might as well just throw in the extra digit on display, it would be a shame to have 28 integers go to waste. At that point you might as well upgrade to a 9-bit system with 512 values. The decoder will have to be a bit larger to accommodate 9 inputs, but the output (if BCD) will be the exact same size, three 4-bit scratch spaces. It will be interesting within a GPU, I'm thinking you could decode the binary data into BCD, then take each nybble of the BCD data (each representing a decimal digit) and run them through a binary to unary decoder. You would only need 10 unary outputs, as each nybble of BCD data is restricted to 0 - 9 or 0000 - 1001, thanks to the rule of adding 3 to any space >4 before the left shift occurs. From there the decoder can output to ROM lines or whatever you have to select which pixels make which number.
It sounds troublesome as you could go from binary straight into unary, but trust me, I'm doing you a huge favor. In pure unary to 7-segment conversion (pretty much the same as what you would be doing, only more simple) you have to build the memory for each digit... every single time. The beauty of the double dabble algorithm in redstone is that the BCD value can go straight into a small unary set up and you only have to build memory for your instructions to each digit once! I have made the mistake of going binary -> unary -> 7-segment before, and it's a horrible idea. It's just inefficient, clunky, and you end up with a parking garage full of redstone.
^^^ This is the one I wanted to show you, but it seems you can only download specific chapters. I have all of them on a flash drive if you would like me to send them in an e-mail. The Elements of Computing Systems teaches from more of a functional point of view, using examples of Boolean Logic.
^^^ This one is also good, but the first several pages aren't really applicable to redstone and don't have much to do with Computer Science itself. The knowledge contained is from an extremely low-level perspective, so it may seem a bit more confusing to someone without experience in the field. I would definitely recommend starting with the first link.
The Meaning of Life, the Universe, and Everything.
i am actually working on a cpu, so far it cant do much, but i have just begun. somewhere on these forums i have seen someone building a cpu with an integrated gpu, and theyhave built a monitor for it using lots of redstone torhes and pistons, but all it can seem to do is simple stuff, still impressive though. they got the monitor to use pistons to make a small movie thingamajig using pistons to pushout certain blocks at certain times