Branch Redirect and Flush
Redirect fetch and invalidate younger work after a taken branch.
Task
Select the next PC between sequential PC+4 and the branch target, and assert flush whenever a branch is taken.
Examples
- branch_taken=0 → next_pc=pc_plus4.
- branch_taken=1 → next_pc=branch_target and flush=1.
Constraints
- Combinational logic.
- Flush exactly follows taken redirect.
- No speculative state beyond the redirect decision.
What this problem is testing
A strong answer translates the requirement into synthesizable state and combinational logic, states reset and update semantics precisely, and handles boundary conditions rather than only the nominal case.
Interview habit: before coding, state reset behavior, the state that must be stored, and the conditions that update each output.
Verification plan
- Check reset or initialization before normal operation.
- Exercise the nominal sequence cycle by cycle.
- Test boundaries such as zero, maximum count, wraparound, full/empty, or simultaneous controls when they apply.
- Prove the design holds state when an enable or qualifying condition is absent.
Common mistakes
- Incomplete combinational assignments that infer a latch.
- Blocking/non-blocking assignments that do not match combinational versus sequential intent.
- Parameter-width and boundary errors.
- Passing the happy path while missing reset, overflow, simultaneous operations, or illegal requests.
Interview follow-up questions
- How would you parameterize this design without introducing width or boundary bugs?
- What changes if reset timing or polarity changes?
Concepts to review
RISC-V, branch, flush
Open Branch Redirect and Flush in the HDLForge interactive editor.
Related problems
- 2:1 Multiplexer — Easy
- Parameterized Counter — Easy
- Priority Encoder — Medium
- Synchronous FIFO — Hard
- Shift Register — Easy
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