Control Flow Unit (CFU)

The Control Flow Unit (CtrlFlowUnit) arbitrates between control-flow redirect notifications from multiple sources – the decode-issue unit (for JAL/JALR) and the control-flow execute unit (for resolved conditional branches) – and selects a single winner each cycle. The winning redirect is broadcast to the fetch unit (to restart instruction fetching at the correct target) and back to the decode-issue unit (to flush the current instruction window). In configurations with branch prediction, the unit also forwards branch-predictor update signals (bp_update_val, taken, pc, target) from the winning source so the fetch unit’s branch predictor can learn from resolved branches and jumps.

The CFU operates on the ControlFlowNotif interface, which carries redirect_val (fetch must redirect), bp_update_val (branch/JAL resolution data is valid for BP update), seq_num, pc, target, and taken.

Arbitration

CtrlFlowUnit arbitrates between p_num_arb redirect sources using a binary-tree structure of pairwise CtrlFlowUnitHelper arbiters. Each helper selects the older of two notifications based on their sequence numbers, using the oldest_seq_num from a single shared SSSeqAge instance for wrap-around-safe age comparison across the p_num_be_lanes commit lanes (see Sequence Numbers). The binary tree is built at elaboration time: leaf nodes connect to the arb interface array, and each level halves the number of candidates until a single winner emerges at the root.

When only one source is valid, it wins unconditionally. When both sources are valid, the older notification (the one with the earlier sequence number) is selected. Unused leaf slots (when p_num_arb is not a power of two) are zero-filled with redirect_val = 0. For the trivial p_num_arb == 1 case, the single source passes through directly with no arbitration hardware.

Helper modules pass through the full ControlFlowNotif payload (pc, target, taken, bp_update_val) from the winning source, so the branch predictor receives update information through the arbitrated grant.

Decoupled Grant for the DIU

The CFU produces two grant outputs:

• ``gnt``: The full arbitration result across all p_num_arb sources, broadcast to the fetch unit and other subscribers.

• ``gnt_excl``: The arbitration result excluding the DIU source (identified by p_diu_idx), connected to the DIU’s own ctrl_flow_sub input.

The decoupled grant breaks the combinational loop that would otherwise form: the DIU publishes a redirect combinationally on its ctrl_flow_pub interface, the CFU arbitrates it into gnt, and gnt would feed back into the DIU. By stripping the DIU’s own redirect out of the path that returns to the DIU, the loop is broken without needing a sequential break in the fetch-redirect path. Both grants use the same SSSeqAge instance for age tracking. The full grant uses a binary tree of arbiters across all sources; the excluded grant uses a second binary tree that omits the DIU source. When p_num_arb == 2 (the common case with one XU source and one DIU source), gnt_excl is a zero-cost passthrough of the single non-DIU source, requiring no additional arbitration hardware.