When completing function selector board I got the idea to make yet one more board for ALU. This last board should provide a single interface through which the ALU would be connected to the rest of the CPU. Also I hoped that magnitude determination circuit would not be too complex and be able to fit on this board. Turned out this wasn't main concern -- I think, it could be fit on that board with place to spare; the main drawback was long chain of OR gates needed which would make signal propagation big issue. So I opted to have a small function of byte sign extend (BSE) which is just copies bit 7 to all the higher bits.
Instead of adding a complex ALU function to the interface board, I opted to make it a display for the ALU: it has LED banks to show inputs and outputs, as well as individual LEDs to show flags and to indicate which instruction ALU is doing at the moment.
Also the board has bus drivers on result output which make use of ALU enable signal -- if it is 0, the ALU output is floating.
Below is board's photo with captions:
Next step is integrating all these boards into functional ALU, and testing it out.