Weird ALU issue that I have no idea how to debug
I’m building and troubleshooting an ALU, and I’ve run into a weird issue.
When the B register is all 0s and the A register is set to 00000001, the SUM register is showing 01000001. This suggests the second LED is stuck high.
I traced the wiring and found that A_6 was incorrectly connected to pin 3 (which is tied to the second LED). I moved it to pin 14 of U20 (where it’s supposed to go), but then the first LED lights up instead of the second.
At this point I’m confused because:
• With B = 00000000 and A = 00000001, the SUM should be 00000001, but I get 01000001.
• When A_6 is floating, I get 10000001.
• When A_6 is connected to pin 14 (correct location), I get 01000001.
Even more strange:
• If I manually set only the second LED on the A register, the output shifts and the first LED lights instead.
• So instead of getting 01000000, I get 10000000.
It seems like the bit positions are shifting or getting swapped somewhere in the SUM output.
Final summary of behavior:
• Rightmost LED ON on A register → SUM looks mostly correct but with an extra second bit high.
• Second LED ON on A register → output shifts left, and the first LED becomes high instead.
What should I check next to debug this?