Control Flow Unit (CFU)

The control flow unit arbitrates between control flow redirect notifications from multiple sources (execute units, the decode-issue unit) 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 to any other units that must react to a pipeline redirect. In processors with branch prediction, the unit also forwards branch predictor update signals (bp_update_val, taken, pc, target) from the winning source so that the fetch unit’s branch predictor can learn from resolved branches and jumps.

All control flow unit variants operate 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.

CtrlFlowUnit L1

The Level 1 Control Flow Unit (CtrlFlowUnitL1) arbitrates between p_num_arb redirect sources using a binary-tree structure of pairwise CtrlFlowUnitL1Helper arbiters. Each helper selects the older of two notifications based on their sequence numbers, using the SeqAge module for wrap-around-safe age comparison. The SeqAge module tracks age via the single-lane CommitNotif interface.

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. 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. 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.

CtrlFlowUnit L1 Chain

The CtrlFlowUnitL1Chain is a variant of the L1 unit that uses a linear chain instead of a binary tree. Starting from source 0, each stage compares the running winner against the next source and carries the older one forward. This produces a longer combinational path (O(n) instead of O(log n)) but uses simpler wiring and may be preferred when the number of sources is small.

CtrlFlowUnit L2

The Level 2 Control Flow Unit (CtrlFlowUnitL2) extends L1 with support for a superscalar backend. Instead of the single-lane SeqAge module, L2 uses SSSeqAge to track the oldest committed sequence number across p_num_be_lanes commit lanes. This allows correct wrap-around-safe age comparison when multiple instructions commit per cycle.

The arbitration structure is the same binary tree of pairwise helpers as L1, but the CtrlFlowUnitL2Helper receives the oldest_seq_num directly from SSSeqAge rather than instantiating its own age tracker. A single SSSeqAge instance is shared across all helpers in the tree.

CtrlFlowUnit L3

The Level 3 Control Flow Unit (CtrlFlowUnitL3) extends L2 with a decoupled grant output for the decode-issue unit, which is necessary when the DIU’s control flow publisher (DIUCtrlFlowPublisher) is combinational. It 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. This breaks the combinational loop that would otherwise form: the DIU publishes a redirect combinationally, the control flow unit arbitrates it, and the result feeds back into the DIU.

Both grants use the same SSSeqAge instance for age tracking. The full grant uses a binary tree of CtrlFlowUnitL3Helper 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.

The L3 helper also passes through the full ControlFlowNotif fields (pc, target, taken, bp_update_val) from the winning source, enabling the branch predictor to receive update information through the arbitrated grant.