Level 8: 8-bit Adder
Task
Build an 8-bit adder (ADDER8) using FullAdders and buses.
Build an 8-bit adder from a chain of eight full adders (FullAdder). Like column addition for 8-bit numbers.
Solution
Notice the carry chain: bit 7 (the most significant) is computed last — the carry must "run" through all 8 adders. That is propagation delay: the longer the chain, the later the result settles and the lower the maximum clock frequency. See the Propagation Delay article.
Step 1 — split the buses: Place TWO Splitters. Feed bus A to the first, bus B to the second. Each Splitter outputs 8 bits (Bit0-LSB ... Bit7-MSB).
Step 2 — place adders: Place 8 FullAdders vertically. Feed each with corresponding bits: - FullAdder#1: A.Bit0 and B.Bit0 - FullAdder#2: A.Bit1 and B.Bit1 - ... - FullAdder#8: A.Bit7 and B.Bit7
Step 3 — carry chain: Connect CarryOut of first adder to CarryIn of second, second CarryOut to third CarryIn, and so on. Leave first CarryIn unconnected (defaults to 0).
Step 4 — collect result: Feed Sum outputs of all 8 adders to Maker inputs (Sum#1→Bit0... Sum#8→Bit7). Maker output is the circuit Sum. Last adder CarryOut is Carry.
