Debugging a Combinational Loop in Forwarding
A pipelined execution unit uses forwarding paths to resolve data hazards by routing computed values back to the input multiplexers. The execution stage computes a result combinationally based on an external input in_a and an operand in_b, then registers the result into out_val.
Expected behavior:
| Cycle | fwd_en | in_a | in_b | Expected out_val | |-------|----------|--------|--------|--------------------| | 1 | 0 | 1 | 0 | 1 | | 2 | 1 | 0 | 1 | 0 | | 3 | 1 | 0 | 0 | 0 | | 4 | 0 | 1 | 1 | 0 |
The synthesis log shows a combinational loop warning on the forwarding multiplexer. During Cycle 2, simulation hangs and outputs an indeterminate state X instead of the expected 0. Inspect the circuit and correct the fault.
Constraints
- Do not alter the boolean logic of the execution stage.
- The forwarding path must originate from a sequential element to break the zero-delay loop.
Topics
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